Quantitative study of interaction forces and friction in aqueous colloidal systems

Friction-force measurements between a silica sphere and a titanium dioxide wafer in electrolyte solutions were made using an atomic force microscope. The effect of electrical double-layer interactions on the adhesion and the friction force were investigated as a function of pH. In contrast to taking friction measurements in air, conducting the study in aqueous solution has allowed the surface separation, adhesion, and applied force to be controlled independently. Friction was found to be dependent only on the intrinsic force. Friction was seen to be independent of pH. When a force law fitted to the measured data was used, the separation as a function of intrinsic force was likewise found by theoretical calculations to be independent of pH. It was concluded that friction was solely dependent on separation and that the effect of applied force and electrical double-layer interactions served merely to change the separation. In addition, it was proposed that a single layer of unbound water molecules effectively lubricated the surfaces.