The Physics of Microdroplets

Preface

Capillary phenomena are intriguing. During the many years I (Jean) have spent with my colleagues working on microsystems for biotechnology, I have observed the difficulty to predict – and sometimes understand – the behavior of droplets and interfaces at the micro scale. First, optical observation is not straightforward – it is not easy to locate an interface seen from above in the microscope. Second, the analysis of the observed phenomena is complicated. In my personal experience, that was the case for pancreatic cells encapsulation in micro-flow-focusing devices, liquid-liquid extraction systems, digital microfluidics, capillary valves, spontaneous capillary flows in closed and open channels, in cracks, and between fibers.

And the difficulty is even more important for the conception of new microsystems. Questions such as “where is the interface going to anchor?” or “will the particles cross the interface?” or “will the interface de-pin when the capsule arrives?” or “will the capillary force be sufficient?” are repeatedly being asked. Although illustrious pioneers such as P-G. de Gennes, D. Quéré, G.M. Whitesides, and others have contributed to the knowledge of interface behaviors on a theoretical standpoint, much is left to understand for the engineer having to design a microchip or the student behind his computer or the biologist at his lab bench.

In this book, Ken and I have attempted to give the reader the tools for solving these capillary and surface tension problems, ...