Research Topic

Modeling of flow over superhydrophobic surfaces under the influence of electric fields

Superhydrophobic Surfaces

Fig. 1: Schematic comparison of a flow over a solid wall and a fluid boundary. Together, they make up the movement over a hydrophobic surface.

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

Fig. 2: Electroosmosis at a superhydrophobic surface

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

Fig. 3: Calculated velocity distribution

We are developing a possibility of nevertheless enhancing electroosmotic flow by using superhydrophobic surfaces.

By setting up a theoretical model of the surface-fluid interaction and performing numerical calculations, a particular setup is designed, which is able to take advantage of the slip provided by the superhydrophobic surface. Taking into account the electric as well as mechanical properties of all participating substances, limitations of commonly employed configurations are explored, and an optimized setup is developed. With this design, it should be possible to achieve a considerable amplification of the electroosmotic velocity, which can be an order of magnitude higher than in a standard setup.

Key Research Area

Multi-Physics – Special Coupled Systems: Fluid- and Electrodynamics

Contact

Clarissa Schönecker
Dipl.-Ing.

 

L1|08 116

Petersenstr. 32
D-64287 Darmstadt

 

Tel.: +49(0)6151/16-2839
Fax: +49(0)6151/16-72021

steffes(at)csi.tu-...

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