Molecular origins of friction: the force on adsorbed layers

Simulations and perturbation theory are used to study the molecular origins of friction in an ideal model system, a layer of adsorbed molecules sliding over a substrate. These calculations reproduce several surprising features of experimental results. In most cases, the frictional force on a solid monolayer has a different form from that observed between macroscopic solids. No threshold force or static friction is needed to initiate sliding; instead, the velocity is proportional to the force. As in experiments, incommensurate solid layers actually slide more readily than fluid layers. A comparison of experiment, simulation, and analytic results shows that dissipation arises from anharmonic coupling between phonon modes and substrate-induced deformations in the adsorbate.