8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P7.66Thu 811:10-14:00Understanding the hydrophobic nature of nano-ruggedsolid surfaces at the molecular scaleFrédéric Leroy 11 Technische Universität Darmstadt, Petersenstrasse 22, 64287, Darmstadt, GermanyWe aim to contribute to the understanding at a molecular level of the origin of the hydrophobicnature of surfaces exhibiting roughness at the nanometer scale. We present molecular dynamicscalculations of the surface free energy of water in contact with graphite-based smooth and modelsurfaces whose roughness dimension stretches from a few Angstroms to a few nanometers. BothCassie and Wenzel wetting states (suspended/collapsed wetting states) are studied. All the ruggedsurfaces are observed to yield higher surface free energy than the perfectly smooth one. The surfacefree energy of Cassie states is predicted from a Cassie-Baxter like equation. The origin ofthe hydrophobic nature of surfaces yielding Cassie states is therefore the reduction of the numberof 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 tothe roughness pattern at the nanometer scale. While graphite is found to be slightly hydrophilic,Wenzel states are dominated by an unfavorable effect arising from the hydrogen-bonding networkperturbation that overcomes the favorable enthalpic effect induced by the surface roughness. Weshow that the solid-liquid surface free energy of Wenzel states linearly varies with the roughnesscontour length per unit area.66