  Friction force acting on silver and nickel nanoparticles sheared on a
graphene sheet in different lateral directions is investigated using classical
molecular dynamics simulations. The results reveal the existence of frictional
anisotropy for both metals. In most cases, the maximum value of the friction
force is about two times larger than the minimum one. The form of dependencies
of instantaneous values of the friction force components on the corresponding
lateral components of the position of the centre of mass of the nanoislands
strongly depends on sliding direction, varying between the sawtooth and the
irregular one. A qualitative explanation of the results based on the "patch"
model is proposed.
