Resource Paper: Sonoluminescence

Although sonoluminescence was discovered relatively early in the 20th century, it did not gain the general interest of the scientific community until sonochemistry was determined to have commercial applications in the latter part of the century. Because sonoluminescence was later believed to have a strong correlation with the sonochemical yield, and provided a noninvasive measure of sonochemical activity, it was studied in some earnest by chemists (although this correlation was later shown to be misleading as certain solutes could quench the sonoluminescence without reducing the sonochemical yield). The sonoluminescence that was produced by a cavitation field is now known as multibubble sonoluminescence (MBSL). The fortuitous discovery of single bubble sonoluminescence (SBSL) ushered in an intense period of research activity, mostly by physicists, and even captured the attention of the wider scientific community when it was proposed to be a product of quantum radiation, and, under certain conditions, could even produce nuclear fusion. In a 15 year period between 1990 and 2005, over 1000 papers were published on the topic. Because the attention to SBSL was so intense, and new discoveries appear every month, this resource paper has listed the publications mostly in terms of their date of publication, with specific reference to topical areas of interest. With such a large number of publications, and the limits on space, the author has used the number of citations as a guide to their inclusion, with the exception of recent papers that have not been in circulation long enough to acquire a relatively large number of citations.

It is perhaps useful to explain that this paper is meant for both the novice who wants to learn something about the topic, and the experienced scientist who wants a listing of the most important publications, including the most recent ones. It is also important to state that this is not a review paper, with critical evaluations of the merits of individual papers. Indeed, many papers are included that propose explanations of sonoluminescence that have been widely discredited by subsequent experiments. The author believes that these papers should be included for historical interest, and also because there are still proponents of some models that are not widely accepted, but still have their fervent constituency. For example, although the explanation of sonoluminescence as quantum radiation has not stood the test of experimentation, it was advocated by a famous Nobel Prize winner in a long series of papers. Although the “hot spot” (thermal) model of sonoluminescence is the most widely accepted model, there are still strong advocates of the electrical discharge model, with some papers receiving over 100 citations, most of which are supportive, and some very recent. The author has also included many of the papers that claim that nuclear fusion can be obtained by acoustic cavitation (and the emission of sonoluminescence used as a time point for the fusion); again, these claims have been largely refuted, but this author knows of at least two companies that are spending millions of dollars trying to achieve “cavitation fusion.” Should the reader find the inclusion of many seemingly contradictory papers confusing, it should be understood that there are differences of opinion from credible scientists and strong advocates for their respective positions.

Finally, for the convenience of the reader, each reference is followed by the letter A (for advanced), I (for intermediate), or E (for elementary).

• 1.

  “Action des ultrasons sur les plaques photographiques,” N. Marinesco and J. Trillat, C. R. Acad. Sci. Paris 196, 858–861 (1933). (E) It appears that this is the first published reference to sonoluminescence, although with the development of large sonar arrays after World War I, it is unlikely that it was not observed earlier. Marinesco and Trillat report that they were trying to accelerate the development of photographic film, and noticed that the film was exposed. It is not clear if they understood that it was cavitation induced sonoluminescence that was responsible.

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• 2.

  “Luminescenz im Ultraschallbeschickten Wasser,” H. Frenzel and H. Schultes, Z. Phys. Chem. B 27: 421–427 (1934). (E). Frenzel and Schultes also hoped to speed the development of photographic film using a sonar transducer. After developing the film, and seeing small dots on the film and many acoustically active bubbles in the liquid, they reasoned that these bubbles were probably causing the exposure and concluded that it was due to friction between the bubbles and the film.

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• 3.

  “The emission of visible light from cavitated liquids,” L. Chambers, J. Chem. Phys. 5, 290–295 (1937) (E) Chambers performed a systematic study of light emission from a transducer at 8.9 kHz, using the dark-adapted eye as a detector (although no mention was made of ear protectors). Light emission were observed for 14 liquids, although none was observed for 22 others. https://doi.org/10.1063/1.1750025

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• 4.

  “Sonoluminescence and sonic chemiluminescence,” E. Newton Harvey, J. Am. Chem. Soc. 61, 2392–2398 (1939). (E) Harvey undertook an extensive study of sonoluminescence, mostly in water, although he added a number of different gases. He concluded that the light emissions were due to electrical discharges. Although Harvey was not the discoverer of sonoluminescence, he was apparently the first to call it by its present name, writing “It is customary to designate a luminescence by the method of excitation, as electroluminescence, photoluminescence, triboluminescence…. Accordingly the luminescence which appears when sound waves pass through liquids has been called acoustic or sonic luminescence, for short, sonoluminescence.” https://doi.org/10.1021/ja01878a037

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• 5.

  “Cavitation produced by ultrasonics,” B. Noltingk and E. Neppiras, Proc. Phys. Soc. B 63, 674–685 (1950). (E) Until Noltingk and Neppiras published this paper on bubble dynamics, the general impression was that sonoluminescence was due to electrical discharge. This paper showed that the gas in the interior of the bubble was most likely heated to incandescence, giving rise to the “hot spot” model of sonoluminescence, the model currently favored by most researchers. https://doi.org/10.1088/0370-1301/63/9/305

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• 6.

  “Sonoluminescence: A discussion,” P. Jarman, J. Acoust. Soc. Am. 32, 1459–1462 (1960). (E) A brief summary of existing knowledge about sonoluminescence is given and this relatively early stage of sonoluminescence research, i.e., before the discovery of SBSL. Various theories of the origin of this luminescence are discussed, including the “hot spot” and the electrical discharge models. The author concludes that sonoluminescence is basically thermal in origin and that it might possibly arise from microshocks within the collapsing cavities, a prescient deduction. (Arguments as to whether true “shock” waves exist within the gas, or they are just intense pressure waves, are ongoing to this day.) https://doi.org/10.1121/1.1907940

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• 7.

  321. “Uber den Zusammenhang Zwischen der Sonolumineszenz and der Schwingungskavitation in Flussigkeiten,” H. Kuttruff, Acustica 12, 230–254 (1962). (E) In this early study of sonoluminescence, Kuttruff was able to demonstrate that the light flashes were coincident with shock waves emitted during bubble collapse, thus establishing a causal relationship between cavitation and sonoluminescence. He also was able to observe sonoluminescence in mercury by using a transparent mechanical oscillator.

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• 8.

  “Sonoluminescence,” A. Walton and G. Reynolds, Adv. Phys. 33, 595–660 (1984). (E) Very extensive review with over 100 references, with descriptions of the many ways of generating sonoluminescence, including non-acoustics means. Good description of bubble dynamics and why it is important in understanding the topic. Highly cited. https://doi.org/10.1080/00018738400101711

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• 9.

  “Acoustic cavitation generated by microsecond pulses of ultrasound,” L. Crum and J. Fowlkes, Nature 319, 52–54 (1986). (E) When the use of diagnostic ultrasound became widespread, concerns were raised as to their potential harmful effects, in particular, acoustic cavitation. Crum and Fowlkes were able to detect sonoluminescence (and presumably cavitation) produced by a single acoustic cycle of ultrasound at diagnostic frequencies. https://doi.org/10.1038/319052a0

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• 10.

  “Studies of the cavitational effects of clinical ultrasound by sonoluminescence: 3. Cavitation from pulses a few microseconds in length,” M. Pickworth, P. Dendy, T. Leighton, E. Worpe, and R. Chivers, Phys. Med. Biol. 34, 1139–1151 (1989). (E) In this study of low frequency clinical ultrasound devices, the authors were able to show that the cavitation generated by these devices produced sonoluminescence. Sonoluminescence was observed for pulses of a few cycles, but the ultrasound intensity threshold for onset increased sharply with decreasing pulse length. https://doi.org/10.1088/0031-9155/34/9/001

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• 11.

  “Sonoluminescence of non-aqueous liquids,” K. Suslick and E. Flint, Nature 330, 553–555 (1987). (E) In a pioneering paper, Suslick and Flint were able to examine the spectra of sonoluminescence in a nonaqueous liquid and were able to correlate the spectra with specific molecular emission bands (the Swan bands), demonstrating rather conclusively that these spectra originate unambiguously from excited-state molecules created during acoustic cavitation. Furthermore, they suggested that these high-energy species probably result from the recombination of radical and atomic species generated during the high temperatures and pressures of cavitation. https://doi.org/10.1038/330553a0

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• 12.

  “Sonoluminescence from nonaqueous liquids: Emission from small molecules,” E. Flint and K. Suslick, J. Am. Chem. Soc. 111, 6987–6992 (1989). (A) Sonoluminescence spectra were observed from several nonaqueous liquids in the presence of various gases. They concluded that sonoluminescence from organic liquids was caused by emission from small free radicals and molecules, such as C2, CN, C02, and Cl. https://doi.org/10.1021/ja00200a014

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• 13.

  “Sonoluminescence,” R. Verrall and C. Sehgal, Ultrasonics 25, 29–30 (1987). (I) Until this paper, there was still uncertainly as to the origins of sonoluminescence. Here, the authors report the first spectrally resolved sonoluminescence spectra from hydrocarbon and halocarbon liquids. These spectra originated unambiguously from excited-state molecules created during acoustic cavitation, and almost certainly resulted from the recombination of radical and atomic species generated during the high temperatures and pressures of cavitation. https://doi.org/10.1016/0041-624X(87)90007-2

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• 14.

  “On the origin of sonoluminescence and sonochemistry,” K. Suslick, S. Doktycz, and E. Flint, Ultrasonics 28, 280–290 (1990). (I) By this date, much of multibubble sonoluminescence and chemiluminescence was understood. The hot-spot model of sonoluminescence was accepted by the overwhelming number of scientists and chemiluminescence was shown to be due to the radiative recombination of free radicals produced by the high pressures and temperatures during the final states of collapse of the bubble. This paper summarizes the state of the knowledge at this time. https://doi.org/10.1016/0041-624X(90)90033-K

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• 15.

  “Sonochemistry and Sonoluminescence,” K. Suslick and L. Crum, in Handbook of Acoustics, edited by M. Crocker (Wiley, Hoboken, NJ, 1998), pp. 242–251. (E) This book is a comprehensive overview of acoustics, with a general review article on sonochemistry and sonoluminescence.

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• 16.

  Sonochemistry and Sonoluminescence, edited by L. Crum, T. Mason, J. Reisse, and K. Suslick (Kluwer Academic, Dordrecht, the Netherlands, 1999). (A) This book consists of the Proceedings of NATO Advanced Study Institute on Sonochemistry and Sonoluminescence, held at Leavenworth, Washington, USA, 18–29 August 1997. It is an excellent compilation of the state of the art of the relevant science at the time, as it assembled over 73 investigators from 19 countries around the world and contains their most recent work.

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• 17.

  Sonoluminescence, F. Ronald Young (CRC, Boco Raton, FL, 2005). (E) A good review with 223 pages and reasonably complete coverage of the various individual topics.

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• 18.

  The Acoustic Bubble, T. G. Leighton (Academic Press, London, 1994). (E) Perhaps the most complete and comprehensive book on bubble acoustics, with an excellent chapter on sonoluminescence.

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• 19.

  Shock Focussing Effect in Medical Science and Sonoluminescence, edited by R. Srivastava, D. Leutloff, K. Takayama, and H. Gronig (Springer, Berlin, 2003). (A) This book contains chapters on cavitation and sonoluminescence, but mainly deals with the focusing aspects of shock waves.

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• 20.

  Mechanoluminescence, Sonoluminescence from Acoustic Cavitation, N. Eddingsaas (ProQuest, Ann Arbor, 2009). (I) Relatively up-to-date coverage of cavitation and bubble dynamics including sonoluminescence.

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• 21.

  “Sonoluminescence,” L. Crum and R. Roy, Phys. Today September, 22–29 (1994). (E) A good early reference article describing sonoluminescence for the informed layman. It gives an introduction to the topic and provides a brief description of SBSL relevant to that time period.

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• 22.

  “Sonoluminescence: Sound into light,” S. Putterman, Sci. Am. February, 32–37 (1995). (E). Excellent article overviewing the status of sonoluminescence research at that time. It also contains in the same issue in The Amateur Scientist section a description of how to construct an apparatus to observe sonoluminescence.

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• 23.

  “Sonoluminescence: Nature's smallest blackbody,” G. Vazquez, C. Camara, S. Putterman, and K. Weninger, Opt. Lett. 26, 575–577 (2001). (E) A short review of the status of sonoluminescence research at that time period, particularly from an optics viewpoint. https://doi.org/10.1364/OL.26.000575

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• 24.

  “Sonoluminescence: The star in the jar,” S. Putterman, Phys. World May, 38–42 (1998). (E) A very nice popular article in Physics World magazine.

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• 25.

  “Sonoluminescence: And there was light!” Robert Apfel, Nature 398, 378–379 (1999). (E) Contains an excellent overview of recent research, describing the contrary views of Hilgenfeld and Lohse that there is a rather simple explanation for SBSL, and that of Putterman that it is much more complicated, involving plasma physics that is still not understood. https://doi.org/10.1038/18786

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• 26.

  “Sonoluminescence: Shocking Revelations,” L. Crum and T. Matula, Science 276, 1348–1351 (1997). (E) Contains a brief review of the latest research at the time, including comments on a paper in the same issue by Moss et al. on computer simulations of sonoluminescence. Using sophisticated computer codes developed for research on inertial confinement fusion, Moss et al. were able to explain the nature of the emitted light. https://doi.org/10.1126/science.276.5317.1348

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• 27.

  “Bubbles hotter than the Sun,” L. Crum and K. Suslick, New Sci. 146, 36–40 (1995). (E) A lay-level description of SBSL, with some interesting figures, including the cover, in which a New Scientist artist depicted his conception of the phenomenon.

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• 28.

  “Physics: Far from the frontier,” Geoff Brumfiel, Nature 437, 1224–1225 (2005). (E) Contains a very nice personal portrayal of Seth Putterman, one of the pioneers of sonoluminescence research. https://doi.org/10.1038/4371224a

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• 29.

  “How snapping shrimp snap: Through cavitating bubbles,” M. Versluis, B. Schmitz, A. von der Heydt, and D. Lohse, Science 289, 2114–2117 (2000). (E) How snapping shrimp kill prey by generating a cavitation bubble and a water jet. https://doi.org/10.1126/science.289.5487.2114

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• 30.

  “Snapping shrimp make flashing bubbles,” D. Lohse, B. Schmitz, and M. Versluis, Nature 413, 477–478 (2001). (E) A remarkable paper that demonstrated that the loud noise produced by Snapping Shrimp is due to the violent collapse of a large cavitation bubble generated under the tensile forces of a high-velocity water jet formed when the shrimp's snapper-claw snaps shut. Furthermore, sonoluminescence is produced by the cavitation collapse, and is appropriately dubbed “shrimpoluminescence.” https://doi.org/10.1038/35097152

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• 31.

