Anisotropy Effects in Atomic-Scale Friction

The static and kinetic friction experienced by a point mass elastically driven at different angles on surface lattices with square, hexagonal, and honeycomb symmetries are estimated by analytical and numeric calculations based on the Prandtl–Tomlinson (PT) model. Assuming a strong surface coupling, the anisotropy of static friction increases from 3.7 up to 46.3% when the density of packing of the surface atoms is reduced, but this is not the case for kinetic friction, the anisotropy of which is maximal on a square lattice. Although these results have not been supported by accurate experimental verifications so far, the PT model was successfully applied to interpret anisotropy effects in the friction force profiles measured, among other surfaces, on rectangular lattices with complex unit cells and on stepped crystal surfaces.