Surface force apparatus and its application to nanorheological studies

Abstract

A surface force apparatus has been adapted to operate as a rheometer on a molecular scale. The way it works and the new possibility given to study dynamic properties of very thin films (<1 μm) squeezed between solid surfaces are discussed. Three kinds of tests, enabling the measurement of the viscoelastic properties of confined media in addition to static interactions between the surfaces, are presented. These are: draining—the surfaces are brought together at constant speed; relaxation—the displacement is stopped and the force is decreasing toward a static force; oscillations—the surfaces are submitted to small axial sinusoidal displacements around an average position. Results obtained with dilute polymer solutions moderately confined are presented. Variations of parameters characterizing the terminal zone such as the zero shear viscosity, the average relaxation time, and a parameter connected to the dispersion of relaxation times are presented and qualitatively discussed as a function of the separation between the solid surfaces.