Quartz-crystal microbalance studies of the velocity dependence of interfacial friction

We have employed a quartz-crystal microbalance technique to measure the vibrational amplitude dependence of quality factor shifts which occur when Kr monolayers adsorb on the microbalance's gold electrodes. Assuming that the friction force is proportional to some power of the sliding velocity of the film relative to its substrate, such measurements allow one to infer whether or not the friction force law is linear in form. We have observed a linear friction law to be valid for a full monolayer of both liquid and solid krypton, for sliding speeds estimated to be in the range 3--8 cm/s. The form of the friction law at lower coverages appears to be nonlinear, consistent with $n<1$ in a more general law, F∝vⁿ.