Nonlinear phenomena in induced charge electroosmosis
Induced charge electroosmosis (ICEO) refers to production of electroosmotic slip by way of induced charges. Unlike fixed-charge-zeta potentials, the induced zeta potentials are proportional to the applied electric field strength which can be very strong in microfluidic devices. As a result, the induced zeta potentials are generally much higher than the thermal voltage (ζ > kT/ze). The linear theory of electrokinetics which is derived under the Debye-Huckel limit (ζ << kT/ze) breaks down for such large induced zeta potentials and predicts unrealistically high magnitudes of ICEO slip velocities. Moreover, many flow characteristics observed in experiments can not be explained by the linear theory. A vast discrepancy between the linear theory and the experiments creates the need for an investigation of the effects which take place at high zeta potentials (called nonlinear effects). This dissertation investigates some of these nonlinear effects by the means of experiments and numerical simulations. An effort has been made to reduce the discrepancy between the theory and the experiments and to explain the previously unexplained experimental flow characteristics.
Induced charge electroosmotic flow was produced on a planar microelectrode with an AC electric field. The experimental velocity was found to be 2 orders of magnitude lower than the predictions of the linear theory. It was also found that the slip velocity saturates at high applied voltages, a feature not predicted by the linear theory.
A nonlinear electrokinetic model was formulated with the intent of explaining the experiments. The nonlinear model is more advanced than the linear model. It solves for the surface conduction of ions through the diffuse layer and also models the double layer as a nonlinear capacitor which requires exponentially large amounts of charge to get charged. Surface conduction refers to excess ionic currents through the diffuse layer. We show that surface conduction through a nanoscale diffuse layer can cause micron scale gradients in the bulk electric field and cause severe reduction in the tangential electric field. We show that these nonlinear effects deteriorate the slip velocity. We are able to reduce the discrepancy between the theory and the experiments by one order of magnitude.
Finally ICED flow is produced on a rough surface with nanoscale roughness. We demonstrate that the roughness of a surface can have dramatic effects on the flow velocity. These effects are explained with the help of fundamental aspects of surface conduction.