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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 20: POSTER: Biological Systems + New Materials

CPP 20.5: Poster

Mittwoch, 28. März 2007, 16:00–18:30, Poster B

Adhesion and wetting: from monolayer to capillary bridges — •Ralf Kaminke and Klaus Mecke — Friedrich-Alexander-Universität Erlangen-Nürnberg, Staudtstr. 7, 91058 Erlangen

Nanomechanical AFM measurements of adhesion forces revealed a strong dependence on humidity for Geckos spatula on various hydrophobic substrates [G. Huber etal, PNAS 102, 16293 (2005)]. Whereas capillary condensed water bridges are unlikely to form between hydrophobic substrates, adsorbed particles may build molecular bridges which seems to be sufficient to explain the increase of adhesion with humidity - even at sub-monolayer coverage. Adsorption isotherms as well as adhesion forces changes qualitatively at the crossover from nanometer thickness of liquid films to monolayers where the details of the substrate potential are relevant. Most density functionals use a hard sphere fluid as a reference system for an expansion of the attractive dispersion forces and model substrates by plane walls. However, close to a solid substrate the entropy of a fluid is dominated by the occupation probability of discrete adsorption sites formed by the corrugated substrate surface. We propose a density functional which captures accurately the monolayer adsorption properties but covers also the growth of mesoscopic films and capillary condensation at hydrophilic conditions. We discuss the dependence of wetting and adhesion on humidity and substrate strength, in particular at the crossover from single particle to capillary condensed bridges.