Nature of Mechanical Instabilities and Their Effect on Kinetic Friction

It has long been recognized that kinetic friction ${F}_{k}$ between two solids must be due to instabilities, sudden ``pops'' of certain degrees of freedom. Here, such pops are studied with a focus on boundary lubrication. The pops' characteristics and consequently the friction-velocity relationship depend qualitatively on dimensionality, commensurability, and details of the lubricant wall interaction. It is found that ${F}_{k}$ should be small between commensurate surfaces. ${F}_{k}$ is large for incommensurate surfaces, unless the lubricant's motion is confined to 1D. The effects of thermal noise are discussed and computer simulations are employed to show the relevance of the predictions to less idealized models.