This chapter is dedicated to the study of droplets and plugs (large droplets contacting the walls) in the geometry of microsystems, i.e. in microchannels and micro-chambers. In micro-wells, the droplet is at rest, and a static approach can be performed. On the other hand, in microchannels, fluid velocities are most of the time small; the capillary number is then much smaller than unity and a quasi-static approach is valid. However, we will discuss when needed the role of the drag force on the droplet. We shall assume gravity is negligible.
In the first part, we focus on the behavior of liquids in micro-wells by analyzing the shape of the surface, which we show depends on evaporation; in the second part, we deal with the shapes and arrangements of droplets in microchannels. In microchannel geometry, we will call the “dispersed” phase the droplets or plugs and the “continuous” phase the carrier fluid.
Micro-wells are largely used in biotechnology as containers for performing chemical and biochemical reactions. Droplets are usually delivered in the wells by hand or by robots (fig. 6.1). In this section, the behavior of liquids in micro-wells is investigated depending on the wetting properties of the walls.