Nanoscale Friction In Simulations of Boundary Lubrication and the Effect of Surface Ordering

Boundary lubrication is a sliding friction regime in which the magnitude of the frictional force depends on the chemical details of the fluid-surface and/or surface-surface interactions. Molecular dynamics simulations were conducted on confined surfactant-like molecules in order to quantify their role in modifying the transmission of normal and shear forces between opposing sliding surfaces. The structure of confined fluid film (i.e. long-range ordering) was insensitive to sliding velocity (0-7.5 m/s), while friction force increased with sliding speed. In one simulation, a subtle yet distinct change in inter-layer ordering was observed within the film, which was accompanied by large changes in instantaneous normal force but small to negligible changes in frictional (shear) force. Findings from this study and implications for surface design will be discussed.