Friction and Capillary Forces at the Nanometer Scale

Abstract

Frictional behaviors between mica surfaces have been investigated with the Surface Force Apparatus under various relative vapor pressures (rvp) of both water and cyclohexane. Stick-slip frictional behaviors were observed only under cyclohexane vapor. At rvp of 20%, stick-slip appeared but faded out with sliding time. At a rvp greater than 50%, the stick-slip pattern was stable without fading. Dependence of sliding velocity and applied load on stick-slip motions indicated that the mechanism of the stick-slips in the high rvp (stable stick-slip) region differs from that of the region of fading stick-slip. In the rvp range between 20% and 50%, where the fading stick-slip is observed, the condensate liquid seeps into the contact area under the influence of the applied tangential force and thus triggers the slip motion. Due to the small condensation volume, the liquid condensed around the contact area is exhausted in the process of repeating stick-slip. At a rvp greater than 50%, where stable sick-slip is observed, the stick-slip caused by essentially the same origin as that observed with mica surfaces sliding in bulk cyclohexane liquid. In vapor, the stick-slip is enhanced by the increase of the negative Laplace pressure in the capillary condensed liquid, thereby forcing the surfaces toward each other more strongly with decreasing rvp.