Investigation of mixing efficiency in electroosmotic flow with non-uniform wall Zeta potential

In this paper, the mixing efficiency in electroosmotic flow inside a micromixer is simulated numerically for different states of non-uniform wall Zeta potential. The geometry of flow is a two-dimensional channel between two parallel plates and the flow is assumed to be incompressible, steady and laminar. The governing equations, including a Laplace equation for the distribution of external electric potential, a Poisson equation for the distribution of electric double layer potential, the Nernst-Planck equation for the distribution of ions concentration, the species convection-diffusion equation, the modified Navier-Stokes equations for the fluid flow field, have been solved using the finite volume numerical method. In order to validate the numerical results, the analytical results of an ideal electroosmotic flow in where throughout the walls are charged is compared with the obtained numerical results. The numerical results show that, by linear-ascending, linear-descending and parabolic changes of the wall Zeta potential at the middle length of the microchannel, the mixing efficiency increases compared to a constant Zeta potential. For the cases of linear changing of Zeta potential, the mixing efficiency increases to 86% and for parabolic change of Zeta potential the mixing efficiency increases to 75%, while the Zeta potential is constant at middle length the maximum of mixing efficiency increases to 64%. In the case that only the upper wall at middle length is charged, the results show that a vortex region is created in the flow. This vortex region causes a maximum (100%) mixing efficiency.