Dynamic effects on force measurements. 2. Lubrication and the atomic force microscope

We present the results of investigations of high-speed drainage of a thin film confined between a microscopic colloidal probe and a substrate performed with a new atomic force microscope-related setup. Theoretical calculations are used to formulate the governing equation (force balance) for instantaneous deflection of a cantilever spring, which is due to both concentrated forces acting on a colloidal probe and viscous drag force on a cantilever itself. The suggested way to subtract the latter contribution allows design of a lubrication experiment. Two pairs of interacting solids, characterized by different wettability and smoothness, immersed into water-electrolyte solutions have been studied. Results for hydrophilic silica surfaces are in excellent agreement with the Reynolds theory of hydrodynamic lubrication. Faster drainage of a thin film confined between hydrophobic rough polystyrene surfaces is consistent with the theory of film drainage between slippery surfaces. The slip lengths are found to be of the order of the size of asperities, and do not depend on the separation and shear rate. The results are important for colloidal dynamics and nanofluidics.