Toward High Net Velocities in AC Electroosmotic Micropumps Based on Asymmetric Coplanar Electrodes

Abstract

The most frequently studied ac electroosmotic micropumps exploit the coplanar asymmetric arrangements of the forcing electrodes. We analyze these systems by means of the following two mathematical models: 1) the classical slip model, which is based on a capacitor-resistor representation of the spatial domain, and 2) the nonslip model, which is based on the Poisson-Navier-Stokes-Nernst-Planck approach to the entire domain, including the electric double layers. Both the models predict similar results in many low-amplitude regimes. However, the nonslip model gives us a much better insight on the high-amplitude (nonlinear) behavior of the micropumps. Our most important findings obtained by the nonslip model can be summarized as follows: 1) There are optimal values of the electrode and gap size ratios that are generally different from those obtained by the slip model; 2) the micropump performance is relatively insensitive with respect to the electrode size ratio; 3) there is an optimal vertical confinement that enables us to attain high net velocities; 4) flow reversals on frequency, amplitude, and certain geometry characteristics are observed; 5) the energy efficiency of these pumps is very low; and 6) the Joule heating effect is negligible. The nonslip model characteristics are also discussed to explain the observed differences between predictions of the models. Convergence analysis dealing with the precision of numerical results obtained by the nonslip model is presented.