Flow Patterns and Electrokinetic Mixing Performance in Heterogeneous Microchannels

Due to the recent advances in microfabrication techniques, it is possible to produce microchannels with positive, negative, or even neutral surface charges. According to several numerical and experimental investigations, such a combination of charge patterns on the microchannel walls results in complex flow fields with circulation zones that are highly desirable for fluid mixing requirements as in lab-on-a-chip devices. In this paper, the mixing efficiency associated with electro-osmotic flows in heterogeneous microchannels is investigated. The Navier-Stokes equations are solved for the flow field along with species transport equations to obtain the concentration field. The effects of the Electric Double Layer (EDL) on the flow field are considered using the Helmholtz-Smoluchowski model in which the EDL effects on the fluid adjacent to the walls are replaced by velocity slip at walls. Different configurations and profiles for the wall charges can be applied to the microchannel walls. In the present study, heterogeneous patterns consisting of different patches with constant zeta-potentials are considered. The flow pattern of a single patch consists of a single vortex attached to the channel wall, which significantly increases the mixing performance. It is expected that a combination of several patches would increase the mixing performance considerably. Therefore, the effects of the size, number, and locations of multiple patches on the mixing performance are investigated in detail. The results for a single patch indicate that the mixing efficiency increases with the size of the patch and its proximity to the microchannel inlet. It is expected that with a suitable combination of patches, an optimized configuration can be found in which the mixing efficiency is maximized and the length of the mixing section is minimized. The results can be applied to the design of micro-mixers to minimize their size while achieving the desired mixing requirements.