Sebastian Dehe


Electroosmotic flow over superhydrophobic surfaces; Electrohydrodynamic instabilities at liquid-liquid interfaces


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fax +49 6151 16-24278

Work L2/06 105
Alarich-Weiss-Str. 10
64287 Darmstadt

The most prominent example of a superhydrophobic surface is the Lotus-leaf, from which water droplets roll-off easily. Artificial superhydrophobic surfaces can be created by patterning surfaces with microstructures, such as pillars. If a liquid rests on top of the structures without replacing the gas in between, less friction acts on the liquid, which can be utilized for drag-reduction applications in pressure-driven flow.

In this project, the response of a superhydrophobic surface to externally applied electric fields is studied. In particular, the wetting state stability of a dual-scale roughness is investigated by microscopical methods, revealing the existence of a range of intermediate wetting states. Also, the electro-osmotic flow over a superhydrophobic surface is characterized. When additional gate electrodes are embedded beneath the surface, an order-of-magnitude increase of the resulting flow rate compared to flat surfaces can be achieved. In addition, the species transport in complex flow fields induced by electro-osmosis is studied, with a focus on sample dispersion.

Experimental methods utilized in this project include soft-lithography, bright-field microscopy and contact angle measurements for the surface production and characterization, as well as fluorescence-microscopy in combination with particle tracking velocimetry for the flow characterization. The governing equations are obtained by analytical methods, and numerical modeling using FEM (Comsol Multiphysics) is used for computing flow fields and concentration distributions.

Electrohydrodynamic instabilities at liquid-liquid interfaces

Liquid-liquid interfaces deform under the actuation by electric fields, and disintegrate at sufficiently high field strengths. The most prominent example is the so-called Taylor cone, where an interface takes a conical shape and ejects a fine jet or small droplets from the tip of the cone. In addition, other electrically triggered instabilities exist, such as the electric analogue of the Faraday instability.

In this project, the response of a liquid-liquid interface is studied versus spatially inhomogeneous DC-fields, as well as spatially homogeneous AC fields. For the spatially inhomogeneous DC-field, the existence of an alternative deformation mode, coexisting with the classical Taylor-cone mode is shown and attributed to the viscous interaction of the emitted charged droplets with the background fluid. For the spatially homogeneous AC fields, the critical field strength as well as the resulting wave-length of the electrically driven Faraday-instability is studied.

In this project, high-speed imaging of the interfacial instabilities is used in conjunction with image processing and electrical current measurements on the nA-scale to characterize the instabilities. The experimental observations are reproduced by numerical simulations in Comsol Multiphysics to verify the driving mechanism. The wavelengths of the Faraday-instability are measured using a refraction-based technique, which allows the reconstruction of the interface.