Introduction to Microfluidics , Patrick Tabeling , (translated from French by Suelin Chen), Oxford U. Press, New York, 2005. $99.50 (301 pp.). ISBN 0-19-856864-9
In preparing to review Patrick Tabeling’s Introduction to Microfluidics , I was curious to learn the number of books on the subject that have appeared since 2000. A simple search on WorldCat, the Online Computer Library Center catalog, generated 11 other books published during this short period. These books collectively document the influences that first spurred interest in small fluidic devices, the hydrodynamic equations and boundary conditions pertinent to small-scale flows, recipe instructions for the design and fabrication of prototypes suitable for biomicro-electromechanical systems, and applications ranging from lab-on-a-chip devices to optofluidic components. Not included in the literature search were 10 or so lengthy review articles, about 20 conference volumes, and 2 journals devoted exclusively to microfluidics— Lab on a Chip , published by the Royal Society of Chemistry, and Microfluidics and Nanofluidics , produced by Springer. There is now even a $5000 prize, sponsored by Lab on a Chip and Corning Inc, awarded to “Pioneers in Miniaturization” under the age of 45.
The almost frantic output from academic and industrial researchers underscores the tremendous potential anticipated for technologies based on flow miniaturization. But gadflies like me remain a bit skeptical as to whether this field holds equal promise from a physics perspective, because most of the mechanisms for generating flow, mixing, or separation are reasonably well understood. Exceptions include subtleties involving multiphase flows, boundary conditions at liquidsolid interfaces, and the crossover from continuum to molecular-length descriptions of flow.
For the most part, an undergraduate course on the fundamentals of fluid, heat, and mass transfer is sufficient for understanding the operational basis of microfluidic devices. In addition, the prevalence of software packages geared toward flow optimization for complex geometries renders the design of even the most esoteric layouts accessible to those with a more limited background. I thus approached Tabeling’s book with bias, hoping to find material suitably challenging for physics students despite the book being an introductory text.
The seven main chapters in Introduction to Microfluidics touch on the subjects of low-Reynolds-number flow and its consequences for miniature systems; transport processes critical to microfluidic devices, such as diffusion, dispersion, mixing, absorption, and separation; electrokinetic flows in the context of lab-on-a-chip systems; heat transfer and efficient thermal control with microscale heat exchangers; and deposition and sealing techniques for constructing microplumbing components. The book is based on graduate courses offered during 2001-03 at the Jussieu campus, the University of Paris VI: Pierre and Marie Curie, and the École Polytechnique. This target audience explains the author’s informal presentation style, which pervades his ambitious attempt to cover all facets of microfluidics in one volume.
Tabeling anchors his whirlwind tour of the field with specific examples: He provides more than 170 drawings illustrating various concepts, geometries, and device realizations, and he includes occasional boxed inserts containing short mathematical derivations. Unfortunately, the presentation does not lend itself to conventional homework problems. Also, the references at the end of each chapter of the book, which was originally published in French in 2003, are not sufficiently comprehensive or up to date for those eager to tackle more advanced or specialized topics.
For example, physics students might enjoy learning more about the forces responsible for electrowetting, one of the more elegant methods for droplet actuation. Electrohydrodynamic forces, when coupled to the dynamics of a moving contact line, reveal some challenging problems; but the book’s discussion of electrowetting, which is poorly described, is far too brief and is limited only to the Lippman equation. The two electrowetting references are geared more toward materials compatibility and device fabrication than fundamentals. The limited references miss substantial developments since 2002 that provide the hydrodynamic basis for the technique. Electrowetting devices represent just one category of a larger class of open microfluidic systems driven by surface acoustic waves, thermocapillary stresses, magnetic forces, and other electrocapillary phenomena—none of which are discussed at any length. Although droplet motion by surface-energy gradients is also considered, the explanation the author provides is not totally accurate. The fluid velocity depends on the gradient of the local curvature, as well as the square of the local droplet thickness, and not simply on the curvature as stated.
Unfortunately, Tabeling’s desire for brevity leads in many cases to misleading descriptions, as, for example, with droplet evaporation. Many studies during the past decade have shown that evaporating droplets are not just subject to diffusion but also undergo complex processes as a result of boundary pinning, substrate wettability, thermo-capillary and Marangoni effects, ambient saturation conditions, and vapor recoil effects, to name a few. Perhaps a second printing of this book will include more detailed descriptions and eliminate distracting misnomers and grammatical errors, due in part to the inexperience of the translator, a graduate student at MIT.
The same qualities then that make the book an entertaining and painless entrée into the field of microfluidics, however, may leave physics students dissatisfied, as several of the presentations are too sketchy, elementary, or misleading in their simplicity. When referring to molecular-dynamics studies of slip boundary conditions, for example, Tabeling incorrectly states that the critical shear rate above which substantial slip is possible at liquid-solid interfaces is of order 1013 s−1, well beyond the realm of commonplace flows. The proper way to make contact between actual laboratory systems and molecular-dynamics simulations is through the relevant dimensionless quantities like the Reynolds, capillary, or Bond numbers. As known, a naive mapping of the length or time scales extracted from a Lennard-Jones interaction potential always leads to ridiculously high or low estimates in comparison to real experimental systems.
Despite these drawbacks, Tabeling’s infectious excitement and broad interest in microfluidics and microhydro-dynamic flows infuse the pages of this easy-to-read book. The illustrations and images are very useful in conveying concepts in a straightforward way. Readers who want a sweeping introduction to the various transport mechanisms and technologies at play will benefit from and enjoy this compact overview of the subject. Introduction to Microfluidics is likely to intrigue those interested in commercial devices who wish to peek under the covers to learn more about the fundamentals governing small-scale flows.