  “Cavitation science: Is there a simple theory of sonoluminescence?,” S. Putterman, P. G. Evans, G. Vazquez, and K. Weninger, Nature 409, 782–783 (2001). (E) It is argued that simple models of SBSL do not work, on the grounds that they cannot account for some well-established observations and that these simple models involve the application of equations outside their range of validity. They believe that there are complex plasma species generated and thus more sophisticated approaches are necessary for an adequate description of the phenomenon. https://doi.org/10.1038/35057317

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• 32.

  Sonoluminescence, M. Goodheart, G. Spearman, L. Ellis, and D. Robinson, audio CD (available from Amazon). An album of electronic music somehow meant to represent sonoluminescence.

• 33.

  “Chain Reaction,” a movie produced by The Zanuck Company, and distributed by 20 Century Fox, starring Keanu Reeves, Morgan Freeman, and Rachel Weisz (1996). In this movie, Morgan Freeman is looking for the “key to energy independence” and refers to sonoluminescence. Much of the scientific equipment used in the sets of the film, which was shot mostly in Chicago, came from the cold storage vault of the School of Chemical Sciences, University of Illinois at Urbana-Champaign.

• 34.

  “Sonoluminescence,” R. Finch, Ultrasonics 1, 87–98 (1963). (E) Although there was some research on sonoluminescence being undertaken at this time (he gives 80 references), the research is mostly in the general area of sonochemistry. There is an excellent discussion of the history, as well as that of the origin of the sonoluminescence emissions—which he describes as probably thermal in origin. https://doi.org/10.1016/0041-624X(63)90060-X

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• 35.

  “Sonoluminescence and sonochemical reactions in cavitation fields. A review,” M. Margulis, Ultrasonics 23, 157–169 (1985). (A) Margulis has been a proponent of the electrical theory of sonoluminescence, in which the (presumably) charged interfaces of the collapsing bubble exhibit electrical discharge and thus light emissions. Although there are still some advocates of this theory, nearly all experimental and theoretical studies discount this concept as a reasonable explanation for the phenomenon. https://doi.org/10.1016/0041-624X(85)90024-1

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• 36.

  “Sonoluminescence, sonochemistry, and sonophysics,” L. Crum, J. Acoust. Soc. Am. 95, 559–562 (1994). (E) A brief review of the role that sonoluminescence plays in sonochemistry and sonophysics, and the similarities and dissimilarities that are involved. https://doi.org/10.1121/1.408351

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• 37.

  “Defining the unknowns of sonoluminescence,” B. Barber, R. Hiller, R. Löfstedt, S. Putterman, and K. Weninger, Phys. Rep. 281, 65–143 (1997). (A). Probably the most complete and detailed review available, at least at the date of publication. In this review, the most interesting details of SBSL are given, including the remarkable degree of energy amplification, details on the speed of the collapsing liquid interface, discussion of the role of noble gases, and the spectrum (including its origin as well as its limited band width due to water absorption). They also speculate on whether SBSL is a classical effect, or whether it may have some more exciting origins. https://doi.org/10.1016/S0370-1573(96)00050-6

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• 38.

  “Comparisons of sonoluminescence from single-bubbles and cavitation fields: Bridging the gap,” T. Matula and R. Roy, Ultrasonics Sonochem. 4, 61–64 (1997). (E) These authors address the concepts of SBSL and MBSL. The latter is typically associated with cavitation fields in which many bubbles are involved. The spectrum differs in that with the single bubble, temperatures are apparently much higher, giving a spectrum that is devoid of the normal chemical bands seen when sonochemistry is investigated. The authors address this issue by comparing the two phenomena with regards to their light-flash durations and emission spectra-leading to some surprising differences and similarities. https://doi.org/10.1016/S1350-4177(97)00005-9

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• 39.

  “Inertial cavitation and single-bubble sonoluminescence,” T. Matula, Philos. Trans. R. Soc. London A 357, 225–249 (1999). (E) This article includes discussions of bubble levitation, the inertial cavitation threshold, the parameter space in which stable SBSL is observed, measurements of the acoustic and electromagnetic emissions from a sonoluminescing bubble, and the effects of impurities on the quality of the light emission from sonoluminescence bubbles. Comparisons are also made between sonoluminescence from a single bubble (SBSL) and sonoluminescence from a cavitation field (MBSL). https://doi.org/10.1098/rsta.1999.0325

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• 40.

  “Sonoluminescence light emission,” S. Hilgenfeldt, S. Grossmann, and D. Lohse, Phys. Fluids 11, 1318–1329 (1999). (A) The authors contend that SBSL is not an exotic phenomenon but can quantitatively be accounted for by applying a few well-known, simple concepts: The Rayleigh–Plesset dynamics of the bubble's radius, polytropic uniform heating of the gas inside the bubble during collapse, the dissociation of molecular gases, and thermal radiation of the remaining hot noble gas, where its finite opacity (transparency for its own radiation) is essential. A system of equations based on these ingredients correctly describes the widths, shapes, intensities, and spectra of the emitted light pulses, all as a function of the experimentally adjustable parameters, namely, driving pressure, driving frequency, water temperature, and the concentration and type of the dissolved gas. They also comment on the concept of optical opacity, i.e., that the hot plasma has such a high electron density that photons cannot penetrate to the exterior. https://doi.org/10.1063/1.869997

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• 41.

  “Bubble dynamics, shock waves and sonoluminescence,” C. Ohl, T. Kurz, R. Geisler, O. Lindau, and W. Lauterborn, Philos. Trans. R. Soc. London A 357, 269–294 (1999). (A) A general review of shock waves and sonoluminescence covering such topics as laser-generated cavitation, jet formation, few and large bubble systems, bubble clouds, collective bubble dynamics, and chaotic oscillations. https://doi.org/10.1098/rsta.1999.0327

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• 42.

  “Experimental and Theoretical Bubble Dynamics,” W. Lauterborn, T. Kurz, R. Mettin, and C. Ohl, in Advances in Chemical Physics, edited by I. Prigogine and S. Rice (Wiley, New York, 1999), pp. 295–380. (E) In this exhaustive overview of bubble dynamics, sonoluminescence is examined from acoustically levitated single bubbles undergoing both symmetrical and asymmetrical collapse, laser induced bubble collapse, as well as a variety of cavitation field systems that represent multibubble sonoluminescence. The juxtaposition between theoretical analysis and experimental measurements make this paper especially revealing concerning the origins of sonoluminescence.

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• 43.

  “Sonoluminescence: How bubbles turn sound into light,” S. Putterman and K. Weninger, Ann. Rev. Fluid Mech. 32, 445–476 (2000). (E). Good review of the state of the art of sonoluminescence research at this time. They observe that although the most rational picture of sonoluminescence involves the creation of a “cold” dense plasma by an imploding shock wave, neither the imploding shock nor the plasma have been directly observed. They note that attempts to attack sonoluminescence from the perspective of continuum mechanics lead to interesting issues related to bubble shape oscillations, shock shape instabilities, and shock propagation through nonideal media, and chemical hydrodynamics. They also point out that the limits of energy focusing that can be ultimately be achieved from collapsing bubbles have yet to be determined either experimentally or theoretically. https://doi.org/10.1146/annurev.fluid.32.1.445

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• 44.

  “Sonoluminescence: How bubbles glow,” D. Hammer and L. Frommhold, J. Mod. Opt. 48, 239–277 (2001). (E) This review attempts to elucidate the phenomenon of sonoluminescence in terms of fundamental principles. They focus mainly on the processes which generate the light, but other relevant facts, such as the bubble dynamics, are considered for the understanding of the physics involved. Their emphasis is on SBSL but they also examine some of the excellent work on MBSL and its spectral characteristics for clues. These authors believe that weakly ionized gas models are remarkably successful when combined with a hydrodynamic bubble model, in terms of reproducing observed spectral shapes, intensities, optical pulse widths and the dependencies of these observables on the experimental parameters. They are also critical of most other models, including the “hot spot model.” https://doi.org/10.1080/09500340117525

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• 45.

  “The effect of surface active solutes on bubbles exposed to ultrasound,” F. Grieser and M. Ashokkumar, Adv. Colloid Interface Sci. 89–90, 423–438 (2001). (A) This review article describes the effect of different solutes on the intensity of both SBSL and MBSL. They observe that much of the effect of the solutes on MBSL intensity is due to the change in bubble cloud behavior, while most of the effect on SBSL intensity is due to the formation and accumulation of decomposition products in the hot core of the bubble. https://doi.org/10.1016/S0001-8686(00)00064-6

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• 46.

  “Sonochemistry and Sonoluminescence,” K. Suslick, in Encyclopedia of Physical Science and Technology, 3rd ed. (Academic, San Diego, 2001), Vol. 17, pp. 363–376. (E) Written mostly for the layman, it is an excellent review of the state of the art of both sonochemistry and sonoluminescence.

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• 47.

  “Single-bubble sonoluminescence,” M. Brenner, S. Hilgenfeldt, and D. Lohse, Rev. Mod. Phys. 74, 425–484 (2002). (A) This review fills in a lot of detail concerning the research discoveries that were made since the 1997 review of Barber et al. At this stage in SBSL research, the available information favored a description of sonoluminescence caused by adiabatic heating of the bubble at collapse, leading to partial ionization of the gas inside the bubble and to thermal emission such as bremsstrahlung. After a brief historical review, the authors survey the major areas of research: Sec. II describes the classical theory of bubble dynamics, as developed by Rayleigh, Plesset, Prosperetti, and others, while Sec. III describes research on the gas dynamics inside the bubble. Shock waves inside the bubble do not seem to play a prominent role in the process. Section IV discusses the hydrodynamic and chemical stability of the bubble. Stable SBSL requires that the bubble be shape stable and diffusively stable and, together with an energy focusing condition, this fixes the parameter space where light emission occurs. Section V describes experiments and models addressing the origin of the light emission. https://doi.org/10.1103/RevModPhys.74.425

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• 48.

  “Contemporary review on nature of sonoluminescence and sonochemical reactions,” M. Margulis and I. Margulis, Ultrasonics Sonochem. 9, 1–10 (2002). (I) A later review of sonochemistry and sonoluminescence from the electrical discharge model. https://doi.org/10.1016/S1350-4177(01)00096-7

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• 49.

  “Single bubble sonoluminescence: A chemist's overview,” M. Ashokkumar and F. Grieser, Chem. Phys. Chem. 5, 439–448 (2004). (E) In part, the discovery of SBSL has been a major contributor to the theoretical and experimental advances that have been made to account for the event. This minireview is from the perspective of a physical chemist and considers the progress that has been made in understanding the role of solutes in affecting the sonoluminescence from a solution exposed to ultrasound, and the physicochemical properties of solutes that are important in controlling both SBSL and MBSL.

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• 50.

  Sonoluminescence,” Kyuichi Yasui, T. Tuziuti, M. Sivakumar, and Y. Iida, Appl. Spectrosc. Rev. 39, 399–436 (2004). (E) K. Yasui has published extensively on sonoluminescence, mostly via theoretical modeling, and he and his co-authors have described much of their work and that of others in this broad review. https://doi.org/10.1081/ASR-200030202

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• 51.

  “Bubbles in an acoustic field: An overview,” M. Ashokkumar, J. Lee, S. Kentish, and F. Grieser, Ultrasonics Sonochem. 14, 470–475 (2007). (E) A nice review of the behavior of bubbles in an acoustic field, although sonoluminescence is only briefly mentioned. https://doi.org/10.1016/j.ultsonch.2006.09.016

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• 52.

  “The effect of surface active solutes on bubbles in an acoustic field,” M. Ashokkumar and F. Grieser, Phys. Chem. Chem. Phys. 9, 5631–5643 (2007). (E) Acoustic cavitation has been used in a number of areas: These include therapeutic applications, contrast imaging, synthesis of nanomaterials, production of nanoemulsions, treatment of food materials, waste-water treatment, etc. In all these applications, a variety of chemical species are present and most of these chemicals are surface active in nature. There is limited literature available on the effect of surface active solutes on acoustic bubbles. This review, containing over 100 references, provides a consolidated overview of experimental investigations on the effect of surface active solutes on cavitation bubbles and the relevance of these studies to some applications, including sonochemistry and sonoluminescence. https://doi.org/10.1039/b707306m

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• 53.

  “Acoustic cavitation, bubble dynamics and sonoluminescence,” W. Lauterborn, T. R. Geisler, D. Schanz, and O. Lindau, Ultrasonics Sonochem. 14, 484–491 (2007). (E) Basic facts on the dynamics of bubbles in water are presented. Measurements on the free and forced radial oscillations of single spherical bubbles and their acoustic (shock waves) and optic (luminescence) emissions are given in photographic series and diagrams. Bubble cloud patterns and their dynamics and light emission in standing acoustic fields are discussed. https://doi.org/10.1016/j.ultsonch.2006.09.017

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• 54.

  “Inside a collapsing bubble: Sonoluminescence and the conditions during cavitation,” K. Suslick and D. Flannigan, Ann. Rev. Phys. Chem. 59, 659–683 (2008). (E) The authors discuss acoustic cavitation as a unique source of energy for driving chemical reactions with sound, a process known as sonochemistry. They report on spectroscopic analyses of sonoluminescence from single bubbles as well as a cloud of bubbles that display both line and band emission, as well as the underlying continuum arising from a plasma. Application of spectrometric methods of pyrometry as well as tools of plasma diagnostics to relative line intensities, profiles, and peak positions allow them to determine intracavity temperatures and pressures. https://doi.org/10.1146/annurev.physchem.59.032607.093739

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• 55.

  “Physics of bubble oscillations,” W. Lauterborn and T. Kurz, Rep. Prog. Phys. 73, 1–88 (2010). (E) A recent and very extensive review of bubble acoustics, including a discussion of SBSL and single cavitation bubble luminescence, with 499 references https://doi.org/10.1088/0034-4885/73/10/106501.

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• 56.

  “The characterization of acoustic cavitation bubbles—An overview,” M. Ashokkumar, Ultrasonics Sonochem. 18, 864–872 (2011). (E) The levitated single bubble that leads to SBSL has been extensively studied both experimentally and theoretically, and has led to a better understanding of both sonoluminescence and acoustic cavitation. However, in a multibubble system, the formation of bubble streamers and clusters makes it difficult to characterize the cumulative properties of these bubbles. In this overview, some recently developed experimental procedures for the characterization of acoustic cavitation bubbles have been discussed, including the sonoluminescence produced by multi-bubble systems. https://doi.org/10.1016/j.ultsonch.2010.11.016

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• 57.

  “Nonlinear Acoustics in Fluids,” W. Lauterborn, T. Kurz, and I. Akhatov, in Springer Handbook of Acoustics, 2nd ed., edited by T. D. Rossing (Springer, Berlin, 2014), pp. 265–314. (E) This chapter contains an excellent description of bubble acoustics and an up-to-date, although brief, overview of sonoluminescence.

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Early investigations of sonoluminescence suggested a variety of models, with the hot spot model coming into general acceptance. The hot spot model simply assumes that as the bubble collapses, the gas that is contained within the bubbles is heated to incandescent temperatures. However, this model is unsatisfactory to some investigators and accordingly they have suggested a variety of alternative models, some of which are included below.

