Active microfluidic mixing based on transverse electro-osmotic flows*

As with their macroscale counterparts, laminar fluid mixing becomes a very important, albeit inherently difficult step at the microscale. Micromixers based on electro-osmotic flow (EOF) rely on either a modification of microchannel geometries or a modification of the $\zeta $-potential of the microchannel surfaces to enhance fluid mixing. Here we present a new method of achieving chaotic advection in microchannels by applying an electric field perpendicular to the mean flow direction driven by a pressure gradient in a planar rectangular microchannel. EOF on microchannel surfaces in a direction orthogonal to the main channel axis is generated via an electric field produced by integrated electrodes at the corners of a microchannel. By using serial combinations of different mixing cycles, we show that complete mixing can occur in straight microchannels of length scales on the order of a millimeter. Computational fluid dynamics (CFD) is used to characterize and optimize the mixing efficiency of the system and to compare with experimental measurements.