The non-monotonic overlapping EDL-induced electroviscous effect with surface charge-dependent slip and its size dependence

On the basis of the analytical solutions of electrical potential and ion distributions within the surface charge-induced overlapping electric double layer (EDL), this present work studies theoretically the overlapping EDL induced-electroviscous effect (EVE) on the pressure-driven flow in a parallel-plate microchannel with both the no-slip condition and the surface charge-dependent slip condition. The results show that the overlapping EDL induced-EVE shows a non-monotonic trend from enhancement to reduction with the gradually increasing absolute value of zeta potential. When the absolute value of zeta potential is large enough, the EVE and the relevant fluid drag will finally show a decreasing trend with the increasing absolute value of zeta potential. In addition and more interesting, the non-monotonicity of overlapping EDL induced-EVE shows a characteristic of size dependence. The critical zeta potential beginning to exhibit the decreasing trend of EVE is related to the size of the microchannel. The overlapping EDL-induced EVE in a microchannel with a smaller height needs a larger critical zeta potential to exhibit the decreasing trend. The mechanisms of these phenomena are analyzed. The subject of the paper is the theoretical results and the corresponding analysis.