Nature of mechanical instabilities and their effect on kinetic friction

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.