Electroosmotic Flow on Superhydrophobic Surfaces
Fast and effective transport of liquids through microchannels presents an important challenge in the development of new microfluidic devices. In this context, the natural no-slip condition at the wall has a dominating retaining effect on the fluid. In order to overcome this impediment, superhydrophobic surfaces possess promising properties, as they are able to provide an apparent slip at the wall. Appropriate surfaces have a very fine roughness, so that a liquid won't wet the entire surface, but air will be entrapped in the gaps. If the liquid resides in such a Cassie state above the wall cavities, the air offers a movable boundary to the fluid (Fig. 1).
Application to Electroosmosis
However, such surfaces do not enhance electroosmotic flow. This is due to the fact, that superhydrophobic surfaces on the one hand enhance the flow along the wall, but on the other hand also reduce the driving force because of zero net charge at the liquid-gas interface. Electroosmotic flows rely on the formation of charge layers, which occur at the interface between two contacting substances. These charges and subsequently the remaining fluid can be set in motion by application of an outer electric field. Yet on the movable air boundary, charges can also travel in the opposite direction, which generally just cancels out the driving force towards the intended direction (Fig. 2). Even for a perfectly slipping liquid-gas interface no gain is to be expected compared to a plain wall.
Enabling Electroosmotic Flow on Superhydrophobic Surfaces
In order to nevertheless make an enhancement of electroosmotic flow possible, the concept is to integrate electrodes within the structure of the superhydrophobic surface. These electrodes will draw ions from the bulk of the liquid towards the interface, where they accumulate until the electric field of the electrodes is shielded. As the fluidic interface is effectively charged now, the driving force of electroosmotic flow is increased, which leads to a much faster flow velocity
A computational model was set up, which determines the amount of charge at the interface and its subsequent effect on the acceleration of the flow. Depending on the employed materials, amplifications of the electroosmotic velocity of one order of magnitude are predicted.
C. Steffes, T. Baier and S. Hardt, “Enabling the Enhancement of Electroosmotic Flow over Superhydrophobic Surfaces by Induced Charges”, Colloids and Surfaces A: Physicochemical and Engineering Aspects, In Press, Abstract