Basic Principles of Microfluidics

Microfluidics is a rapidly evolving multidisciplinary field that relies on the manipulation of minute volumes of fluids in submillimeter-sized channels with application in chemistry, biotechnology, and health, among other relevant areas. At this scale, a paradigm shift takes place, as phenomena that are unnoticed at the macroscale emerge, since viscous, capillary, and surface tension effects tend to overcome gravity and inertial effects. Therefore, flow behavior often contradicts our macroscopic intuition, yet may easily become more predictable and thus controllable. This chapter focuses on the physics aspects underlying microfluidics. Thus, the basics of fluid mechanics are introduced and the governing equations for fluid flow are presented. Mass transfer and in particular diffusion and diffusion advection aspect are introduced and discussed and complemented with a brief incursion in the strategies for mixing at the microscale, developed to overcome the limitations of diffusion-based processes. Surface tension and capillarity effects are described and their significance in microfluidics is elucidated, also considering their impact in droplets/bubbles formation and multiphase flow.