The Physics of Microdroplets

Chapter 4

Sessile Droplets

4.1 Abstract

This chapter is dedicated to the analysis of the behavior of sessile droplets. A sessile droplet is a droplet sitting on a solid substrate (fig. 4.1).

Figure 4.1 Simulation of a sessile droplet on a flat substrate.

The volume and surface area of a sessile droplet on a horizontal, uniform and smooth substrate have been derived in the preceding chapter. In this chapter we analyze more complicated situations where the sessile droplet is placed on an inclined, or an inhomogeneous, or rough substrate. Many different situations may occur: a droplet placed on an inclined plane, or at the boundary between a hydrophilic and hydrophobic substrate, or on a vertical step of the substrate. We derive the notions of pinning and canthotaxis, which may contribute to immobilize a droplet that would otherwise move. Finally, the case of a droplet on a chemically or morphologically inhomogeneous substrate is investigated, and the fundamental Wenzel and Cassie-Baxter laws are presented. These considerations lead to the notions of super-hydrophobicity and super-hydrophilicity.

4.2 Droplet Self-motion Under the Effect of a Contrast or Gradient of Wettability

Microsystem substrates are often composed of different materials which have different wettabilities. Capillary forces are then locally different. Droplets deposited on such inhomogeneous substrates experience ...