Lack of Pinning for Rigid Sliding Monolayers in Microbalance Experiments

Recent work on the dynamics of monolayers on a metallic substrate attached to a quartz oscillator has provided interesting data on kinetic friction at the microscopic level. Sliding of the film relative to the substrate is often observed even in situations in which theory seems to predict that the film should be pinned by substrate imperfections. In this letter we propose, in order to attempt to resolve this issue, that if the defect potentials have a range of a little more than an atomic spacing, the net forces on the film due to the defects are likely to be quite small due to cancellations.