What controls the thicknesses of wetting layers?

Abstract

Wetting layers can form on solid and fluid substrates when the long-ranged part of the effective intermolecular potential (which tends to thicken the layers) competes with gravity (which tends to thin the layers). Authors have reported layers whose thicknesses range from 7 to 600 nm on a variety of substrates. Here we use the theory of dispersion forces to make simple estimates of a layer's thickness in equilibrium. The estimates imply that, in certain cases, the measured thickness differs by a factor of 10 from the thickness expected in equilibrium. To resolve these differences, we have estimated the rates of several hydrodynamic processes (including diffusion) which occur as the layers approach equilibrium in common experimental configurations. When the layers are formed from mixtures of partially miscible liquids, equilibration can take months in cells with dimensions on the order of 1 cm. When mixtures are cooled or stirred continuously, a steady state can be reached; however, the layer's thickness will not be the equilibrium thickness. In contrast, layers formed by liquid–vapour systems can approach their equilibrium thickness in hours when thermal conduction through the vapour phase is favourable.