Relation between adhesion and friction forces across thin films

Abstract

We report the results of measurements relating the normal forces ( e.g. adhesion forces) between two atomically smooth mica surfaces separated by molecularly thin liquid films to their lateral forces ( e.g. friction or shear forces). The normal forces across simple liquids generally exhibit spatial oscillations, alternating between maxima and minima with a periodicity equal to the mean molecular diameter and a peak-to-peak amplitude that decays roughly exponentially with distance. Such oscillations are indicative of out-of-plane molecular ordering into discrete molecular layers within the film. The critical shear force needed to initiate lateral motion (the static friction force) was found to be smaller the farther out the potential minimum in which the motion was initiated. Such yield points for shear motion are indicative of epitaxially induced in-plane molecular ordering within the film. We have investigated whether the lateral friction forces are related to the normal oscillatory force-law between two mica surfaces across tetradecane. The results indicate that such a relation does exist, which was further explored by studying the effects of water and the “twist angle” between the two surfaces lattices on these interactions.