The Physics of Microdroplets

Chapter 7

Capillary Effects: Capillary Rise, Capillary Pumping, and Capillary Valve

7.1 Abstract

In this chapter, we investigate the effect of capillarity forces on liquid rise, liquid pumping and valving. In the first part, we analyze how capillary forces oppose gravity to trigger capillary rise in and around solid structures such as plates, hollow pillars and bundle of solid pillars. In the second part, the theory of capillary pumping is developed and examples of capillary pumps are given. In the third part, the effectiveness of capillary valves is analyzed. Note that capillary pumping and valving are wide-spread for small scale microsystems because of their simplicity. For example, in biotechnology, most point of care (POC) microsystems use capillarity and wetting for filling of components with liquid samples.

7.2 Capillary Rise

In the introduction of this book, we have shown that capillary forces are usually negligible at the macroscale compared to other forces such as gravity, and inertia, but become important as the geometric scales are reduced. They are often dominant at the microscale and nearly always at the nanoscale. In this first section, we investigate how capillary forces can make liquids rise against gravitational forces. It is shown that the height that the liquid can reach is considerable.

7.2.1 Cylindrical Tubes: Jurin’s Law

When a capillary tube is plunged into a volume of wetting liquid, the liquid rises inside the tube under the effect of capillary forces ( ...