Effect of ionic concentration on electrokinetic instability in a cross-shaped microchannel

Abstract

This paper performs numerical and experimental investigations into electrokinetic instability (EKI) effects to accomplish mixing of multiple solutions with different electric conductivities in a cross-shaped microchannel. This study considers two multiple-species, namely two aqueous electrolyte solutions and three electrolyte solutions with conductivity ratios ranging between 1 and 10, respectively. A stratified flow condition is formed when the intensity of the applied DC electrical field is below a certain threshold value. However, as the intensity increased, various EKI phenomena are induced, including a series of flow recirculations at the interfaces of neighboring species flows, a string of pearl-like flow structures aligned with the low-conductivity species stream, and a wavy perturbation of the species interfaces. The EKI phenomena are clarified in terms of the respective axial velocities and specie flow pressure gradients. In practice, the nature of the EKI effect depends upon the relative directions of the conductivity gradients within the microchannel. Analyzing the EKI phenomena effects in mixing multiple-species, it is found that the mixing performance obtained when the conductivity gradients are orientated in opposing directions is higher than that achieved when the conductivity gradients are aligned. Furthermore, the optimal mixing index is achieved when the conductivity gradients are directed away from one another (i.e. from the center of the microchannel toward the microchannel walls) rather than toward one another (i.e. from the microchannel walls toward the center of the microchannel).