From molecular to multi-asperity contacts: how roughness bridges the friction scale gap
While friction stems from the fundamental interactions between atoms at a contact interface, its best descriptions at the macroscopic scale remain phenomenological. The so called "rate-and-state" models, which specify the friction response in terms of the relative sliding velocity and the "age" of the contact interface, fail to uncover the nano-scale mechanisms governing the macro-scale response, while models of friction at the atomic scale often overlook how roughness can alter the friction behavior. Here we bridge this gap between nano and macro descriptions of friction by correlating the physical origin of macroscopic friction to the existence, due to nanometric roughness, of contact junctions between adsorbed monolayers. Their dynamics, as we show, emerges from molecular motion. Through coupled experimental and atomic simulations, we highlight that transient friction overshoots its steady-state value after the system is allowed to rest, with the friction force decaying to a steady-state value over a distance of a few nanometers, much smaller than the junction size, even with a root-mean-square roughness of 0.6 nm. We demonstrate how this transient decay is intrinsically related to the evolution of the number of cross-surface attractive physical links between adsorbed molecules on rough surfaces. We also show that roughness is a sufficient condition for the appearance of frictional aging. In systems that show structural aging, this paints contact junctions as a key component in the observation of the transient friction overshoot, and shows how infrajunction molecular motion can control the macroscopic response.