Electro-osmotic flow enhancement over superhydrophobic surfaces

Electro-osmotic flow is a well-established and efficient method for driving microchannel flows that relies on the interaction of an externally applied electric field with charge arising at the interface between the liquid and the channel walls. However, its relatively low velocities together with its dependence on the pH of the liquid severely limit its utility. Here we experimentally demonstrate fast electro-osmotic flow over microstructured superhydrophobic surfaces. By suspending the electrolyte in a Cassie-Baxter state over hierarchical surfaces, we create stable gas-liquid interfaces on which we induce charge through a gate electrode. We provide a detailed investigation and characterization of the electro-osmotic velocity as a function of the surface geometry by utilizing particle tracking velocimetry in a microfluidic device and show that the resulting electro-osmotic velocity scales with the ratio of slip length to double-layer thickness. Compared to no-slip surfaces, we demonstrate an order of magnitude enhancement in velocity and complete pH independence, enabling wider utility of electro-osmotic flow in manipulation of microscale flows.