9 - Introduction to Droplet Microfluidics and Multiphase Microflows

Abstract

This chapter provides an introduction to droplet microfluidics and multiphase microflows. It focuses on the flow of liquid plugs in capillary tubes. Microflows are not always constituted by a single liquid. They deal with small quantities of liquid in the form of liquid plugs moving inside capillary tubes, and separated by inert, biocompatible, nonmiscible plugs of another liquid. Conceptually, liquid plugs may be seen as a transitional state between microflows and digital microfluidics. The shape of a liquid plug in a capillary tube depends on the capillary forces. The contact angle formed between a flowing liquid front and a solid surface is not constant, but reflects the balance between capillary forces and viscous forces. Bernoulli's equation combined with Tanner's law explains the main features of the behavior of liquid plugs moving inside capillary tubes. Flow regions may be decomposed in to two—the regions where a fluid moves inside the capillary inducing a friction pressure drop, and the interfaces that induce a capillary pressure drop. The capillary pressure drop is due to the difference of the capillary forces between advancing and receding fronts, because of the two different contact angles. The chapter also focuses on the extraction of target molecules from a primary liquid and concentration of these molecules in a secondary liquid; and on the formation and behavior of droplets in a microflow, and especially on the emission of monodispersed droplets.