Enabling the enhancement of electroosmotic flow over superhydrophobic surfaces by induced charges

Abstract

Fast and effective transport of liquids through microchannels presents an important challenge in the development of novel microfluidic devices. In this context, the natural no-slip condition at the wall has a dominating retaining effect on the fluid. Superhydrophobic surfaces possess promising properties to overcome this impediment, as they are able to provide an apparent slip at the wall. If the liquid resides in a Cassie state above the wall cavities, the gas filling the gaps offers a movable boundary to the fluid. However, such surfaces do not enhance electroosmotic flow due to a lack of net charge at the liquid–gas interfaces, which consequently reduces the driving force. We present a possibility of enhancing electroosmotic flow over superhydrophobic surfaces. By setting up a theoretical model of the surface–fluid interaction and performing numerical calculations, a particular setup is proposed that utilizes the apparent slip of a superhydrophobic surface to increase the electroosmotic flow velocity. Firstly, the driving force is magnified by employing a special electrode configuration which increases the ion concentration at the liquid–gas interface and thus leads to an increased flow rate. Furthermore, by analyzing the effects of material properties, limitations of the common air-filled cavities are pointed out, and an optimized setup utilizing non-standard materials is proposed. With such an optimized design, considerable amplifications of the electroosmotic velocity are predicted that can reach a factor of 10.