The Physics of Microdroplets

Chapter 9

Droplets, Particles and Interfaces

9.1 Abstract

Solid particles, bubbles, droplets or gelled droplets are commonly found in microfluidic systems. Very often, these objects interact together or contact existing interfaces.

The different combinations of contact are numerous. In this chapter we restrict ourselves to the study of the behavior of liquid droplets in contact with other droplets and interfaces, and on solid spheres at interfaces. In the first part, we focus on the question of engulfment of a liquid droplet by another droplet or in another liquid phase across an interface. On one hand, engulfment of a droplet immersed in a carrier liquid into another liquid across the interface is crucial in encapsulation applications [1, 2]; on the other hand, engulfment of a droplet in another droplet is of importance in chemical micro-reactors using reactions performed inside droplets [3–6].

In a second section, we focus on solid spheres and their behavior in contact with droplets and interfaces (fig. 9.1). In particular, the crossing of an interface by a solid sphere is presented. There are many occurrences of particles at an interface in the domain of microfluidic systems; for example, the understanding of interaction of solid spheres and interfaces is essential in self-assembly applications [7].

Figure 9.1 Example of PMMA (polymethyl methacrylate) particles stuck on the surface of an oil droplet (image inspired by the Manoharan Lab at Harvard).

9.2 Neumann’s Construction ...