Molecular simulations of sliding on SDS surfactant films

Abstract: We use molecular dynamics simulations to study the frictional response of the anionic surfactant sodium dodecyl sulfate (SDS) monolayers and hemicylindrical aggregates physisorbed on gold. Our simulations of a sliding spherical asperity reveals two friction regimes: At low loads, the films show Amontons' friction with a friction force that rises linearly with normal load. At high loads, the friction force is independent of load as long as no direct solid-solid contact occurs. The transition between these two regimes happens when only a single molecular layer is confined in the gap between the sliding bodies. The friction force at high loads on a monolayer rises monotonically with film density and drops slightly with the transition to hemicylindrical aggregates. This monotonous increase of friction force is compatible with a traditional plowing model of sliding friction. At low loads, the friction coefficient reaches a minimum at intermediate surface concentrations. We attribute this behavior to a competition between adhesive forces, repulsion of the compressed film, and the onset of plowing.