Mixing in Microscale

Mixing in microscale is one of the key technologies for miniaturized analysis systems. Miniaturization brings advantages to a wide range of traditional fields such as chemical industry, pharmaceutical industry, analytical chemistry, biochemical analysis, and high-throughput synthesis. Due to the dominant surface effects, miniaturization also brings in challenges that do not occur in macroscale. For instance, turbulence used in macroscale to improve mixing is not possible in microscale. The laminar, deterministic flow caused by the dominant viscous effects makes turbulence impractical in microscale. Molecular diffusion may be improved by the shorter mixing path. However, the higher flow velocity leads to dominant advective effects and consequently the longer residence time and the longer mixing channel. These challenges will be discussed later with the help of nondimensional numbers. Looking at the evolution of micromixer designs in the last decade, it is apparent that a number of design techniques has been employed to improve mixing. This chapter will review these several concepts – from diffusion-based micromixers in the early stage to the recently reported micromixers based on chaotic advection. In general, this chapter classifies micromixers into two types: passive and active.

Passive micromixers do not need moving parts and actuators, the mixing concept is only based on molecular diffusion as well as chaotic advection. Thus, the two major subgroups of the passive concept are micromixers based on molecular diffusion and micromixers based on chaotic advection. For mixing of large molecules with small diffusion coefficients, chaotic advection is the more favorable concept. Mixing concepts based on molecular diffusion can be further categorized based on the arrangement of the mixed phases as parallel lamination, serial lamination, serial segmentation, and injection.

Active micromixers require external disturbance to improve mixing. Based on the source of the disturbance, active mixing is categorized here as pressure-driven, temperature-induced, electrohydrodynamic, dielectrophoretic, electrokinetic, magnetohydrodynamic, and acoustic concepts.