Lattice model of mobility at interfaces: Free surfaces, substrates, and bilayers

A simple kinetic lattice model of free volume and mobility transport in fluids is applied to study the enhancement of mobility at a free surface in thin fluid films, as well as proximity effects in fluid bilayers consisting of materials with different local mobility. Consistent with experimental observations on fluid and polymeric thin films, our model predicts the presence of a mobile layer of material near the free surface of a kinetically arrested (glassy) film. The mobile layer extends deeper into the film, in front-like fashion, as the sample approaches the transition to complete fluidity. The extent of enhanced mobility is independent of film thickness at a given temperature, thus we find that thinner films have more suppressed sample-average glass transition temperatures compared to bulk material. This theme repeats itself in our simulations of fluid bilayers; slabs of material with suppressed or enhanced mobility respectively cause premature or delayed glassification of the whole system.