• 58.

  “Confined electron model for single-bubble sonoluminescence,” L. Bernstein and M. Zakin, J. Phys. Chem. 99, 14619–14627 (1995). (A) This model assumes that the emission arises from electrons confined to voids in the hot, dense fluid formed during the final stages of bubble collapse. These electrons are produced by high-temperature ionization of the bubble constituents. https://doi.org/10.1021/j100040a008

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• 59.

  “Sonoluminescence: An alternative ‘electrohydrodynamic’ hypothesis,” T. Lepoint, D. De Pauw, F. Lepoint-Mullie, M. Goldman, and A. Goldman, J. Acoust. Soc. Am. 101, 2012–2030 (1997). (E) A variation of the electrical discharge model is presented. https://doi.org/10.1121/1.418242

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• 60.

  “A new mechanism for sonoluminescence,” A. Prosperetti, J. Acoust. Soc. Am. 101, 2003–2007 (1997). (A) It is proposed that eventually the bubble must collapse asymmetrically, and introduce a reentrant jet that will impact the opposite wall. This impact will result in a fracturing of the liquid that cannot flow during the extremely short time scale over which pressure is applied, resulting in light emission. https://doi.org/10.1121/1.418133

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• 61.

  “Electron-atom Bremsstrahlung and the sonoluminescence of rare gas bubbles,” L. Frommhold, Phys. Rev. E 58, 1899–1904 (1998). (A) The author suggests that electron–neutral-atom bremsstrahlung may be a principal mechanism of the light emission of rare gas SL bubbles. https://doi.org/10.1103/PhysRevE.58.1899

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• 62.

  “Sonoluminescence: Proton-tunneling radiation,” J. Willison, Phys. Rev. Lett. 81, 5430–5433 (1998). (E). Proton tunneling occurs as water undergoes a phase transition in the abrupt pressure transient coincident with the bubble reaching minimum size. A classical electrodynamic treatment of the current impulses of proton tunneling proposed by the authors provided close agreement with observed spectra of SBSL. https://doi.org/10.1103/PhysRevLett.81.5430

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• 63.

  “A confined electron spherical void model in sonoluminescence,” Z. Liu, L. Yu, and J. Luo, J. Phys. Chem. A 105, 1267–1269 (2001). (A) A confined electron spherical void model generalized from the rectangular box one model is used for the discussion of the radical spectrum emitted from nonhomogeneous hot dense helium gas in sonoluminescence. Their theoretical analysis showed that the gas can emit a continuous spectrum from atomic states 3s to 2p that fits with the experimental data in SBSL. https://doi.org/10.1021/jp000686h

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• 64.

  “Mechanism of sonochemical reaction and sonoluminescence,” M. Margulis and I. Margulis, High Energy Chem. 38, 285–294 (2002). (E) The authors examine sonochemistry and sonoluminescence and determine that only the electrical theory has full applicability. This theory assumes that during cavitation collapse, electrical charges on the surface generate light and chemical reactions https://doi.org/10.1023/B:HIEC.0000041338.11770.74.

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• 65.

  “The cavitation induced Becquerel effect and the hot spot theory of sonoluminescence,” T. Prevenslik, Ultrasonics 41, 313–317 (2003). (A) The author proposes that the cavitation induced Becquerel effect, most likely caused by cavitation, induces the observed light emission. https://doi.org/10.1016/S0041-624X(02)00458-4

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Although the hot spot model was favored by most researchers, and has withstood the test of time, there were a number of different models that showed remarkable agreement with the experimental data. Moreover, the excitement generated by SBSL, and the apparent mysteries associated with its origin, stimulated Julian Schwinger (Nobel prize in physics, 1965) to propose that SBSL originated from a dynamic Casimir effect, a type of quantum radiation.

• 66.

  “Casimir energy for dielectrics,” J. Schwinger, Proc. Natl. Acad. Sci. 89, 4091–4093 (1992). (A) https://doi.org/10.1073/pnas.89.9.4091

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• 69.

  “Casimir light: The source,” J. Schwinger, Proc. Natl. Acad. Sci. 90, 2105–2106 (1993). (A) https://doi.org/10.1073/pnas.90.6.2105

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  “Casimir light: Field pressure,” J. Schwinger, Proc. Natl. Acad. Sci. 91, 6473–6475 (1994). (A) https://doi.org/10.1073/pnas.91.14.6473

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The idea of Schwinger that sonoluminescence was some form of quantum radiation was exciting in itself and prompted a number of investigations to expand and extend the idea of Schwinger.

• 73.

  “Sonoluminescence as quantum vacuum radiation,” C. Eberlein, Phys. Rev. Lett. 76, 3842–3845 (1996). (A) Sonoluminescence was explained in terms of quantum vacuum radiation by moving interfaces between media of different polarizability. It was considered as a dynamic Casimir effect, in the sense that it was a consequence of the imbalance of the zero-point fluctuations of the electromagnetic field during the noninertial motion of the boundary. https://doi.org/10.1103/PhysRevLett.76.3842

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  “Theory of quantum radiation observed as sonoluminescence,” C. Eberlein, Phys. Rev. A 53, 2772–2787 (1996). (A) Eberlein followed her initial paper with a much more detailed analysis in which five separate agreements with experiment were presented. https://doi.org/10.1103/PhysRevA.53.2772

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• 75.

  “Casimir energy for a spherical cavity in a dielectric: Applications to sonoluminescence,” K. Milton and Y. Ng, Phys. Rev. E 55, 4207–4216 (1997). (A) The authors conclude that the energy (from the Casimir effect) is too small to account for the large burst of photons seen in sonoluminescence. https://doi.org/10.1103/PhysRevE.55.4207

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• 76.

  “Comment on ‘Sonoluminescence as quantum vacuum radiation,’ ” A. Lambrecht, M. Jaekel, and S. Reynaud, Phys. Rev. Lett. 78, 2267 (1997). (E) These authors also conclude the effect is too small. https://doi.org/10.1103/PhysRevLett.78.2267

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  “Observability of the bulk Casimir effect: Can the dynamical Casimir effect be relevant to sonoluminescence?,” K. Milton and Y. Ng, Phys. Rev. E 57, 5504–5510 (1998). (A) Also found the effect too small to explain SBSL. https://doi.org/10.1103/PhysRevE.57.5504

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• 78.

  “Identity of the van der Waals force and the Casimir effect and the irrelevance of these phenomena to sonoluminescence,” I. Brevik, V. Marachevsky, and K. Milton, Phys. Rev. Lett. 82, 3948–3951 (1999). (A) Showed that the Casimir, or zero-point energy, of a spherical bubble in a dielectric medium coincided with the sum of the van der Waals energies between the molecules that make up the medium, and was 10 orders of magnitude too small to account for sonoluminescence. https://doi.org/10.1103/PhysRevLett.82.3948

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• 79.

  “Dynamic Casimir effect in single bubble sonoluminescence,” W. Chen and R. Wei, Chin. Phys. Lett. 16, 767–769 (1999). (A). The bubble dynamics for SBSL was coupled with the dynamic Casimir effect due to the quantum vacuum fluctuation. The numerical results showed that the power radiated by the Casimir effect was only on the order of 10⁻³⁸ W and thus much too small to account for SBSL. https://doi.org/10.1088/0256-307X/16/10/024

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• 80.

  “Sonoluminescence as a QED vacuum effect: Probing Schwinger's proposal,” S. Liberati, M. Visser, F. Belgiorno, and D. Sciama, J. Phys. A: Math. Gen. 33, 2251–2272 (2000). (A) The authors report that the timescales required to implement Schwinger's original suggestion are not physically relevant to sonoluminescence. https://doi.org/10.1088/0305-4470/33/11/307

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• 81.

  “Sonoluminescence as a QED vacuum effect. I. The physical scenario,” S. Liberati, M. Visser, F. Belgiorno, and D. Sciama, Phys. Rev. D 61, 085023 (2000). (A) These authors doubt that this effect can explain SBSL. https://doi.org/10.1103/PhysRevD.61.085023

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• 82.

  “Sonoluminescence: Two-photon correlations as a test of thermality,” F. Belgiorno, S. Liberati, M. Visser, and D. Sciama, Phys. Lett. A 271, 308–313 (2000). (A) The authors suggest a method for testing the quantum radiation hypothesis. https://doi.org/10.1016/S0375-9601(00)00394-7

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Relatively recently, there arose a strong interest in sonochemistry, in which commercial products were produced exclusively by the application of intense ultrasonic fields to liquids. In many cases, these strong fields produced acoustic cavitation and its associated sonoluminescence. Since the light emissions were thought to be a key to the chemical dynamics, a considerable interest was also generated into what is now called multibubble sonoluminescence (MBSL).

The origin of the “light from sound” was not intuitively obvious and there are still some disagreement as to the actual mechanism that generates the light.

• 83.

  “Measurements of sonoluminescence from pure liquids and some aqueous solutions,” P. Jarman, Proc. Phys. Soc. 73, 628–640 (1959). (E) Jarman made extensive measurements of the timing of the sonoluminescence flash with respect to the period of the sound field and found that a flash occurred every acoustic cycle. He concluded that the origin of sonoluminescence was electrical discharge, which, by the way, he discarded in terms of the hot spot theory the next year (see Ref. 6). https://doi.org/10.1088/0370-1328/73/4/312

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• 84.

  “Sonoluminescence,” R. Verrall and C. Sehgal, Ultrasonics 25, 29–30 (1987). (I) Verrall and Sehgal were some of the first to conclude that sonochemiluminescense was due to molecular transitions induced by the high temperatures and pressures within the collapsing cavitation bubble. https://doi.org/10.1016/0041-624X(87)90007-2

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• 85.

  “Sonoluminescence from nonaqueous liquids: Emission from small molecules,” E. Flint and K. Suslick, J. Am. Chem. Soc. 111, 6987–6992 (1989). (A) Sonoluminescence spectra were obtained from several nonaqueous liquids in the presence of various gases. They were able to conclude that sonoluminescence from organic liquids was caused by emission from small free radicals and molecules, such as C2, CN, C02, and Cl, and that the principal source of the sonoluminescence was not blackbody radiation or electrical discharge but by the chemical reactions of high energy species during cavitation collapse. https://doi.org/10.1021/ja00200a014

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• 86.

  “Sonoluminescence from alkali-metal salt solutions,” E. Flint and K. Suslick, J. Phys. Chem. 95, 1484–1488 (1991). (A) The authors report that excited-state alkali-metal atoms are the result of secondary reactions of metal ions with high-energy radicals formed directly in the cavitation bubble, and that the large line width is caused by rapid collisional deactivation of the excited-state atom. https://doi.org/10.1021/j100156a084

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• 87.

  “The effect of ultrasound power on water sonoluminescence,” Y. Didenko, T. Gordeychuk, and V. Koretz, J. Sound Vib. 147, 409–416 (1991). (I) Sonoluminescence was observed from water saturated with argon and xenon for the 200–500 nm wavelength interval when these solutions were insonified at a frequency of 22 kHz. The total intensity of the light emission in the Xe-saturated water was stronger than in the Ar-saturated water. The spectrum of Xe-saturated water had a strong band near 260 nm. The sonoluminescence spectra did not change markedly with increasing ultrasound power. https://doi.org/10.1016/0022-460X(91)90489-7

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• 88.

  “Sonoluminescence spectrum of seawater,” L. Becker, J. Bada, K. Kemper, and K. Suslick, Marine Chem. 40, 315–320 (1992). (E) The sonoluminescence spectra of both seawater and NaCl in water were characterized by an emission line at 589 nm from excited-state sodium. https://doi.org/10.1016/0304-4203(92)90029-A

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• 89.

  “Sonoluminescence from metal carbonyls,” K. Suslick, E. Flint, M. Grinstaff, and K. Kemper, J. Phys. Chem. 97, 3098–3099 (1993). (A) Spectrally resolved sonoluminescence was observed from the ultrasonic irradiation of organometallic compounds in silicone oil solutions. Specifically, ultrasonic irradiation of solutions of Fe(CO)5, Cr(C0)6, Mo(C0)6, and W(CO) produced atomic line emission from metal atom excited states. The intensity of this sonoluminescence was the highest yet observed. https://doi.org/10.1021/j100115a007

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• 90.

  “Sonochemistry and sonoluminescence: Effects of external pressure,” A. Henglein and M. Gutierrez, J. Phys. Chem. 97, 158–162 (1993). (A) The oxidation of iodide by 1-MHz continuous ultrasound and the sonoluminescence in water created by pulsed ultrasound were investigated in the pressure range from 0.7 to 3 bar. At low intensities, the chemical yield and the luminescence intensity decreased with increasing external pressure. At higher intensities the yields increased moderately with increasing pressure. https://doi.org/10.1021/j100103a027

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• 91.

  “The effect of bulk solution temperature on the intensity and spectra of water sonoluminescence,” Y. Didenko, D. Nastich, S. P. Pugach, Y. Polovinka, and V. Kvochka, Ultrasonics 32, 71–76 (1994). (A) The intensity of the light emission decreased with an increase in temperature, but the temperature dependencies were different for high and low frequencies. https://doi.org/10.1016/0041-624X(94)90083-3

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• 92.

  “Spectra of water sonoluminescence,” Y. Didenko and S. Pugach, J. Phys. Chem. 98, 9742–9749 (1994). (A) The sonoluminescence spectra of water saturated with helium, argon, krypton, and xenon gases were studied in a 220–500 nm interval. Total light power of sonoluminescence and the yield of hydrogen peroxide increased in the order He < Ar < Kr < Xe. https://doi.org/10.1021/j100090a006

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• 93.

  “Sonoluminescence from aqueous alcohol and surfactant solutions,” M. Ashokkumar, R. Hall, P. Mulvaney, and F. Grieser, J. Phys. Chem. B 101, 10845–10850 (1997). (I) The authors observed a rather complicated dependence on these solutes. https://doi.org/10.1021/jp972477b

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• 94.

  “Sonophotoluminescence: Pyranine emission induced by ultrasound,” M. Ashokkumar and F. Grieser, Chem. Commun. 5, 561–562 (1998). (E) Sonoluminescence was used to excite the water soluble acid–base fluorescent indicator trisodium 8-hydroxy-1,3,6-pyrenetrisulfonate, leading to light emission (sonophotoluminescence).

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• 95.

  “The effect of pH on multibubble sonoluminescence from aqueous solutions containing simple organic weak acids and bases,” M. Ashokkumar, P. Mulvaney, and F. Grieser, J. Am. Chem. Soc. 121, 7355–7359 (1999). (I) The authors examined the effect of pH on MBSL and determined that in the pH ranges where sonoluminescence quenching occurs, the longer the alkyl chain length the greater the effect. https://doi.org/10.1021/ja990482i

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• 96.

  “Sonochemistry and sonoluminescence in aqueous AuCl4—Solutions in the presence of surface-active solutes,” K. Barbour, M. Ashokkumar, R. Caruso, and F. Grieser, J. Phys. Chem. B 103, 9231–9236 (1999). (I) This paper examined the role of both alcohols and surfactants on quenching the MBSL intensity. https://doi.org/10.1021/jp990811t

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• 97.

