Complete and incomplete wetting by adsorbed solids

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Abstract
We carry out two calculations which show that a substrate which attracts a solid adsorbate too strongly prevents that adsorbate from wetting it. In the first, the adsorbed film is modeled microscopically as a set of layers in registry with one another. The appropriate thermodynamic potential is minimized at zero temperature with respect to the common lateral spacing and the individual layer heights for systems of up to 20 layers. For Xe, Kr, and Ar adsorbed on graphite, we find that the film thickness should exceed 20 layers, in agreement with experiment. A similar result is found for Ne, in disagreement with experiment. For atoms attracted very strongly, we find a thickness less than 20 layers, and therefore incomplete wetting behavior. In the second calculation, the film is modeled as an elastic continuum. We show that strain caused by the substrate potential induces a long-range force which, in general, prevents complete wetting of a solid film.