When explaining the origin of friction, it is important to answer the question of how two
dissimilar solids manage to interlock. It has been shown that incommensurate interfaces, where the particles belonging to the two surfaces in contact are out-of-register, can slide at extremely low lateral forces. One would, thus, be tempted to attribute ubiquitous static friction to commensurate interfaces, except that most surfaces in contact form incommensurate interfaces. Understanding the microscopic mechanisms that govern friction represents a fundamental scientific problem with important practical applications.
The transition from static to dynamic friction is not completely well defined, because even when the lateral force is below the nominal static friction, a body can slowly creep forward due to thermal activation. In addition, direct visualization of the contact area at the onset of slip and numerical simulations indicate the formation and propagation of detachment fronts fracturing the multicontact interface. These findings suggest that the onset of slip is due to microscopic processes, ultimately due to the interactions between individual atoms lying on the surfaces in contact, propagating up to the macroscale to yield collective sliding.
We show that the size dependence of friction in xenon clusters adsorbed on several substrates by means of classical MD simulations and QCM expertiments; that the friction is strictly related to the commensurability between the rare gas atoms adsorbed cluster and the substrate and, during dynamics simulations, these atoms coalesce in domain with the same commensurability; and that it is possible to model force fields between adsorbed monolayers and metal substrate in classical MD simulations.
||M. Pierno, L. Bignardi, M. C. Righi, L. Bruschi, S. Gottardi, M. Stohr, P. Silvestrelli, P. Rudolf and G. Mistura Thermolubricity of Xe monolayers on graphene, Nanoscale, 6, 8062 (2014). (PDF)