  “Sonophotoluminescence from aqueous and non-aqueous solutions,” M. Ashokkumar and F. Grieser, Ultrasonics Sonochem. 6, 1–5 (1999). (I) The authors observed that the sonoluminescence generated in air-saturated aqueous and nonaqueous solutions using 515 kHz ultrasound could be used to vibronically excite several fluorescent solutes, namely, fluorescein, eosin, pyranine, and pyrene. https://doi.org/10.1016/S1350-4177(98)00038-8

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• 98.

  “Multibubble sonoluminescence in aqueous salt solutions,” M. Wall, M. Ashokkumar, R. Tronson, and F. Grieser, Ultrasonics Sonochem. 6, 7–14 (1999). (I) The authors observed a good correlation of the increase in the sonoluminescence signal with the extent of gas solubilisation in the solutions with changing salt concentration. https://doi.org/10.1016/S1350-4177(98)00037-6

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• 99.

  “Roles of cavitation and acoustic streaming in megasonic cleaning,” G. Gale and A. Busnaina, Part. Sci. Technol.: Int. J. 17, 229–238 (1999). (E) Acoustic emissions and sonoluminescence were used to determine the ability of these devices to clean substrates. https://doi.org/10.1080/02726359908906815

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• 100.

  “Impact of ultrasonic frequency on aqueous sonoluminescence and sonochemistry,” M. Beckett and I. Hua, J. Phys. Chem. A 105, 3796–3802 (2001). (A) Their results indicated that nonlinear bubble implosions play a more significant role at lower frequencies whereas higher species flux rates influence chemical reactivity at higher frequencies. https://doi.org/10.1021/jp003226x

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• 101.

  “Influence of bubble clustering on multibubble sonoluminescence,” S. Hatanaka, K. Yasui, T. Kozuka, T. Tuziuti, and H. Mitome, Ultrasonics 40, 655–660 (2002). (A) It was determined that when sonoluminescence was quenched suddenly at excessive ultrasonic power, the behavior of bubbles clearly changed, indicating that bubble cloud dynamics influenced sonoluminescence emissions. https://doi.org/10.1016/S0041-624X(02)00193-2

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• 102.

  “Sonochemistry and sonoluminescence of room-temperature ionic liquids,” J. Oxley, T. Prozorov, and K. Suslick, J. Am. Chem. Soc. 125, 11138–11139 (2003). (A) This paper obtained correlations between sonochemistry and MBSL for ionic liquids. https://doi.org/10.1021/ja029830y

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• 103.

  “Pressure during sonoluminescence,” W. McNamara III, Y. Didenko, and K. Suslick, J. Phys. Chem. B 107, 7303–7306 (2003). (A) Using synthetic spectra, they were able to measure the pressure within a collapsing cavitation bubble, obtaining a value of 30 MPa. https://doi.org/10.1021/jp034236b

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• 104.

  “The influence of acoustic power on multibubble sonoluminescence in aqueous solutions containing organic solutes,” D. Sunartio, M. Ashokkumar, and F. Grieser, J. Phys. Chem. B 109, 20044–20050 (2005). (I) The authors obtained mixed results in terms of using more power. https://doi.org/10.1021/jp052747n

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• 105.

  “Proton transfer between organic acids and bases at the acoustic bubble-aqueous solution interface,” M. Ashokkumar and F. Grieser, J. Phys. Chem. B 109, 19356–19359 (2005). (A) The multibubble sonoluminescence emission intensity from aqueous solutions containing simple aliphatic organic acids (RCOOH) and bases (RNH2) and mixtures of the two types of solutes were examined as a function of pH. https://doi.org/10.1021/jp054490z

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• 106.

  “Acoustic emission from cavitating solutions: Implications for the mechanisms of sonochemical reactions,” G. Price, M. Ashokkumar, M. Hodnett, B. Zequiri, and F. Grieser, J. Phys. Chem. B 109, 17799–17801 (2005). (I) The acoustic emissions from collapsing cavitation bubbles generated using ultrasound of 20 and 515 kHz frequencies in water were measured and correlated with sonoluminescence and hydroxyl radical production to yield further information on the frequency dependence of sonochemical reactions. https://doi.org/10.1021/jp0543227

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• 107.

  “Sonochemistry and sonoluminescence in ionic liquids, molten salts, and concentrated electrolyte solutions,” D. Flannigan, S. Hopkins, and K. Suslick, J. Organometallic Chem. 690, 3513–3517 (2005). (A) This is a review of their previous results on the effects of cavitation on some room-temperature ionic liquids, including the sonoluminescence spectra of molten salt eutectics and concentrated aqueous electrolyte solutions. https://doi.org/10.1016/j.jorganchem.2005.04.024

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• 108.

  “Mechanoluminescence: Light from sonication of crystal slurries,” N. Eddingsaas and K. Suslick, Nature 444, 163 (2006). (E) Luminescence in cavitating clouds can also be generated by high speed collisions between solid particles in slurries, generated mechanoluminescence (also known as fractoluminescence or triboluminescence). https://doi.org/10.1038/444163a

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• 109.

  “Effect of water-soluble solutes on sonoluminescence under dual-frequency sonication,” A. Brotchie, M. Ashokkumar, and F. Grieser, J. Phys. Chem. C 111, 3066–3070 (2007). (A) In this paper, it was demonstrated that the synergy effect of dual-frequency ultrasound on sonoluminescence observed in water could be further enhanced by the presence of several different solutes. https://doi.org/10.1021/jp067524r

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• 110.

  “Correlation between Na* emission and ‘chemically active’ acoustic cavitation bubbles,” D. Sunartio, K. Yasui, T. Tuziuti, T. Kozuka, Y. Iida, A. Ashokkumar, and F. Grieser, Chem. Phys. Chem. 8, 2331–2335 (2007). (I) Evidence is presented that strongly suggests Na* emission arises from a population of bubbles that are sonochemically active but not producing sonoluminescence.

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• 111.

  “Evidence for a plasma core during multibubble sonoluminescence in sulfuric acid,” N. Eddingsaas and K. Suslick, J. Am. Chem. Soc. 129, 3838–3839 (2007). (A) The spectrum of multibubble sonoluminescence in concentrated sulfuric acid sparged with Ar, at relatively low acoustic power, consists of a broad continuum extending into the UV with SO and Ar emission lines on top of this continuum. The observation of the Ar lines indicates that an optically opaque plasma is probably generated inside the bubble during cavitation. https://doi.org/10.1021/ja070192z

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• 112.

  “Sonoluminescence, sonochemistry (H2O2 yield) and bubble dynamics: Frequency and power effects,” P. Kanthale, M. Ashokkumar, and F. Grieser, Ultrasonics Sonochem. 15, 143–150 (2008). (A) In this paper, a comprehensive experimental and numerical investigations on the effects of ultrasound frequency and acoustic power on sonoluminescence and H2O2 yields was undertaken. https://doi.org/10.1016/j.ultsonch.2007.03.003

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• 113.

  “Experimental and theoretical investigations of sonoluminescence under dual frequency conditions,” P. Kanthale, A. Brotchie, M. Ashokkumar, and F. Grieser, Ultrasonics Sonochem. 15, 629–635 (2008). (A). A synergistic enhancement of the sonoluminescence (SL) signal, >30-fold, at low powers (4.6 W) of the higher frequency was observed. At a higher acoustic power level (15.8 W) the dual frequency operation produced a decrease in the sonoluminescence signal. https://doi.org/10.1016/j.ultsonch.2007.08.006

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• 114.

  “Sonochemistry and sonoluminescence under simultaneous high- and low-frequency irradiation,” A. Brotchie, M. Ashokkumar, and F. Grieser, J. Phys. Chem. C 112, 8343–8348 (2008). (A) In these studies, it was determined that although dual-frequency sonication causes a decrease in the integrated sonoluminescence intensity and sonochemical efficiency in water, in the presence of certain solutes, a significant enhancement in activity could be attained. https://doi.org/10.1021/jp8006987

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• 115.

  “Effect of power and frequency on bubble-size distributions in acoustic cavitation,” A. Brotchie, F. Grieser, and M. Ashokkumar, Phys. Rev. Lett. 102, 084302 (2009). (A) Acoustic bubble-size distributions were determined using a pulsed ultrasound method at different ultrasound powers and frequencies. It was observed that the mean bubble size increased with increasing acoustic power and decreased with increasing ultrasound frequency. https://doi.org/10.1103/PhysRevLett.102.084302

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• 116.

  “The detection and control of stable and transient acoustic cavitation bubbles,” M. Ashokkumar, J. Lee, Y. Iida, K. Yasui, T. Kozuka, T. Tuziutib, and A. Towata, Phys. Chem. Chem. Phys. 11, 10118–10121 (2009). (A) The authors report on a multibubble sonoluminescence-based experimental technique for the detection and the control of a type of cavitation at low and high ultrasound frequencies. It was observed that the use of a horn-type sonicator operating at 20 kHz primarily generates transient cavitation bubbles. However, the use of plate type transducers at low frequencies (25 and 37 kHz) generated a significant amount of stable cavitation, as evidenced by the quenching of sonoluminescence by volatile solutes. https://doi.org/10.1039/b915715h

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• 117.

  “Spatial distribution of acoustic cavitation bubbles at different ultrasound frequencies,” M. Ashokkumar, J. Lee, Y. Iida, K. Yasui, T. Kozuka, T. Tuziuti, and A. Towata, Chem. Phys. Chem. 11, 1680–1684 (2010). (I) Images of sonoluminescence, sonophotoluminescence, and sonochemiluminescence were recorded in order to semi-quantitatively compare the spatial distribution of the cavitation activity at three different ultrasound frequencies (170, 440, and 700 kHz) and at various acoustic amplitudes. At all ultrasound frequencies investigated, the sonochemically active cavitation zones were much larger than the cavitation zones where sonoluminescence was observed.

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• 118.

  “Characterization of acoustic cavitation bubbles in different sound fields,” A. Brotchie, F. Grieser, and M. Ashokkumar, J. Phys. Chem. B 114, 11010–11016 (2010). (I) The authors determined that the relative extent of bubble coalescence in a dual-frequency field correlated strongly with the synergistic enhancement of the sonochemical reaction rates. https://doi.org/10.1021/jp105618q

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• 119.

  “Sonoluminescence from OH(C²Σ⁺) and OH(A²Σ⁺) radicals in water: Evidence for plasma formation during multibubble cavitation,” R. Pflieger, H.-P. Brau, and S. Nikitenko, Chem. Eur. J. 16, 11801–11803 (2010). (A) The authors contend that their data clearly point to non-thermal plasma formation during multibubble cavitation in water in the presence of Kr and Xe. https://doi.org/10.1002/chem.201002170

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• 120.

  “Inertially confined plasma in an imploding bubble,” D. Flannigan and K. Suslick, Nat. Phys. 6, 598–601 (2010). (E) By accounting for the temporal profile of the sonoluminescence pulse and the potential optical opacity of the plasma, their results suggest that the ultimate conditions generated inside a collapsing bubble may far exceed those determined from emission from the transparent outer region of the light-emitting volume. https://doi.org/10.1038/nphys1701

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• 121.

  “The characterization of acoustic cavitation bubbles—An overview,” M. Ashokkumar, Ultrasonics Sonochem. 18, 864–872 (2011). (E) This paper provides a nice overview of the acoustic behavior of bubbles in a sound field and the effect of bubble-bubble interactions on the threshold and intensity of sonoluminescence. https://doi.org/10.1016/j.ultsonch.2010.11.016

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• 122.

  “Geometric optimization of sonoreactors for the enhancement of sonochemical activity,” Y. Son, M. Lim, M. Ashokkumar, and J. Khim, J. Phys. Chem. C 115, 4096–4103 (2011). (E) This paper observed sonoluminescence images when various reactors are used, demonstrating that sonoluminescence can provide evidence of standing waves and traveling waves and lack of intense cavitation fields. https://doi.org/10.1021/jp110319y

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• 123.

  “Extreme conditions during multibubble cavitation: Sonoluminescence as a spectroscopic probe,” K. Suslick, N. Eddingsaas, D. Flannigan, S. Hopkins, and H. Xu, Ultrasonics Sonochem. 18, 842–846 (2011). (A) The paper is an excellent overview of the existence of extreme conditions that develop during both single bubble and multibubble sonoluminescence, and offers an explanation for the origin of these conditions. https://doi.org/10.1016/j.ultsonch.2010.12.012

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• 124.

  “Sonoluminescence and sonochemiluminescence from a microreactor,” D. Rivas, M. Ashokkumar, T. Leong, K. Yasui, T. Tuziuti, S. Kentish, D. Lohse, and H. Gardeniers, Ultrasonics Sonochem. 19, 1252–1259 (2012). (A) In this collaborative effort between several groups, micro-machined pits were placed on a silicone substrate and these pits were able to generate sonoluminescence and sonochemiluminescence at a more controlled rate than when there were no pits. https://doi.org/10.1016/j.ultsonch.2012.04.008

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• 125.

  “The behavior of acoustic bubbles in aqueous solutions containing soluble polymers,” R. Tronson, M. F. Tchea, M. Ashokkumar, and F. Grieser, J. Phys. Chem. B 116, 13806–13811 (2012). (I) The intensity of the sonoluminescence was observed for aqueous solutions containing PVP/surfactant and PVP/alcohol mixtures. It was observed that PVP enhances multibubble sonoluminescence by increasing the number of active bubbles in the system by hindering bubble-bubble coalescence processes. https://doi.org/10.1021/jp308897c

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• 126.

  “Correlation between sonochemistry and sonoluminescence at various frequencies,” M. Zhou, N. Yusofab, and M. Ashokkuma (Royal Society Chemistry), RSC Adv. 3, 9319–9324 (2013). (I) It was observed that the chemically active cavitation bubble population is dominant in the “open” configuration and the sonoluminescing bubble population is dominant in the “closed” configuration. A possible reason for such differences is the existence of a well-defined standing wave pattern in the closed system. https://doi.org/10.1039/c3ra41123k

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Because MBSL was generally thought of as a thermal effect, an analysis of it could provide a probe of the bubble's interior—especially the temperature. Thus, understanding the nature of MBSL meant a better understanding of cavitation and thus improved sonochemical yields. The following papers sought to use MBSL, and perhaps its spectrum, to provide information about the physical chemistry within the bubble's interior.

• 127.

  “The temperature of cavitation,” E. Flint and K. Suslick, Science 253, 1397–1399 (1991). (E) As a spectroscopic probe of the cavitation event, sonoluminescence provides a solution. Sonoluminescence spectra from silicone oil were reported and analyzed. From comparison of synthetic to observed spectra, the effective cavitation temperature was found to be 5075 ± 156 K. https://doi.org/10.1126/science.253.5026.1397

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• 128.

