Berlin 2012 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 36: Wetting, Micro and nanofluidics I

CPP 36.10: Talk

Thursday, March 29, 2012, 17:45–18:00, C 264

The influence of nano-roughness on solid-liquid surface free energies. A molecular dynamics simulations contribution. — •Frédéric Leroy — TU-Darmstadt, Darmstadt, Germany

We address the question of the determination of the surface free energies of rugged solid-liquid interfaces. We aim to describe interfaces whose roughness dimension is of the order of 1-10 nanometers. Although such surfaces are ubiquitous, the way nanostructures influence the solid-liquid surface free energy remains largely unexplored. This is mainly due to the impossibility to carry out direct measurements of this quantity. Its estimation relies on droplets contact angles measurements and on the use of equations to connect surface free energies and contact angles. There is experimental evidence that this task is not always straightforward to be performed. It is also legitimate to wonder whether continuum equations have the ability to describe situations where molecular details may matter.

We introduce a thermodynamic integration algorithm to determine the surface free energy of smooth and rugged interfacial systems. We perform molecular dynamics calculations dealing with water in collapsed wetting states on model rugged graphite-based surfaces simultaneously exhibiting roughness and chemical heterogeneity. We show that the related continuum theory is able to predict the nanometer scale results under the condition that water does not experience too strong confinement. Our results also evidences that the origin of surface free energy, at least in the case of water on non-polar attractive substrates, is to be found in the very few first layers of adsorbed water.