Nature of mechanical instabilities and their effect on kinetic friction

TL;DR

This paper investigates the microscopic instabilities influencing kinetic friction, especially in boundary lubricants, revealing critical parameter thresholds where instability behavior and friction dependence on velocity change.

Contribution

It characterizes the nature of microscopic instabilities affecting kinetic friction and identifies parameter thresholds that alter their behavior and velocity dependence.

Findings

Identification of critical parameter values where instability behavior changes

Demonstration of changes in frictional force dependence on sliding velocity

Validation of theoretical predictions through molecular dynamics simulations

Abstract

It has long been recognized that the key to understand kinetic friction force $F_k$ is the analysis of microscopic instabilities that lead to sudden irreversible "pops" of certain degrees of freedom. In this Letter, the nature of such instabilities is characterized with an emphasis on boundary lubricants. It is shown that there are certain critical values of the parameters defining our model Hamiltonian, where the behavior of the instabilities changes qualitatively. Simultaneously, the functional dependence of $F_k$ on the sliding velocity $v_0$ changes. The relevant parameters studied here are dimensionality of the interface, degree of commensurability, first higher harmonic in the lubricant wall potential, and temperature. Molecular dynamics simulations are carried out to test whether the predictions made on the basis of the simple model also hold in less idealized circumstances.