Basic Microfluidics Theory
Flow in microsystems behaves very different than flow on the macroscale, i.e., the flow we are used to in our everyday life. The most obvious difference is that the chaotic turbulent flow we most often observe, e.g., rivers flowing or tap water running does not appear on the microscale. Here, the flow is more smooth and most often what we call laminar flow. Other parameters considered important on the macroscale such as inertia are insignificant on the microscale, whereas viscosity becomes extremely important. Diffusion, which on large scale is a hopeless parameter to use for transport, becomes significant on microscale. The surface of your system has to be considered more carefully as the surface to volume ratio (S/V) increases dramatically as you downscale your system. Take for example a cubic macrosystem with sides of 1 m, here S/V = 6 m−1, whereas for a system with sides of 1 μm the V/S = 600,000 m−1, which is a huge difference and has a large impact on flow behavior. In this chapter the basic microfluidic theory will be presented, enabling the reader to gain a comprehensive understanding of how liquids behave at the microscale, enough to be able to engage in design of micro systems and to support the theory used in other chapters in the book, but without going into the deep underlying theoretical approach.