AC electroosmotic generated in-plane microvortices for stationary or continuous fluid mixing

This paper proposes a novel design of the patterned AC electroosmotic flows (AC-EOF) by various electrode configurations to locally and simultaneously generate in-plane microvortices of clockwise, counter-clockwise, or pair rotation in a microchannel for micromixing applications. The device was operated under the application of an AC signal with a voltage and frequency below 20 Vp–p and 10 kHz, respectively, for all cases. Microparticle image velocimetry (μPIV) was used to characterize the flow fields of the generated in-plane microvortices. The sizes of the active regions and the magnitudes of the rotational strength of microvortices were found to increase with the increase of the applied voltages and an optimal value was achieved at an appropriately applied frequency. By employing the microvortices, AC-EOF micromixers were successfully carried out for either rapid local mixing of two stationary fluids with high viscosity ratio or continuous mixing of two flowing fluids introduced into a Y-shaped microchannel of 400 μm wide. The mixing times for the stationary fluids with viscosity ratio of 450 and the continuously flowing fluids were 2 min and 0.86 s, respectively, which are 15-fold and 30-fold faster than that of mixing directly by diffusion. The AC-EOF micromixer provides a great mixing enhancement with a shorter mixing length for the application of flows in low Peclet numbers (Pe < 3 × 103). The AC-EOF micromixer can be employed locally and simultaneously for either stationary or continuous flows in microchannels, and integrated into a lab-on-a-chip system.