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| - n-Octadecyltriethoxysilane Monolayer Coated Surfaces in Humid Atmospheres: Influenceof Capillary Condensation on Surface Deformation and Adhesion
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| - We have investigated the effect of humidity on surface deformation and adhesion of mica surfaces coatedwith n-octadecyltriethoxysilane self-assembled monolayers using a surface forces apparatus. The Maugis modelof contact elasticity based on linear elastic fracture mechanics is used to analyze the results. The Laplacepressure is assumed to act in the “Dugdale” zone outside the contact area to account for capillary condensation.We measure the radius of the contact area as a function of applied load and use the model to obtain thesurface energy and elastic constant of these surfaces for humidities ranging from 0 to 99%. The limitingvalues in dry and near-saturated conditions are as expected from well-known theories. A significant result isthat we also obtain the surface energy for intermediate humidities. Increasing humidity modifies the deformedshape of the surfaces in contact due to capillary condensation. The sharp bifurcation at the edge of the contactzone for low humidities (JKR-type contact) is replaced by rounded edges (DMT-type contact) with increasinghumidity. This is predicted by the Maugis model and is experimentally observed using optical interferencefringes of equal chromatic order. We are able to separate the capillary condensation and solid−solidcontributions to the adhesive force because the Maugis model allows a direct calculation of the area onwhich the Laplace pressure acts. At humidities approaching saturation the forces due to capillary condensationdominate monolayer−monolayer adhesion. At lower humidities both capillary condensation and directmonolayer−monolayer interaction contribute to the overall adhesion.
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