  “Sonoluminescence temperatures during multi-bubble cavitation,” W. McNamara, III, Y. Didenko, and K. Suslick, Nature 401, 772–775 (1999). (A) By using measured spectra and comparing it with synthetic spectra, the authors were able to determine the role of various chemical and physical parameters in sonoluminescence emissions and importantly to measure the emission temperature of the atomic and molecular excited states produced during cavitation https://doi.org/10.1038/44536.

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• 129.

  “Cavitation thermometry using molecular and continuum sonoluminescence,” L. Bernstein, M. Zakin, E. Flint, and K. Suslick, J. Phys. Chem. 100, 6612–6619 (1996). (A) The use of molecular and continuum emission spectra from MBSL and SBSL was explored as a probe of bubble temperature during cavitation collapse. https://doi.org/10.1021/jp953643n

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• 130.

  “Temperature of multibubble sonoluminescence in water,” Y. Didenko, W. McNamara III, and K. Suslick, J. Phys. Chem. A 103, 10783–10788 (1999). (A) The authors used sonoluminescence from excited states of metal atoms, and synthetic spectra, as a spectroscopic probe of temperatures inside cavitation bubbles. https://doi.org/10.1021/jp991524s

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• 131.

  “Effect of noble gases on sonoluminescence temperatures during multibubble cavitation,” Y. Didenko, W. McNamara III, and K. Suslick, Phys. Rev. Lett. 84, 777–780 (2000). (A) The authors observed that noble gases can raise the internal gas temperature but is limited in that this energy will be spent in the excitation and destruction of polyatomic vapor rather than in raising the translational temperature of species within the bubble. https://doi.org/10.1103/PhysRevLett.84.777

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• 132.

  “Temperature in multibubble sonoluminescence,” K. Yasui, J. Chem. Phys. 115, 2893–2896 (2001). (A) The authors contends that SBSL is brighter than MBSL because in the latter there are considerable amounts of vapor in the bubble during collapse. https://doi.org/10.1063/1.1395056

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• 133.

  “A Comparison between multibubble sonoluminescence intensity and the temperature within cavitation bubbles,” M. Ashokkumar and Franz Grieser, J. Am. Chem. Soc. 127, 5326–5327 (2005). (A) A report on how the presence of aliphatic alcohols, sodium dodecyl sulfate, and sodium chloride affect the temperature within the bubble. https://doi.org/10.1021/ja050804k

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• 134.

  “Evidence for a plasma core during multibubble sonoluminescence in sulfuric acid,” N. Eddingsaas and K. Suslick, J. Am. Chem. Soc. 129, 3838–3829 (2007). (A) This paper presented the first demonstration of an optically opaque core within bubbles during MBSL under the most severe conditions. https://doi.org/10.1021/ja070192z

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• 135.

  “Temperature inhomogeneity during multibubble sonoluminescence,” H. Xu, N. Glumac, and K. Suslick, Angew. Chem. Intl. Ed. 8, 1079–1082 (2010). (A) This paper demonstrates that sonoluminescence can have multiple apparent temperatures depending on the location and time during bubble compression from which the emission is occurring https://doi.org/10.1002/anie.200905754.

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Sonoluminescence can also be used to learn things about the behavior of medical devices that used high intensity ultrasound.

• 136.

  “Studies of the cavitational effects of clinical ultrasound by sonoluminescence: 2. Thresholds for sonoluminescence from a therapeutic ultrasound beam and the effect of temperature and duty cycle,” M. Pickworth, P. Dendy, T. Leighton, and A. Walton, Phys. Med. Biol. 33, 1239–1248 (1988). (E) The clinical device had a operating frequency of 1 MHz, a maximum output intensity of 3 W/cm², and with varying duty cycles.

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• 137.

  “Acoustic emission and sonoluminescence due to cavitation at the beam focus of an electrohydraulic shock wave lithotripter,” A. Coleman, M. Choi, J. Saunders, and T. Leighton, Ultrasound Med. Biol. 18, 267–281 (1992). (A) Lithotripters generate copious amounts of cavitation, and thus also sonoluminescence due to the long negative pressure tail of the shock wave. One can use these emissions to learn things about lithotripter operation. https://doi.org/10.1016/0301-5629(92)90096-S

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• 138.

  “The spatial distribution of cavitation induced acoustic emission, sonoluminescence and cell lysis in the field of a shock wave lithotripter,” A. Coleman, M. Whitlock, T. Leighton, and J. Saunders, Phys. Med. Biol. 38, 1545–1560 (1993). (A) This study examined the spatial distribution of various properties attributed to the cavitation field generated by a shock wave lithotripter. The acoustic emissions detected with a 1 MHz, 12 cm diameter focused hydrophone occurred in two distinct bursts. The immediate signal was emitted from a small region contained within the 4 MPa peak negative pressure contour. A second, delayed, burst was emitted from a region extending further along the beam axis. https://doi.org/10.1088/0031-9155/38/11/001

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• 139.

  “Sonoluminescence as an indicator of cell membrane disruption by acoustic cavitation,” S. Cochran and M. Prausnitz, Ultrasound Med. Biol. 27, 841–850 (2001). (E) This paper established correlations between the amount of light produced by sonoluminescence and both molecular uptake and cell viability. https://doi.org/10.1016/S0301-5629(01)00382-9

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Because sonoluminescence is due to cavitation, experts in cavitation bubble dynamics bought their powerful computation capabilities to show how collapsing bubbles could generated high temperatures and pressures, which lead to the molecular dissociation and radiative recombination representative of sonoluminescence emissions.

• 140.

  “A theoretical study of sonoluminescence,” V. Kamath, A. Prosperetti, and F. Egolfopoulos, J. Acoust. Soc. Am. 94, 248–260 (1993). (A) This paper is an excellent and detailed theoretical analysis of the entire bubble dynamics problem as related to MBSL. https://doi.org/10.1121/1.407083

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• 141.

  “Influence of ultrasonic frequency on multibubble sonoluminescence,” K. Yasui, J. Acoust. Soc. Am. 112, 1405–1413 (2002). (A) Computer simulations of bubble oscillations are performed under conditions of MBSL in water for various ultrasonic frequencies. https://doi.org/10.1121/1.1502898

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• 142.

  “Effects of thermal conduction on bubble dynamics near the sonoluminescence threshold,” K. Yasui, J. Acoust. Soc. Am. 98, 2772–2782 (1995). (A) One of the first papers to examine the effects of thermal conduction on the generation of sonoluminescence. https://doi.org/10.1121/1.413242

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• 143.

  “Vapor supersaturation in collapsing bubbles. Relevance to the mechanisms of sonochemistry and sonoluminescence,” A. Colussi and M. Hoffmann, Phys. Chem. A 103, 11336–11339 (1999). (A) The authors concluded that water vapor, rather than any particular gas, was the main component of collapsing bubbles, and its large heat capacity and atomization energies precluded reaching uniform peak temperatures exceeding 5 K. https://doi.org/10.1021/jp9927202

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If sonoluminescence were a valid indicator of sonochemical efficiency (and it is often not), then it would be helpful if one could determine which materials quenched sonoluminescence, and thus sonochemical yields. Several studies of the effect of quenching have been published.

• 144.

  “Selective quenching of species that produce sonoluminescence,” C. Sehgal, R. Sutherland, and R. Verrall, J. Phys. Chem. 84, 529–531 (1980). (I) One of the earliest reports that certain compounds could quench sonoluminescence. https://doi.org/10.1021/j100442a016

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• 145.

  “Sonoluminescence quenching in aqueous solutions containing weak organic acids and bases and its relevance to sonochemistry,” M. Ashokkumar, K. Vinodgopal, and F. Grieser, J. Phys. Chem. B 104, 6447–6451 (2000). (I) The addition of low levels of unsubstituted or 2-chloro-substituted aliphatic carboxylic acids and the aromatic solutes, phenol and aniline, significantly led to quenching, although changing the pH could lead to restoration. https://doi.org/10.1021/jp9937407

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• 146.

  “Quenching mechanism of multibubble sonoluminescence at excessive sound pressure,” S. Hatanaka, K. Yasui, T. Tuziuti, T. Kozuka, and H. Mitome, Jpn. J. Appl. Phys. 40, 3856–3860 (2001). (A) It was argued that the expulsion of bubbles from the pressure antinode in the field was responsible for the decrease in MBSL intensity at high ultrasonic intensity. https://doi.org/10.1143/JJAP.40.3856

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• 147.

  “Sonoluminescence quenching by organic acids in aqueous solution: pH and frequency effects,” G. Price, M. Ashokkumar, T. Cowan, and F. Grieser, Chem. Commun. 16, 1740–1741 (2002). (I) The quenching of sonoluminescence from aqueous solutions by acrylic and methacrylic acids was dependent on whether the acid was ionized in solution, as controlled by the pH, and also by the frequency of ultrasound used.

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• 148.

  “Comparison of the effects of water-soluble solutes on multibubble sonoluminescence generated in aqueous solutions by 20- and 515-kHz pulsed ultrasound,” R. Tronson, M. Ashokkumar, and F. Grieser, J. Phys. Chem. B 106, 11064–11068 (2002). (A) The influence of aliphatic alcohols, sodium dodecyl sulfate, and sodium chloride on the intensity of sonoluminescence was examined. https://doi.org/10.1021/jp020363g

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• 149.

  “Effect of volatile solutes on sonoluminescence,” K. Yasui, J. Chem. Phys. 116, 2945–2954 (2002). (A) Computer simulations of bubble oscillations in an aqueous methanol solution showed the conditions under which sonoluminescence was quenched. https://doi.org/10.1063/1.1436122

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• 150.

  “Effect of surfactants, polymers, and alcohol on single bubble dynamics and sonoluminescence,” M. Ashokkumar, J. Guan, R. Tronson, T. Matula, J. Nuske, and F. Grieser, Phys. Rev. E 65, 046310 (2002). (A) In this paper, it was observed that nonvolatile solutes, in the low concentration range used, did not significantly affect the radial dynamics nor the SL intensity of a single bubble in water. In contrast, the addition of micromolar quantities of a volatile solute, pentanol, quenched 90% of the SL without affecting the radial dynamics of the bubble. https://doi.org/10.1103/PhysRevE.65.046310

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• 151.

  “Multibubble sonoluminescence from aqueous solutions containing mixtures of surface active solutes,” R. Tronson, M. Ashokkumar, and F. Grieser, J. Phys. Chem. B 107, 7307–7311 (2003). (I) The reason for quenching was determined to be through both inter- and intra-bubble processes. https://doi.org/10.1021/jp034360v

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• 152.

  “Sonoluminescence emission from aqueous solutions of organic monomers,” G. Price, M. Ashokkumar, and F. Grieser, J. Phys. Chem. B 107, 14124–14129 (2003). (I) The authors observed that two separate frequencies gave varying degrees of quenching. https://doi.org/10.1021/jp034375t

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• 153.

  “Sonoluminescence quenching of organic compounds in aqueous solution: Frequency effects and implications for sonochemistry,” G. Price, M. Ashokkumar, and F. Grieser, J. Am. Chem. Soc. 126, 2755–2762 (2004). (A) It was concluded that the effect of the solutes on the sonoluminescence signal from aqueous solutions at two frequencies was primarily due to the balance of two factors; namely, the incorporation of solute within the bubble, leading to quenching, and the prevention of coalescence of the bubbles, leading to enhancement. https://doi.org/10.1021/ja0389624

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• 154.

  “The effect of surface-active solutes on bubble coalescence in the presence of ultrasound,” J. Lee, S. Kentish, and M. Ashokkumar, J. Phys. Chem. B 109, 5095–5099 (2005). (E) This paper examined the effect of surface-active solutes on such bubble coalescence in an ultrasonic field, with only passing reference to sonoluminescence. https://doi.org/10.1021/jp0476444

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• 155.

  “Effect of surfactants on inertial cavitation activity in a pulsed acoustic field,” J. Lee, S. Kentish, T. Matula, and M. Ashokkumar, J. Phys. Chem. B 109, 16860–16865 (2005). (A) In this study, both sonoluminescence and passive cavitation detection (PCD) were used to examine the acoustic cavitation field generated at different acoustic pulse lengths in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS). https://doi.org/10.1021/jp0533271

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• 156.

  “The influence of acoustic power on multibubble sonoluminescence in aqueous solution containing organic solutes,” D. Sunartio, M. Ashokkumar, and F. Grieser, J. Phys. Chem. B 109, 20044–20050 (2005). (I) The effect of varying the applied acoustic power on the extent to which the addition of water-soluble solutes affect the intensity of aqueous multibubble sonoluminescence (MBSL) was investigated. https://doi.org/10.1021/jp052747n

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• 157.

  “Determination of the size distribution of sonoluminescence bubbles in a pulsed acoustic field,” J. Lee, M. Ashokkumar, S. Kentish, and F. Grieser, J. Am. Chem. Soc. 127, 16810–16811 (2005). (I) It was shown how a simple pulsed SL technique can be employed for determining the size distribution of sonoluminescence bubbles in aqueous solutions. https://doi.org/10.1021/ja0566432

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• 158.

  “Effect of surfactants on the rate of growth of an air bubble by rectified diffusion,” J. Lee, S. Kentish, and M. Ashokkumar, J. Phys. Chem. B 109, 14595–14598 (2005). (A) The authors demonstrate that surfactants have an effect on bubble growth and likewise on sonoluminescence. https://doi.org/10.1021/jp051758d

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• 159.

  “Effect of alcohols on the initial growth of multibubble sonoluminescence,” J. Lee, M. Ashokkumar, S. Kentish, and F. Grieser, J. Phys. Chem. B 110, 17282–17285 (2006). (A) The outcome of this study indicates that the extent of SL quenching by alcohols was significantly higher than previously realized if the increase in the active bubble population due to coalescence hindrance was taken into account. https://doi.org/10.1021/jp063320z

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• 160.

  “Sonoluminescence quenching in aqueous solutions of aliphatic diols and glycerol,” D. Sunartio, F. Grieser, and M. Ashokkumar, Ultrasonics Sonochem. 16, 23–27 (2009). (A) MBSL signals that were generated using 358 kHz ultrasound in aqueous solutions of ethylene glycol, 1,3-propanediol, 1,4-butanediol and glycerol, over a range of concentrations. It was found that the intensity of the MBSL was either reduced or enhanced, relative to the signal in water, depending on the concentration of the solute. https://doi.org/10.1016/j.ultsonch.2008.06.005

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• 161.

  “Frequency effects during acoustic cavitation in surfactant solutions,” S. Wu, T. Leong, S. Kentish, and M. Ashokkumar, J. Phys. Chem. B 113, 16569–16573 (2009). (A) The acoustic cavitation-induced events, MBSL, and initial growth of MBSL were studied in surfactant solutions and correlated with bubble coalescence data at three different ultrasound frequencies. The total bubble volume generated for a fixed sonication time, which was indirectly related to bubble coalescence, similarly fell as surfactant concentration increased and then rose again.

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• 162.

