The velocity dependence of dry sliding friction at the nano-scale

TL;DR

This study uses molecular dynamics simulations to show that at the nanoscale, dry sliding friction depends linearly on velocity, contrasting macroscopic laws, due to thermal surface fluctuations causing surface corrugation.

Contribution

The paper reveals a linear velocity dependence of nanoscale dry friction and attributes it to thermal surface fluctuations, challenging classical friction laws.

Findings

Friction force linearly depends on sliding velocity

Thermal surface fluctuations cause surface corrugation

Contrasts with macroscopic Amontons-Coulomb laws

Abstract

We performed molecular dynamics (MD) experiments to explore dry sliding friction at the nanoscale. We used the setup comprised of a spherical particle built up of 32,000 aluminium atoms, resting on a semi-space with a free surface, modelled by a stack of merged graphene layers. We utilized LAMMPS with the COMB3 many-body potentials for the inter-atomic interactions and Langevin thermostat which kept the system at $300 K$. We varied the normal load on the particle and applied different tangential force, which caused the particle sliding. Based on the simulation data, we demonstrate that the friction force $F_{\rm fr}$ linearly depends on the sliding velocity $v$, that is, $F_{\rm fr}=-\gamma v$, where $\gamma$ is the friction coefficient. The observed dependence is in a sharp contrast with the macroscopic Amontons-Coulomb laws, which predict the velocity independence of sliding friction. We explain such a dependence by surface fluctuations of the thermal origin, which give rise to surface corrugation hindering sliding motion. This mechanism is similar to that of the viscous friction force exerted on a body moving in viscous fluid.