A numerical study of 3D electrokinetic flows through micromixers has been performed. The micromixers considered here consist of heterogeneous rectangular microchannels with prescribed patterns of zeta-potential on their walls. Numerical simulation of electroosmotic flows within heterogeneous channels requires the solution of the Navier-Stokes, Ernest-Plank and species concentration equations. As expected, a 3D solution of these equations is computationally very intensive. Therefore, the well known Helmholtz-Smoluchowski model is often used in the numerical simulation of electroosmotic flows. According to 2D studies on electrokinetic mixing inside heterogeneous channels, the existence of vortices within the flow field always increase the mixing performance. Hence, it may be expected that similar observations pertain to mixing in 3D flows as well. However, investigations on 3D micromixers identified situations in which the existence of vortices has little or no significant benefit to the mixing performance. Findings of the present work indicate that the degree of flow asymmetry is a key parameter for the mixing performance. Since 3D flows are more capable of developing asymmetrical flow patterns, they are expected to have better mixing performance as compared to their 2D counterparts. The results presented here for different 3D cases show that mixing performance can be improved significantly depending on the alignment of the vortex plane relative to the mixing interface of the fluids. These observations confirm that 2D simulations of mixing cannot fully explain the behavior of passive micromixers.

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