Enhanced Electrokinetic Micromixing by Optimal Patterning of Heterogeneous Surface Charge

Microfluidic devices require efficient micromixing for fast analysis in biological applications such as enzyme assays, biochemical reaction and detection etc. For electrokinetic micromixers, the performance of a homogeneous T-mixer can be enhanced significantly by addition of heterogeneous surface charge on sidewalls and/or bottom of the microchannel. Previous studies have shown that localized circulations or non-axial flow induced due to presence of heterogeneity augment micromixing performance. However, the effect of heterogeneous charge patterns on mixing performance is not systematically studied. In this computational study, a binary numerical optimization problem is formulated to achieve best mixing performance by identifying the optimal heterogeneous charge pattern. With the assumption of thin electric double layer (EDL), electrokinetic flow is modeled using Helmholtz-Smoluchowski slip condition within a three dimensional finite-element model. The resulting optimal design generates the most favorable transverse flow structure to provide optimal mixing performance. Various other pattern configurations (staggered, herringbone etc.) are examined for a range of operating conditions. The optimal design is found to be superior for all operating conditions with over three-fold improvement in mixing performance with respect to homogeneous T-mixer at higher electric fields (200-400 V/cm).