Transverse diffusion in microfluidic systems

Abstract

This chapter discusses the importance of diffusion in micro devices, most specifically diffusion perpendicular to the direction of flow of fluid. The transport of molecules in fluids in the absence of flow is carried out exclusively by diffusion, which is the random motion of all particles driven by thermal excitation. The negative side of diffusion in microfluidics is that it is generally nontrivial to fully mix two solutions in systems operating at low Reynolds numbers. There are different types of mixtures, namely, the “Manz mixer”, the “T-shaped mixer”, “The flip turn device,” and “Chaotic adjective mixers.” The T-shape mixer is the simplest fluidic device for mixing two solutions. The Manz mixer has low dispersion for use with pulsatile or bolus samples. Flip-turn mixers revert the fluid to the original side-by-side configuration. The “chaotic adjective mixers” allow mixing of simple fluids to occur in rectangular channels when operated at Reynolds numbers between 1 and 100. The power and versatility of microfluidics is directly attributable to its operating in a size range where diffusion is a dominant means of transport. Microfluidic devices generally operate in the presence of flow, so diffusional transport is complemented by convectional transport. However, at low Reynolds number conditions, the absence of turbulence both simplifies the transport and slows it substantially; transport perpendicular to flow relies entirely on diffusion.