The Physics of Microdroplets

Chapter 3

Droplets: Shape, Surface and Volume

3.1 Abstract

Microfluidics is a science dedicated not only to the study of continuous microflows in microchannels but also to the study of micro-drops. As a matter of fact, many applications of microfluidics in biotechnology use droplets. This is the case in planar microfluidics or digital microfluidics, where discrete droplets are moved individually on a locally planar surface [1,2], and in droplet microfluidics, where droplets are produced in microdevices such as T-junctions or flow focusing devices (FFDs) to form capsules or emulsions [2–6]. On the other hand, individual droplets might be dispensed from a robot tip by electric means [7,8]. For biological purposes, droplets provide a small, bounded environment for the study of cells (fig. 3.1). The behavior of micro-droplets depends on their surface energy and volume. This is the reason why this chapter is devoted to the determination of the shape, surface area and volume of static droplets.

Figure 3.1 Cells contained in droplets (deposited on a substrate by means of ECC), reprinted with permission from [8], ©NSTI, 2003).

3.2 The Shape of Micro-drops

3.2.1 Sessile Droplets – the Bond Number

Large droplets deposited on horizontal surfaces have a flattened shape, whereas small droplets have a spherical shape (fig. 3.2). This observation stems from a balance between gravity and surface ...