Observation of depinning phenomena in the sliding friction of Kr films on gold

We have used quartz crystal microbalance to study the sliding friction of atomically thin Kr films adsorbed on gold. We have observed sharp pinning transitions separating a low-coverage region, characterized by slippage at the solid-fluid boundary, from a high-coverage region where the film is locked to the oscillating electrodes. Furthermore, we have induced sliding of a film by slowly increasing the amplitude of the substrate oscillations. Such a depinning transition is characterized by hysteresis as the amplitude is decreased back to zero. These observations have been obtained with two different quartz plates and are similar to those recently reported in the literature. A more quantitative comparison among the different experimental runs indicate that the location of these dynamical transitions is strongly dependent on the state of the substrate surface.