Method to measure the viscosity of nanometer liquid films from the surface fluctuations

Abstract

We describe a method to measure the viscosity of polystyrene liquid films with thicknesses ∼5 and ∼80 nm spin-cast on oxide-coated silicon. In this method, temporal evolution of the film surface is monitored and modeled according to the dynamics of the surface capillary waves. Viscosities obtained from the ∼80 nm films display an excellent agreement with those of the bulk polymer, but those from the ∼5 nm films are up to 106 times reduced. By modeling the data to the Vogel-Fulcher-Tammann relation, we find that the observations are consistent with the thickness dependence of the glass transition temperature previously reported of these films.