  “Development and optimization of acoustic bubble structures at high frequencies,” J. Lee, M. Ashokkumar, K. Yasui, T. Tuziuti, T. Kozuka, A. Towata, and Y. Iida, Ultrasonics Sonochemistry 18, 92–98 (2011). (A) This paper describes the role of acoustics structures in the acoustic field, showing that when the bubbles get too plentiful, sonoluminescence is greatly reduced, if not quenched. https://doi.org/10.1016/j.ultsonch.2010.03.004

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• 163.

  “The role of surfactant headgroup, chain length, and cavitation microstreaming on the growth of bubbles by rectified diffusion,” T. Leong, J. Collis, R. Manasseh, A. Ooi, A. Novell, A. Bouakaz, M. Ashokkumar, and S. Kentish, J. Phys. Chem. C 115, 24310–24316 (2011). (A) Because bubble dynamics is such an important factor in sonoluminescence, this paper examined the role of various solutes and cavitation effects on rectified diffusion, and thus also on sonoluminescence. https://doi.org/10.1021/jp208862p

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• 164.

  “Sonoluminescence quenching and cavitation bubble temperature measurements in an ionic liquid,” P. M. Kanthale, A. Brotchie, F. Grieser, and M. Ashokkumar, Ultrasonics Sonochem. 20, 47–51 (2013). (A) A comparison between the temperatures within imploding acoustic cavitation bubbles and the extent of sonoluminescence quenching by C1–C5 aliphatic alcohols in 1-ethyl-3-methylimidazolium ethylsulfate was obtained. It was also found that the SL intensity decreased with increasing concentration (up to 1 M). https://doi.org/10.1016/j.ultsonch.2012.05.011

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• 165.

  “Multibubble sonoluminescence in ethylene glycol/water mixtures,” M. Bradley, M. Ashokkumar, and F. Grieser, J. Phys. Chem. B 118, 337–343 (2014). (I) This study examined the varied behavior of aliphatic alcohol solutes on MBSL in terms of their influence on inter- and intrabubble effects experienced by the bubbles in the ultrasound field. https://doi.org/10.1021/jp409075n

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• 166.

  “Effect of surfactants on single bubble sonoluminescence behavior and bubble surface stability,” T. Leong, K. Yasui, K. Kato, D. Harvie, M. Ashokkumar, and S. Kentish, Phys. Rev. E 89, 043007 (2014). (A) It was observed that an increase in the surfactant concentration led to a decline in the oscillation amplitude of the bubbles and hence also the light emission intensity. https://doi.org/10.1103/PhysRevE.89.043007

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The discovery of single bubble sonoluminescence (SBSL) ushered in an exciting period of research in sonoluminescence, resulting in over 1000 publications in the next 15 years. Former Nobel Prize winners suggested it was quantum radiation, papers were actually published claiming nuclear fusion, songs were written, and albums were sold—a multi-million dollar movie was even made about it. It attracted the public's attention and hundreds of high school students have used it for their science fair projects. Many of the mysteries of its origin were ascertained in these few years, but some aspects are still unknown and although it has lost its appeal as a scientific topic, there will very likely be more amazing discoveries of this remarkable phenomenon in the future. The evolution of this research is best examined in a historical perspective as it shows a steady scientific progress toward a clearer understanding of the phenomenon. While sonoluminescence was known and studied for over 50 years, it was associated with cavitation fields containing many bubbles, and it was only seen by the dark-adapted naked eye. Although SBSL may have been seen 20 years earlier (see Ref. 171), it was not fully appreciated until Felipe Gaitan was levitating a single bubble in an acoustic resonator and suddenly noticed a steady emission of light from what appeared as a point source. When he performed light scattering measurements and obtained a crude radius-time curve, it was obvious that it was a single bubble; furthermore, examining the light with a photomultiplier indicated that the light emission occurred each and every cycle. Probably the most intriguing aspect was that the system appeared to be self-stable, in that one could leave the apparatus in the evening and the light-emitting bubble would be still there the next morning. When Putterman and his group discovered that the light flash lasted only 10s of picoseconds, SBSL soon became a hot topic.

• 167.

  “Observation of sonoluminescence from a single stable cavitation bubble in a water/glycerine mixture,” L. Crum and D. Gaitan, in Frontiers of Nonlinear Acoustics, 12th ISNA, edited by M. F. Hamilton and D. T. Blackstock (Elsevier Applied Science, New York, 1990), p. 459–463. (E) The first publication of the observation of SBSL.

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• 168.

  “Sonoluminescence from single bubbles,” D. Gaitan and L. Crum, J. Acoust. Soc. Am. 87, S141 (1990). (E) Abstract of a paper presented at a meeting of the Acoustical Society of America reporting the observation of SBSL. https://doi.org/10.1121/1.2027990

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• 169.

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• 170.

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• 171.

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When Gaitan first observed SBSL, he tried to measure the pulse length of the flash, and determined it to be on the order of microseconds; however, when Putterman's group first made similar measurements, he found it to be much shorter, which prompted Gaitan to discover that the rise time of his PMT was restricting his measurement. Putterman's group worked to obtain the fastest PMT in existence and only then could they get a reasonable value of 50 ps, which they described as an upper bound. Because the bubble collapse time was nearly 1000 times this value, the pulse width seemed to have the key to understanding SBSL, and thus there were several attempts to measure the pulse length with some accuracy.

• 172.

  “Observation of synchronous picosecond sonoluminescence,” B. Barber and S. Putterman, Nature 352, 318–320 (1991). (E). The authors reported that sonoluminescence flashes, which comprised over 105 photons, were too fast to be resolved by the fastest photomultiplier tubes available. Furthermore, when sonoluminescence was driven by a resonant sound field, the bursts occurred in a continuously repeating, regular fashion. These precise “clock-like” emissions continued for hours at drive frequencies ranging from audible to ultrasonic. https://doi.org/10.1038/352318a0

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  “Resolving the picosecond characteristics of synchronous sonoluminescence,” B. Barber, R. Hiller, K. Arisaka, H. Fetterman, and S. Putterman, J. Acoust. Soc. Am. 91, 3061–3070 (1992). (E) The resolution with which the synchronous picosecond flashes of acoustically generated light can be measured was greatly improved over their previous method. The flash widths were found to be considerably less than 50 ps and the jitter in the time between flashes was substantially less than 50 ps. https://doi.org/10.1121/1.402942

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  “Calculated pulse widths and spectra of a single sonoluminescing bubble,” C. Moss, D. Clarke, and D. Young, Science 276, 1398–1401 (1997). (I) A sonoluminescing bubble was modeled as a thermally conducting, partially ionized, two-component plasma. The model showed that the measured picosecond pulse widths were due to electron conduction and the rapidly changing opacity of the plasma, and that these mechanisms were also responsible for the absence of an “afterglow” subsequent to the sonoluminescence flash while the hot bubble expands and cools. The calculated spectra for sonoluminescing nitrogen and argon bubbles suggest that a sonoluminescing air bubble probably contains only argon, in agreement with a recent theoretical analysis. https://doi.org/10.1126/science.276.5317.1398

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• 175.

  “Resolving sonoluminescence pulse width with time-correlated single photon counting,” B. Gompf, R. Günther, G. Nick, R. Pecha, and W. Eisenmenger, Phys. Rev. Lett. 79, 1405–1408 (1997). (A) The width of the sonoluminescence pulse was measured for the first time using time-correlated single photon counting. The pulse width at room temperature increased from about 60 ps at low gas concentrations and low driving pressures to more than 250 ps at high gas concentrations and driving pressures. https://doi.org/10.1103/PhysRevLett.79.1405

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• 176.

  “Resolving the sonoluminescence pulse shape with a streak camera,” R. Pecha, B. Gompf, G. Nick, Z. Q. Wang, and W. Eisenmenger, Phys. Rev. Lett. 81, 717–720 (1998). (E) The authors repeated their earlier measurements of the sonoluminescence pulse, this time determining the pulse shape. https://doi.org/10.1103/PhysRevLett.81.717

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• 178.

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• 179.

  “Measurements of sonoluminescence temporal pulse shape,” M. Moran and D. Sweider, Phys. Rev. Lett. 80, 4987–4990 (1998). (A) In this paper, the pulse width and shape were examined as a function of various parameters such as temperature and driving pressure. https://doi.org/10.1103/PhysRevLett.80.4987

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• 180.

  “Measured pulse width of sonoluminescence flashes in the form of resonance radiation,” A. Giri and V. Arakeri, Phys. Rev. E 58, 2713–2716 (1998). (A) These pulse width results were for MBSL and reported values in the nanosecond range.

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  “Computed optical emissions from a sonoluminescing bubble,” W. Moss, D. Young, J. Harte, J. Levatin, B. Rozsnyai, G. Zimmerman, and I. Zimmerman, Phys. Rev. E 59, 2986–2992 (1999). (A) A sonoluminescing bubble was modeled as a thermally conducting, partially ionized plasma. The model accounted for most of the observed experimental trends, including (i) the asymmetric pulse shape, (ii) the temperature and driving pressure dependence of the pulse width and intensity, (iii) spectral shapes, in particular, the 300 nm peak in the spectrum of xenon sonoluminescence, and (iv) a hydrodynamic explanation of why water is the “friendliest” liquid in which sonoluminescence occurs. The agreement between the calculations and the data suggest that the spectral and temporal properties of the emissions of a sonoluminescing bubble are due to adiabatic- or shock-initiated thermal emission from a cool dense plasma. https://doi.org/10.1103/PhysRevE.59.2986

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• 182.

  “Time-resolved spectra of single-bubble sonoluminescence in sulfuric acid with a streak camera,” W. Chen, W. Huang, Y. Liang, X. Gao, and W. Cui, Phys. Rev. E 78, 035301 (2008). (A) The time-resolved spectra of SBSL in sulfuric acid was observed with a streak camera after a spectrograph. The spectral center evolved from infrared to ultraviolet gradually within a SBSL duration, which corresponded to an increase of temperature. The peak temperature within one sonoluminescence duration was 5–9 times higher than the average temperature based on the average spectrum in their experiment. https://doi.org/10.1103/PhysRevE.78.035301

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The observation by Barber and Putterman that the pulse length of SBSL was in the picosecond range stirred much of the excitement about SBSL. The fact that light was emitted only during a small fraction of the collapse time, and that the pulses were observed to have a jitter of less that 50 ps, indicated that SBSL was truly a remarkable physical phenomenon. This discovery led to several publications on the measurement of the important characteristics of SBSL.

• 183.

  “Light scattering measurements of the repetitive supersonic implosion of a sonoluminescing bubble,” B. Barber and S. Putterman, Phys. Rev. Lett. 69, 3839–3842 (1992). (E) Light scattering was used to measure the dynamics of the repetitive collapse of a sonoluminescing bubble of gas trapped in water. It was found that the surface of the bubble was collapsing with a supersonic velocity at about the time of light emission which in turn preceded the minimum bubble radius by about 0.03% of the period of the acoustic drive. https://doi.org/10.1103/PhysRevLett.69.3839

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  “Toward a hydrodynamic theory of sonoluminescence,” R. Löfstedt, B. Barber, and S. Putterman, Phys. Fluids A 5, 2911–2928 (1993). (A) At small Mach numbers the Rayleigh–Plesset equations (modified to include acoustic radiation damping) provided the hydrodynamic description of a bubble's breathing motion. These equations indicated that in the presence of sonoluminescence, the ratio of maximum to minimum bubble radius was about 100, and thus a change of 10⁶ in volume. https://doi.org/10.1063/1.858700

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• 185.

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  “Physical processes for single bubble sonoluminescence,” H. Kwak and J. Na, Phys. Soc. Jpn. 66, 3074–3083 (1997). (A) Their calculations predict that sonoluminescence should occur just prior to the bubble collapse, its duration should be less than 300 ps, and that the increase and subsequent rapid decrease in bubble wall acceleration induces the quenching of the light emission. https://doi.org/10.1143/JPSJ.66.3074

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• 188.

  “A simple explanation of light emission in sonoluminescence,” S. Hilgenfeldt, S. Grossmann, and D. Lohse, Nature 398, 402–405 (1999). (A) Contended that using only hydrodynamic theory, one could explain how the pulse length could be computed based on an assumption of noble gas within the interior and an accounting for the thermodynamics.

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• 189.

  “Modeling of spherical gas bubble oscillations and sonoluminescence,” A. Prosperetti and Y. Hao, Philos. Trans. R. Soc. London A 357, 203–223 (1999). (A) Produced a refined model of bubble dynamics and showed that the parameter space for SBSL could be enlarged over earlier estimates. https://doi.org/10.1098/rsta.1999.0324

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  “Mechanism of single-bubble sonoluminescence,” K. Yasui, Phys. Rev. E 60, 1754–1758 (1999). (A) In this paper, Yasui introduced a quasiadiabatic compression model, and concluded that SBSL is not blackbody radiation but thermal radiation. https://doi.org/10.1103/PhysRevE.60.1754

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  “Energy focusing in a converging fluid flow: Implications for sonoluminescence,” K. Weninger, C. Camara, and S. Putterman, Phys. Rev. Lett. 83, 2081–2084 (1999) (A) Although most of sonoluminescence research has been associated with acoustic cavitation, sonoluminescence can also be generated by hydrodynamic flow. https://doi.org/10.1103/PhysRevLett.83.2081

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• 192.

  “Squeezing alcohols into sonoluminescing bubbles: The universal role of surfactants,” R. Tögel, S. Hilgenfeldt, and D. Lohse, Phys. Rev. Lett. 84, 2509–2512 (2000). (I) Surfactants reduce the intensity of SBSL. This effect was explained by a theoretical model in which the alcohols are assumed to be mechanically forced into the bubble at collapse, modifying the adiabatic exponent of the gas. https://doi.org/10.1103/PhysRevLett.84.2509

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  “Size of the light-emitting region in a sonoluminescing bubble,” J. Dam and M. Levinsen, Phys. Rev. Lett. 92, 144301 (2004). (E) Using a technique borrowed from astronomy, the authors determined the size of the sonoluminesing region in SBSL to be about 200 nm. https://doi.org/10.1103/PhysRevLett.92.144301

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• 194.

  “Focus: Plasma extremes seen through a gas bubble,” A. Bataller, B. Kappus, C. Camara, and S. Putterman, Phys. Rev. Lett. 113, 024301 (2014). (A) In this paper the authors contend that sound-stimulated gas bubbles in liquid become tiny plasmas and may provide a test bed for plasma physics theories. With their model, they calculated an electron density of 10²¹ electrons per cubic centimeter, which agreed with a previous sonoluminescence experiment in sulfuric acid that used a different technique. By comparison, inertial confinement fusion researchers aim for around 10²⁴ electrons per cubic centimeter to ignite fusion. https://doi.org/10.1103/PhysRevLett.113.024301

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Once SBSL was discovered, there was an attempt to define the parameter space for its existence. Many papers were written that sought to define this space.

