Many applications of ultrasound, such as welding plastics or sterilization of medical equipment, exploit cavitation, the rapid expansion and collapse of bubbles as sound waves pulse through a liquid. This phenomenon that has proved to be so utilitarian is also the source of great fascination and mystery: During cavitation the collapsing bubbles develop extremely high temperatures and pressures and emit visible radiation. Recent experiments have elucidated just how extreme the properties of cavitation bubbles are and have also suggested what potential new applications the bubbles might have. In one paper, Bradley Barber and Seth Putterman of the University of California at Los Angeles report that the light emitted during cavitation—known as sonoluminescence—emerges in flashes that are less than 50 picoseconds long, far shorter than anyone had expected, and that the pulses repeat with clocklike precision. In other work, Edward Flint and Kenneth Suslick of the University of Illinois at Urbana‐Champaign have confirmed, by the most direct measurements to date, the long‐standing expectation that the temperature during cavitation soars above 5000 K. A number of researchers have been exploiting these high temperatures for several years by using acoustic waves to accelerate various chemical reactions. Suslick and his colleagues recently reported a new application along these lines—the formation of amorphous iron.

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