Optimization of a diverging micromixer driven by periodic electroosmotics

Abstract

In this study, we investigate the mixing performance of a diverging microchannel driven by periodic electroosmotics. Applying two square-wave electric fields to the two inlets, the influences of the half-angle of the diverging section and the phase difference of the two actuating voltages are explored using different voltage configurations and superimposed pressure heads. Based on the analysis and flow visualization results, we find that two mixing mechanisms are present in the diverging micromixer driven by periodic electroosmotics: flow instability and fluid lamination. Furthermore, both the alternation of electric field and conductivity gradient present in the fluid may contribute to the onset of flow instability. On the other hand, mixing is enhanced by fluid lamination when the actuating voltages are not in-phase. For ź ź 0, fluid tends to flow first from one inlet directly into the other before moving to the opposite side of the diverging section in the following cycle to form fluid striations. As the phase difference increases, mixing is improved due to a longer active duration of this mechanism. We find that the mixing performance always peaks at a phase difference of 0.75ź, and a diverging micromixer with a half-angle larger than 35° is preferred. Comparing to the unipolar actuation, bipolar actuation strengthens fluid lamellae and hence leads to higher mixing efficiency. Increasing the superimposed pressure head, on the other hand, weakens molecular diffusion and deteriorates mixing performance drastically.