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  “Bubble shape oscillations and the onset of sonoluminescence,” M. Brenner, D. Lohse, and T. Dupont, Phys. Rev. Lett. 75, 954–957 (1995). (A). Two different instability mechanisms (the Rayleigh–Taylor instability and parametric instability) cause deviations from sphericity. Distinguishing these mechanisms allowed an explanation of many features of recent experiments on SBSL. https://doi.org/10.1103/PhysRevLett.75.954

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  “Observation of a new phase of sonoluminescence at low partial pressures,” B. Barber, K. Weninger, R. Löfstedt, and S. Putterman, Phys. Rev. Lett. 74, 5276–5279 (1995). (A) Their analysis indicated that previously investigated sonoluminescence from bubbles at 200 Torr required a nondiffusive mass flow mechanism, and thus anticipated the dissociation hypothesis. https://doi.org/10.1103/PhysRevLett.74.5276

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• 197.

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  “Angular correlations in sonoluminescence: Diagnostic for the sphericity of a collapsing bubble,” K. Weninger, S. Putterman, and B. Barber, Phys. Rev. E 54, R2205–R2208 (1996). (I) Deviations from isotropic emission in sonoluminescence were resolved to about one part per thousand. States with larger dipole components were characterized by large fluctuations in the intensity of sonoluminescence. https://doi.org/10.1103/PhysRevE.54.R2205

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• 200.

  “Phase diagrams for sonoluminescing bubbles,” S. Hilgenfeldt, D. Lohse, and M. Brenner, Phys. Fluids. 8, 2808–2826 (1996). (I) The authors presented phase diagrams in the gas concentration versus forcing pressure state space and also in the ambient radius versus gas concentration and versus forcing pressure state spaces. These phase diagrams were based on the thresholds for energy focusing in the bubble and two kinds of instabilities, namely (i) shape instabilities and (ii) diffusive instabilities. https://doi.org/10.1063/1.869131

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  “Shape and extinction thresholds in sonoluminescence parameter space,” J. Ketterling and R. Apfel, J. Acoust. Soc. Am. 107, L13–L18 (2000). (E) The authors determined the conditions under which shape oscillations of the bubble would lead to SBSL extinction. https://doi.org/10.1121/1.428370

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• 205.

  “Extensive experimental mapping of sonoluminescence parameter space,” J. Ketterling and R. Apfel, Phys. Rev. E 61, 3832–3837 (2000). (E) This paper is a very detailed study that provides a collection of data that represented a complete mapping of the parameter space of sonoluminescence at a single frequency. https://doi.org/10.1103/PhysRevE.61.3832

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  “Phase diagrams for sonoluminescing bubbles: A comparison between experiment and theory,” R. Toegel and D. Lohse, J. Chem. Phys. 118, 1863–1875 (2003). (A) Developed a model to predict the boundaries of SBSL. https://doi.org/10.1063/1.1531610

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  “Phase transition to an opaque plasma in a sonoluminescing bubble,” B. Kappus, S. Khalid, A. Chakravarty, and S. Putterman, Phys. Rev. Lett. 106, 234302 (2011). (A). According to the model developed in this paper, sonoluminescence originates in a new phase of matter with high ionization, further evidence that the dense plasma inside the bubble is optically opaque. https://doi.org/10.1103/PhysRevLett.106.234302

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  “Non-Boltzmann population distributions during single-bubble sonoluminescence,” D. Flannigan and K. Suslick, J. Phys. Chem. B 117, 15886–15893 (2013). (A) Excited neon atom emission observed from an 80 wt. % solution of sulfuric acid gave an estimated temperature of only 3400 K, indicative of emission from a cool outer shell at the interfacial region. The neon atom excited-state population were best-described by a non-Boltzmann distribution. These observations were consistent with SBSL emission having both a spatial and temporal component. https://doi.org/10.1021/jp409222x

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  “Single bubble perturbation in cavitation proximity of solid glass: Hot spot versus distance,” D. Radziuk, H. Moehwald, and K. Suslick, Phys. Chem. Chem. Phys. 16, 3534–3541 (2014).(A) Physical perturbation of a single bubble was demonstrated by introduction of a glass fiber. The more perturbed the bubble, the lower its emission temperature. https://doi.org/10.1039/C3CP52850B

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When the SBSL pulse length was shown to be very short with respect to the time of bubble collapse, several papers suggested that shock waves must be generated in the bubble's interior, and thus examined SBSL bubble dynamics from a theoretical viewpoint, particularly within the bubble itself.

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  “On sonoluminescence of an oscillating gas bubble,” H. Greenspan and A. Nadim, Phys. Fluids A 5, 1065–1067 (1993). (A) It was proposed that shock dynamics within the gas of a small bubble explains sonoluminescence—the emission of visible radiation in response to spherically symmetric, ultrasonic excitation of a gas bubble in a liquid. Sufficiently high temperatures were predicted to explain the emission of light by the gas molecules. https://doi.org/10.1063/1.858619

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  “A model of sonoluminescence,” C. Wu and P. Roberts, Proc. R. Soc. London A 445, 323–349 (1994). (A) This paper provided details of the potential for shock waves within the gas in the bubble to generate sonoluminescence. https://doi.org/10.1098/rspa.1994.0064

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  “Theoretical studies of sonoluminescence radiation: Radiative transfer and parametric dependence,” L. Kondić, J. Gersten, and C. Yuan, Phys. Rev. E 52, 4976–4990 (1995). (A) The authors found that the inclusion of certain loss terms, especially those due to radiation, is necessary in order to understand the bubble dynamics, the resulting sonoluminescence pulse, and some recent experiments. https://doi.org/10.1103/PhysRevE.52.4976

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  “Instability of converging shock waves and sonoluminescence,” A. Evans, Phys. Rev. E 54, 5004–5011 (1996). (A) Evans examined the stability of a nearly spherical converging shock wave in a van der Waals gas and considered the implications for sonoluminescence. He determined that the instability was weak, although not as weak as in an ideal gas. https://doi.org/10.1103/PhysRevE.54.5004

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  “Mixture segregation within sonoluminescence bubbles,” B. Storey and A. Szeri, J. Fluid Mech. 396, 203–221 (1999). (A) Showed that water vapor can be trapped inside a collapsing bubble and to lead to important consequences in bubble dynamics—such as preventing the development of shock waves. https://doi.org/10.1017/S0022112099005984

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  “Molecular dynamics approach to single-bubble sonoluminescence,” B. Metten and W. Lauterborn, in Nonlinear Acoustics at the Turn of the Millennium: ISNA 15, edited by W. Lauterborn and T. Kurz, AIP Conf. Proc. 524, 429–432 (2000) (A). Using a molecular dynamics simulation, they were able to observe shock waves in the gas.

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  “Molecular dynamics simulation of the response of a gas to a spherical piston: Implications for sonoluminescence,” S. Ruuth, S. Putterman, and B. Merriman, Phys. Rev. E 66, 036310 (2002). (A) Their model showed strong energy focusing within the bubble, including the formation of shocks, strong ionization, and temperatures in the range of 50 000–500 000 K. https://doi.org/10.1103/PhysRevE.66.036310

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One of the most intriguing aspects of SBSL was that the spectrum appeared to be sensitive to the gas contents as if it were a noble gas. This discovery led to a series of papers examining the effects of the gas (and vapor) within the bubble's interior, and particularly the emission spectrum.

• 227.

  “Effect of noble gas doping in single-bubble sonoluminescence,” R. Hiller, K. Weninger, S. Putterman, and B. Barber, Science 266(5183), 248–250 (1994). (A) Sonoluminescence was found to be extremely sensitive to doping with a noble gas. Increasing the noble gas content of a nitrogen bubble to about 1% dramatically stabilized the bubble motion and increased the light emission by over an order of magnitude to a value that exceeded the sonoluminescence intensity of either gas alone. https://doi.org/10.1126/science.266.5183.248

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• 228.

  “Observation of isotope effects in sonoluminescence,” R. Hiller and S. Putterman, Phys. Rev. Lett. 75, 3549–3552 (1995). (A) The spectrum of sonoluminescence emitted by single bubbles of H2, D2, He3, and He4 trapped in H2O and D2O was measured. They found that heavy water had a dramatic effect on the spectrum of hydrogenic gases, yielding a blackbody-type spectrum with a spectral peak at about 400 nm. https://doi.org/10.1103/PhysRevLett.75.3549

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• 229.

  “Water vapour, sonoluminescence and sonochemistry,” B. Storey and A. Szeri, Proc. R. Soc. London A 456, 1685–1709 (2000). (A) This paper describes the phenomenon of “water vapor trapping,” in which, due to the rapid speed of collapse, water vapor does not have enough time to diffuse through the gas and condense. This trapping results in significant limitations on the temperature achievable. https://doi.org/10.1098/rspa.2000.0582

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• 230.

  “Does water vapor prevent upscaling sonoluminescence?,” R. Toegel, B. Gompf, R. Pecha, and D. Lohse, Phys. Rev. Lett. 85, 3165–3168 (2000). (A) It was observed that at room temperature in the liquid, water-vapor trapping could significant limit the temperature increase in the gas, but reducing the liquid temperature could significantly reduce the effect of water vapor trapping. https://doi.org/10.1103/PhysRevLett.85.3165

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• 231.

  “Temperature and pressure dependence of sonoluminescence,” G. Vazquez and S. Putterman, Phys. Rev. Lett. 85, 3037–3040 (2000). (E) It was observed that as water was cooled, there occurred a 100-fold increase in light emission which were accompanied by only slight changes in the ambient radius of the pulsating bubble. https://doi.org/10.1103/PhysRevLett.85.3037

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  “The effects of surfactant additives on the acoustic and light emissions from a single stable sonoluminescing bubble,” T. Stottlemyer and R. Apfel, J. Acoust. Soc. Am. 102, 1418–1423 (1997) (I) Showed that surfactants reduced SBSL intensities, but BSA increased it. https://doi.org/10.1121/1.420100

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• 233.

  “Effect of surfactants on single-bubble sonoluminescence,” K. Yasui, Phys. Rev. E 58, 4560–4567 (1998). (A) In this paper, Yasui showed that the addition of a surfactant resulted in lowering the achieved temperature inside the bubble due to the inhibition of condensation of water vapor at the bubble wall. https://doi.org/10.1103/PhysRevE.58.4560

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  “Effect of liquid temperature on sonoluminescence,” K. Yasui, Phys. Rev. E 64, 016310 (2001). (A) Yasui determined that at lower liquid temperatures, the presence of water vapor in the bubble was reduced and therefore the SBSL intensity was increased. https://doi.org/10.1103/PhysRevE.64.016310

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  “Single-bubble sonoluminescence from noble gases,” K. Yasui, Phys. Rev. E 63, 035301 (2001). (I) It was reported that in spite of the larger thermal conductivity of lighter noble gases, the maximum temperature in a SBSL bubble of lighter noble gases was higher due to the segregation of the water vapor from the noble gas. https://doi.org/10.1103/PhysRevE.63.035301

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  “Sonoluminescence: Cavitation hots up,” Detlef Lohse, Nature 434, 33–34 (2005). (E) A comment paper in Nature that called attention to the paper in the same issue by Flannigan and Suslick, who were able to use synthetic spectra to match experimental data and thus constitute the first direct measurement of the temperature of sonoluminescence. https://doi.org/10.1038/434033a

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  “Stable single-bubble sonoluminescence without the presence of noble gases,” M. Levinsen and J. Dam, Europhys. Lett. 80, 27004 (2007). (A) The authors report that in spite of the commonly accepted view that stable SBSL can only be achieved in water in the presence of a noble gas or hydrogen, long term stable SBSL can in fact be sustained with only diatomic gases like, e.g., nitrogen being present. https://doi.org/10.1209/0295-5075/80/27004

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Once SBSL was discovered, there was a search for it in liquids other than water. That proved to be difficult, until investigators found it in a wide variety of alcohols, and in concentrated sulfuric acid, which proved to be an important probe of the bubble's interior.

• 238.

  “Sonoluminescence from single bubbles in nonaqueous liquids: New parameter space for sonochemistry,” K. Weninger, R. Hiller, B. Barber, D. Lacoste, and S. Putterman, J. Phys. Chem. 99, 14195–14197 (1995). (E) The emission of light by a single pulsating bubble of xenon trapped in nonaqueous fluids was measured. The intensity of sonoluminescence displayed a remarkable structure as a function of the temperature and partial pressure of the solution. Unexpectedly, Swan lines were absent from the spectrum of SBSL in organic liquids. https://doi.org/10.1021/j100039a001

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  “Spectra of single-bubble sonoluminescence in water and glycerin-water mixtures,” D. Gaitan, A. Atchley, S. Lewia, J. Carlson, X. Maruyama, M. Moran, and D. Sweider, Phys. Rev. E 54, 525–528 (1996). (I) They concluded that the spectrum for air bubbles in water is consistent with that previously reported, the radiated energy decreases as the glycerin concentration increases, and the peak of the spectrum appears to shift to longer wavelengths for the water-glycerin mixtures. https://doi.org/10.1103/PhysRevE.54.525

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  “Molecular emission from single bubble sonoluminescence,” Y. Didenko, W. McNamara, and K. Suslick, Nature 407, 877–879 (2000). (E) Previously, molecular spectra were observed only in MBSL; in this paper, they observed that SBSL in sulfuric acid generated both a strong continuum plus molecular emissions.

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• 241.

  “Detailed simulations of sonoluminescence spectra,” P. Burnett, D. Chambers, D. Heading, A. Machacek, M. Schnittker, W. Moss, P. Young, S. Rose, R. Lee, and J. Wark, J. Phys. B: At. Mol. Opt. Phys. 34, L511–L518 (2001). (A) It was predicted that while the majority of the optical emission corresponds to bound-free transitions, there remains the possibility of observing broad line emission in both the UV and IR regions of the spectrum. It was also argued that it is likely that there is a optically opaque core with a luminous outer shell within the bubble. https://doi.org/10.1088/0953-4075/34/16/102

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• 242.

  “Plasma formation and temperature measurement during single-bubble cavitation,” D. Flannigan and K. Suslick, Nature 434, 52–55 (2005). (E) The authors observed strong emission lines in SBSL in sulfuric acid and concluded that the emitting species must originate from collisions with high-energy electrons, ions or particles from a hot plasma core. This was the first experimental observation for the presence of a plasma core during SBSL. https://doi.org/10.1038/nature03361

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  “The energy efficiency of formation of photons, radicals and ions during single-bubble cavitation,” Y. Didenko and K. Suslick, Nature 418, 394–397 (2002). (E) The authors observed the existence of chemical reactions within a single cavitating bubble in a nonaqueous liquid, and quantified the sources of energy dissipation during bubble collapse. https://doi.org/10.1038/nature00895

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• 244.

  “Plasma formation and temperature measurement during single-bubble cavitation,” D. Flannigan and K. Suslick, Nature 434, 52–55 (2005). (E) Here the authors report on their observations of atomic (Ar) emission and extensive molecular (SO) and ionic (O²⁺) progressions in SBSL spectra from concentrated aqueous H₂SO₄ solutions. They conclude that these emitting species must originate from collisions with high-energy electrons, ions or particles from a hot plasma core. Emission temperatures of 20 000 K were reported. https://doi.org/10.1038/nature03361

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• 245.

