2.1 Introduction

In Chapter 1, we dealt with microflows. Another aspect of microfluidics is the physics of microdrops. Microdrops are a common feature in biotechnology. For example, DNA microarrays are comprised of hundreds to thousands of microwells, each containing a drop of biologic liquid, and a drop dispenser is used to deposit the liquid in each well (Figure 2.1). In such a case, it is essential to understand how the drop relocates in a well.

Figure 2.1: (Left) DNA microarray made by SCIENION; and (right) the principle of microdispensing of liquid into wells of a microplate.

Another relatively new concept in microfluidics is the use of microdrops to perform the same bioanalysis and biorecognition operations as classical microflows devices do. There are some arguments about whether it is better to use microflows or microdrops in microsystems for biotechnology. The choice is complex because it depends on many factors.

First, microflows are more commonly used, better known, and more familiar to the developers. Micropumps and microvalves are now available, and microchannel etching in silicon, plastic, or glass is now a standard technique. Second, microdrop behavior is often complex and puzzling as we shall see in this chapter. However, microdrops have the big advantage of minimizing the surface between the liquid and the solid walls. In biotechnology, it is a real advantage since "nonspecific" adsorptions contact and adherence of the target particles on solid walls at unwanted places in the microsystem are a constant drawback. If these particles are marked with a fluorescent marker, they...