Molecular Simulations of Kinetic-Friction Modification in Nanoscale Fluid Layers

Molecular simulations are used to explore kinetic-friction modification in nanoscale fluid layers of oil and additive confined between sheared parallel walls. The molecules are represented by coarse-grained bead-spring models that reflect the essential solvophilic and solvophobic natures of the chemical groups. The degree of friction modification is surveyed as a function of wall separation, sliding velocity, additive molecular weight and architecture, and oil-additive composition. As a rule, the kinetic-friction coefficient is found to increase first linearly and then logarithmically with increasing sliding velocity. From the results for different additive molecules, some subtle but systematic effects are found that point towards an optimum molecular weight and architecture.