A comparative discussion of different designs of passive micromixers: specific sensitivities of mixing efficiency on Reynolds numbers and fluid properties

Fluid mixing enhancement is an important topic of microfluidics and microreactor technologies. Though many micromixers based on different mechanisms have been reported so far, most investigations were conducted case by case due to the diversity of applications. For a more comprehensive understanding of the problem, this paper presents a comparative discussion of five different passive micromixers, including a planar spiral mixer, a planar split-and-recombine (SAR) mixer, a 3D serpentine mixer, a 3D SAR mixer, and a 3D mixer with divergent geometries. The involved mixing mechanisms include Dean flow/secondary flow, SAR process and viscous interfacial instabilities. From a geometric perspective, this study compares (i) planar and fully 3D structures, (ii) single channels whose cross section preserve connectedness and interconnected multiple sub-channels, (iii) microchannels with and without divergent structures. Key characteristics of the designs are highlighted. The influence of Reynolds number, fluid viscosity and viscosity ratio are also discussed. As the discussed micromixers cover a wide range of fluid mixing conditions, it also serves as a mini-review, providing useful reference on the selection of optimum design for different applications.