Stationary Electro-osmotic Flow Driven by ac Fields around Insulators

Abstract

Electric fields are commonly used for manipulating particles and liquids in microfluidic systems. In this work, we report stationary electro-osmotic flow vortices around dielectric micropillars induced by ac electric fields in electrolytes. The flow characteristics are theoretically predicted based on the well-known phenomena of surface conductance and concentration polarization around a charged object. The stationary flows arise from two distinct contributions working together: an oscillating nonuniform zeta potential induced around the pillar and a rectified electric field induced by the ion concentration gradients. We refer to this fluid flow as concentration-polarization electro-osmosis (CPEO). We present experimental data in support of the theoretical predictions. The magnitude and frequency dependence of the electro-osmotic velocity are in agreement with the theoretical estimates and are significantly different from predictions based on the standard theory for induced-charge electro-osmosis, which has previously been postulated as the origin of the stationary flow around dielectric objects. In addition to furthering our understanding of the influence of ac fields on fluid flows, we anticipate that this work will also expand the use of ac fields for flow control in microfluidic systems.