Initiation of wetting, filmwise condensation and condensate drainage from a surface in a gravity field.

The zeta adsorption isotherm is based on the hypothesis that a vapour adsorbed on a solid surface consists of a collection of molecular clusters. We use this isotherm to propose a method for determining the wetting condition on a vertically oriented silicon surface exposed to heptane in a gravity field. Measurements indicate the amount adsorbed is larger at positions of smaller potential energy. The wetting condition is taken to be reached when the adsorbed vapour is transformed into the adsorbed liquid phase: adsorption lowers the surface tension of Si from the value in the absence of adsorption to that of liquid heptane at wetting, and then as the Si-heptane is cooled further it is reduced to zero, at a subcooling of 3.7 K. The expectation is that when this subcooling is reached, gravity would cause the larger molecular clusters to drain down the surface. This prediction is supported by experimental observations.