Dresden 2011 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 13: Solid / liquid interfaces I

O 13.4: Vortrag

Montag, 14. März 2011, 15:45–16:00, WIL B321

Understanding the hydrophobic nature of nano-rugged solid surfaces at the molecular scale — •Frédéric Leroy — Eduard-Zintl-Institut für Anorganische und Physikalische Chemie and Center for Smart Interfaces, TU-Darmstadt, Darmstadt, Germany

We aim to contribute to the understanding at a molecular level of the origin of the hydrophobic nature of surfaces exhibiting roughness at the nanometer scale. We present molecular dynamics calculations of the surface free energy of water in contact with graphite-based smooth and model surfaces whose roughness dimension stretches from a few Angstroms to a few nanometers.

Both Cassie and Wenzel wetting states are studied. All the rugged surfaces are observed to yield higher surface free energy than the perfectly smooth one.

The surface free energy of Cassie states is predicted from a Cassie-Baxter like equation. The origin of the hydrophobic nature of surfaces yielding Cassie states is therefore the reduction of the number of interactions between water and the solid surface having atomic defects.

On the contrary, Wenzel's theory fails to predict the variation of the solid-liquid surface free energy with respect to the roughness pattern. While graphite is found to be slightly hydrophilic, Wenzel states are dominated by an unfavorable effect arising from the hydrogen-bonding network perturbation that overcomes the favorable enthalpic effect induced by the surface roughness.