Interfacial premelting and the thermomolecular force: thermodynamic buoyancy.

The presence of a substrate can alter the equilibrium state of another material near their common boundary. Examples include wetting and interfacial premelting. In the latter case, temperature gradients induce spatial variations in the thickness of the premelted film that reflect changes in the strength of the repulsion between the substrate and the solid. We show that the net thermomolecular force on a macroscopic substrate is equivalent to a thermodynamic buoyancy force-proportional to the mass of solid that can occupy the volume enclosed by the substrate and the temperature gradient.