Enhanced electro-osmotic microfluidic mixing in an obstacle-embedded converging–diverging channel

Abstract

This work presents an electro-osmotic micromixer that contains an obstacle-embedded converging-diverging channel for the purpose of enhancing fluid mixing at the microscale. Based on the finite element method, a two-dimensional transient model for Alternating current (AC) electro-osmosis and a three-dimensional model for Direct current (DC) electro-osmosis were developed and solved using COMSOL Multiphysics. In this paper, several geometric parameters of the micromixer are systematically studied; these include the distance between inlets, vertical spacing between obstacles, width of the connective channel in converging-diverging configuration, and obstacle angle. Several scenarios were compared to find out the best geometry. It follows from the observations made that the obstacles' vertical spacing and angles significantly affect the overall mixing efficiency, and appropriate values may remarkably enhance the performance. After optimizing the geometrical parameters, the performance of mixing was furthermore investigated for various inlet velocities, applied AC and DC voltages, and frequency. These results indicate that during the early stage of the process (up to 1 s), an inlet velocity of 3 mm/s exhibits superior performance; however, at later times, the lower inlet velocity of 0.5 mm/s becomes dominant due to the stronger development of electro-osmotic effects. Applying a voltage of 16 V yields an average mixing index (MI) of approximately 91%, representing improvements of about 37% and 17% compared to 0.5 V and 4 V, respectively. Moreover, a frequency of 16 Hz provides optimal performance with a maximum average MI of 78%, outperforming 4 Hz (71.2%) and 32 Hz (68%).