Hydrodynamic interactions for the measurement of thin film elastic properties

Abstract

We study the elasto-hydrodynamic (EHD) interaction of a sphere with a flat elastic surface in the prospect of measuring the elastic moduli of soft supported thin films, with non-contact dynamic surface forces or atomic force microscopy measurements. When the sphere is oscillated at a very small amplitude close to the surface, the linear force response undergoes a dynamic transition from a viscous-dominated behaviour at large distance to an elastic-dominated behaviour at short distance. In the limit of very thin or very thick supported layers, we show that the force response obeys simple scaling laws which allow to unambiguously determine the absolute elastic modulus of the layer. In the general case, we establish the very rich phase diagram of the EHD interaction and discuss its application for optimizing experimental parameters.