  “Dynamics of a sonoluminescing bubble in sulfuric acid,” S. Hopkins, S. Putterman, B. Kappus, K. Suslick, and C. Camara, Phys. Rev. Lett. 95, 254301 (2005). (E) Evidence for energy focusing nonlinear processes within the bubble is provided here by the observation that the implosion velocity must exceed a sharp threshold in order for a sonoluminescence flash to be emitted. Imposition of a restriction that the radius of the emitting hot spot is smaller than the radius of the gas bubble implies the presence of a surface emitting blackbody which masks the, as yet, unmeasured core temperature of sonoluminescence. https://doi.org/10.1103/PhysRevLett.95.254301

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  “Moving single bubble sonoluminescence in phosphoric acid and sulphuric acid solutions,” A. Troia, D. Ripa, and R. Spagnolo, Ultrasonics Sonochem. 13, 278–282 (2006). (I) These authors describe some experiments conducted in aqueous solutions of phosphoric and sulphuric acid. In these liquid media, it is possible to reproduce MSBSL (moving single bubble sonoluminescence) and luminescence is emitted even if a trapped bubble is subjected to a strong shape instability, named in the literature the “jittering phase.” https://doi.org/10.1016/j.ultsonch.2005.06.002

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• 247.

  “Measurement of pressure and density inside a single sonoluminescing bubble,” D. Flannigan, S. Hopkins, C. Camara, S. Putterman, and K. Suslick, Phys. Rev. Lett. 96, 204301 (2006). (E) The average pressure inside a sonoluminescing bubble in sulfuric acid was determined by two independent techniques: (1) Plasma diagnostics applied to Ar atom emission lines and (2) light scattering measurements of bubble radius vs time. The results have important Implications for a hot inner core within the bubble. https://doi.org/10.1103/PhysRevLett.96.204301

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• 248.

  “Plasma quenching by air during single-bubble sonoluminescence,” D. Flannigan and K. Suslick, J. Phys. Chem. A 110, 9315–9318 (2006). (A) The authors report the observation of sudden and dramatic changes in SBSL intensity and spectral profiles at a critical acoustic pressure for solutions of sulfuric acid containing mixtures of air and noble gas. https://doi.org/10.1021/jp063023u

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• 249.

  “Emission from electronically excited metal atoms during single-bubble sonoluminescence,” D. Flannigan and K. Suslick, Phys. Rev. Lett. 99, 134301 (2007). (A) These results provide a direct experimental link between single and multibubble SL and point toward the origins of sonochemical reactivity of nonvolatile species. https://doi.org/10.1103/PhysRevLett.99.134301

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  “Temperature nonequilibration during single-bubble sonoluminescence,” D. Flannigan and K. Suslick, J. Phys. Chem. Lett. 3, 2401–2404 (2012). (A). During SBSL from aqueous sulfuric acid containing dissolved neon, rovibronic emission spectra revealed vibrationally hot sulfur monoxide (Tv = 2100 K) that was also rotationally cold (Tr = 290 K). https://doi.org/10.1021/jz301100j

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SBSL was first thought to be radically different from MBSL. Actually, the difference was not that major; it was found to be mostly due to different bubble dynamics.

• 251.

  “Comparison of multibubble and single-bubble sonoluminescence spectra,” T. Matula, R. Roy, P. Mourad, W. McNamara III, and K. Suslick, Phys. Rev. Lett. 75, 2602–2605 (1995). (A) Comparisons of the spectral characteristics of sonoluminescence from cavitation in bubble fields (MBSL) versus cavitation of single bubbles (SBSL) were made for aqueous solutions under similar experimental conditions. In particular, alkali metal chloride solutions exhibited sonoluminescence emission from excited state Na or K atoms in MBSL, while SBSL exhibits no such emission. https://doi.org/10.1103/PhysRevLett.75.2602

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• 252.

  “Single-bubble and multibubble sonoluminescence,” K. Yasui, Phys. Rev. Lett. 83, 4297–4300 (1999). (A) Yasui determined that emissions from excited molecules were strongly quenched by high pressure and temperature inside a SBSL bubble and that sonoluminescence originated in the emissions from a plasma. https://doi.org/10.1103/PhysRevLett.83.4297

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  “Observation of bubble dynamics within luminescent cavitation clouds: Sonoluminescence at the nano-scale,” K. R. Weninger, C. G. Camara, and S. J. Putterman, Phys. Rev. E 63, 016310 (2000). (E) The authors observed that in inertial cavitation in multibubble conditions, the emission region was hotter than that for SBSL, although comparative rate thermometry and spectroscopic thermometry of line emissions observed in some multibubble systems showed lower temperatures. Why this was so is still not understood. https://doi.org/10.1103/PhysRevE.63.016310

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• 254.

  “Multibubble sonoluminescence spectra of water which resemble single-bubble sonoluminescence,” Y. Didenko and T. Gordeychuk, Phys. Rev. Lett. 84, 5640–5643 (2000). (A) In this paper, their results show that at high acoustic intensity and with xenon as a dissolved gas, the emission of the OH radical becomes indiscernible from the continuum. https://doi.org/10.1103/PhysRevLett.84.5640

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Some papers showed SBSL to be dependent on a variety of different phenomenon.

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  “Sonoluminescence in high magnetic fields,” J. Young, T. Schmiedel, and W. Kang, Phys. Rev. Lett. 77, 4816–4819 (1996). (E) The authors reported that in magnetic field sweeps at constant levels of acoustic drive, sonoluminescence disappears above a pressure-dependent threshold magnetic field. These results indicated a dramatic modification of bubble dynamics by magnetic fields. https://doi.org/10.1103/PhysRevLett.77.4816

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  “The role of surface tension in stable single-bubble sonoluminescence,” I. Akhatov, N. Gumerov, C. Ohl, U. Parlitz, and W. Lauterborn, Phys. Rev. Lett. 78, 227–230 (1997). (A) A theory for stable bubble oscillations was presented that is based on the strong influence of the surface tension on the dynamics of small bubbles and takes into account rectified diffusion and the resonance-like response of small bubbles to very strong acoustic pressure amplitudes. This theory provided an explanation for the existence of small, stably oscillating bubbles that have been observed in experiments on sonoluminescence. https://doi.org/10.1103/PhysRevLett.78.227

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  “Bjerknes force and bubble levitation under single-bubble sonoluminescence conditions,” T. Matula, S. Cordry, R. Roy, and L. Crum, J. Acoust. Soc. Am. 102, 1522–1527 (1997). (I) In this paper, their measurements indicated that the equilibrium position of the bubble shifted away from the pressure antinode as the drive pressure increased, in qualitative agreement with nonlinear calculations, but opposite that of linear theory. https://doi.org/10.1121/1.420065

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  “Sonoluminescence from an isolated bubble on a solid surface,” K. Weninger, H. Cho, R. Hiller, S. Putterman, and G. Williams, Phys. Rev. E 56, 6745–6749 (1997). (A) The hemispherical cavities reached maximum radii about five times larger than were realized in sonoluminescence from single bubbles; in addition, the sonoluminescence intensity continued to increase as the liquids were cooled to temperatures as low as 160 K. https://doi.org/10.1103/PhysRevE.56.6745

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• 259.

  “Water temperature dependence of single bubble sonoluminescence,” S. Hilgenfeldt, D. Lohse, and W. Moss, Phys. Rev. Lett. 80, 1332–1335 (1998). (A) The authors examined the temperature dependence of SBSL, showing that lower temperatures gave stronger sonoluminescence. https://doi.org/10.1103/PhysRevLett.80.1332

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  “Bubble shape instability and sonoluminescence,” C. Wu and P. Roberts, Phys. Lett. A 250,131–136 (1998). (A) The authors examined the effect of viscosity on bubble shape instabilities, and found that higher viscosities tended to dampen the higher modes. https://doi.org/10.1016/S0375-9601(98)00834-2

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  “Effect of surfactants on single-bubble sonoluminescence,” K. Yasui, Phys. Rev. E 58, 4560–4567 (1998). (A) Yasui suggested that the reduction of the magnitude of SBSL light by the addition of certain surfactants was due to the increase in the amount of endothermal chemical reactions, which resulted in a lower maximum temperature inside the bubble. https://doi.org/10.1103/PhysRevE.58.4560

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  “Effect of non-equilibrium evaporation and condensation on bubble dynamics near the sonoluminescence threshold,” K. Yasui, Ultrasonics 36, 575–580 (1998). (A) Yasui suggested that water vapor played an important role in SBSL emissions, such that when the temperature was reduced, which lowered the vapor pressure, the intensity of sonoluminescence was increased. https://doi.org/10.1016/S0041-624X(97)00107-8

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  “Ambient pressure and single-bubble sonoluminescence,” L. Kondic, C. Yuan, and C. Chan, Phys. Rev. E 57, R32–R35 (1998). (I) The author's calculations suggested that ambient pressure could have a significant effect on SBSL emissions, predicting a 200% increase in SBSL radiation if the ambient pressure were decreased by only 5%. https://doi.org/10.1103/PhysRevE.57.R32

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  “Ambient pressure effect on single-bubble sonoluminescence,” M. Dan, J. D. N. Cheeke, and L. Kondic, Phys. Rev. Lett. 83, 1870–1873 (1999). (A) This paper is a follow-up on their previous paper, presenting experimental results that supported their earlier predictions. https://doi.org/10.1103/PhysRevLett.83.1870

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  “Single-bubble sonoluminescence in microgravity,” T. Matula, Ultrasonics 38, 559–565 (2000). (E) In what might be considered a total dedication to science, Matuta flew a SBSL system in the so-called “vomit comet” and determined that there was an increase in SBSL intensity in microgravity, probably due to the change in buoyancy of the bubble. https://doi.org/10.1016/S0041-624X(99)00217-6

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One of the most intriguing aspects of SBSL is that the bubble can remain stable for long periods of time. This stability apparently violates normal bubble dynamics in that the bubble should either dissolve due to surface tension or grow due to rectified diffusion. The explanation for this stability was given by Lohse et al. (Ref. 266) in that when the gas heats up in the bubbles interior, molecular oxygen, and nitrogen dissociate and form nitrogen compounds that are soluble in the liquid. After a short period of time, the only gas remaining within the bubble is argon. Thus, the “dissociation hypothesis” explained many aspects of SBSL dynamics. These and related papers were very instrumental in leading to an “understanding” of SBSL.

• 266.

  “Sonoluminescing air bubbles rectify argon,” D. Lohse, M. Brenner, T. Dupont, S. Hilgenfeldt, and B. Johnston, Phys. Rev. Lett. 78, 1359–1362 (1997). (E) First to propose the “dissociation hypothesis,” viz., that nitrogen and oxygen dissociation and subsequent reaction to water soluble gases implied that strongly forced air bubbles eventually consisted of pure argon. Thus, it is the partial argon (or any other inert gas) pressure which is relevant for stability. This hypothesis was subsequently proven to be supported by the experimental results. https://doi.org/10.1103/PhysRevLett.78.1359

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  “Experimental validation of the dissociation hypothesis for single bubble sonoluminescence,” J. Ketterling and R. Apfel, Phys. Rev. Lett. 81, 4991–4994 (1998). (A) Another experiment showing support for the “dissociation hypothesis.” https://doi.org/10.1103/PhysRevLett.81.4991

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  “Suppressing dissociation in sonoluminescing bubbles: The effect of excluded volume,” R. Toegel, S. Hilgenfeldt, and D. Lohse, Phys. Rev. Lett. 88, 034301 (2002). (I) The authors argued that one can solve the water vapor energy-robbing problem, by simply reducing the temperature of the host liquid. https://doi.org/10.1103/PhysRevLett.88.034301

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Because sonoluminescence showed such extreme conditions, even though the acoustic driving pressure amplitude was only on the order of 0.1 MPa (while some medical devices have amplitudes as high as 100 MPa), a group of investigators sought ways of increasing the intensity of the light emissions, thus hopefully amplifying also the associated pressures and temperatures.

• 274.

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  “The radial motion of a sonoluminescence bubble driven with multiple harmonics,” K. Hargreaves and T. Matula, J. Acoust. Soc. Am. 107, 1774–1776 (2000). (E) The authors showed that driving the bubble with harmonic excitation could increase the SBSL intensity, but driving with a “spike,” as suggested by Moss, would not work. https://doi.org/10.1121/1.428575

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  “Role of very-high-frequency excitation in single-bubble sonoluminescence,” F. Moraga, R. Taleyarkhan, R. Lahey, Jr., and F. Bonetto, Phys. Rev. E 62, 2233–2237 (2000). (A) By varying the phase difference between the fundamental and a 10th harmonic, it was possible to enhance the sonoluminescence light emission by as much as a factor of 2.7 compared with single-frequency excitation. https://doi.org/10.1103/PhysRevE.62.2233

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  “Boosting sonoluminescence with a high-intensity ultrasonic pulse focused on the bubble by an adaptive array,” J. Thomas, Y. Forterre, and M. Fink, Phys. Rev. Lett. 88, 074302 (2002). (E) A new experimental approach was presented in which an ultrasonic pulse of high frequency was adaptively focused on the bubble during the collapse. Using an array of eight transmitters, a pressure pulse of 0.7 MPa doubled the flash intensity. https://doi.org/10.1103/PhysRevLett.88.074302

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  “Single bubble sonoluminescence driven by non-simple-harmonic ultrasound,” W. Chen, X. Chen, M. Lu, G. Miao, and R. Wei, J. Acoust. Soc. Am. 111, 2632–2637 (2002). (I) Three types of non-simple-harmonic waves, the rectangular, triangular, and as well as the sinusoidal wave with a pulse were used to drive SBSL and showed a significant increase in SBSL intensity. https://doi.org/10.1121/1.1480417

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  “Cavitation luminescence in a water hammer: Upscaling sonoluminescence,” C. Su, C. Camara, B. Kappus, and S. Putterman, Phys. Fluids 15, 1457–1461 (2003) (I) Using a ultralow frequency, they were able to observe over 10⁸ photons for a single collapse with a peak power of over 0.4 W. https://doi.org/10.1063/1.1572493

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One way of learning something about cavitation is to measure the acoustic emissions that are generated by the collapsing bubble—sometimes called a passive cavitation detector. Interestingly, even a small 5 μm bubble undergoing SBSL generates these emissions.

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Although MBSL was normally associated with sonochemistry, researchers soon discovered that SBSL could have relevance also in sonochemistry.

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Sonoluminescence emissions have even been shown to replicate the nonlinear dynamics concept of deterministic chaos.

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Because the actual temperatures produced within the bubble could not directly be measured, and because many models were not accurate enough to calculate the temperature—although some models gave values near 1 × 10⁶ degrees—there were speculations that if the relevant hydrogen isotopes were contained within the collapsing bubble, nuclear fusion could occur. A number of papers were published on this topic which led to a major controversy. At this time, after many papers and many efforts, there have been no independently replicated studies of cavitation-induced fusion.

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