Liquid–Liquid Flow at Nanoscale: Slip and Hydrodynamic Boundary Conditions

Nonequilibrium molecular dynamics (NEMD) simulations have been performed to describe the flow of a fluid nanolayer confined by another fluid. The results show that the behavior of liquids can still be described by the Navier–Stokes equation at the nanoscale, i.e., when only few molecular layers are involved. NEMD furthermore gives additional knowledge on flow. Indeed, while a very small slip is evidenced for a solid–liquid interface as, e.g., in lubrication, the slip lengths are significantly larger at the liquid–liquid interface, as encountered, e.g., in droplet coalescence. The slip lengths of the two fluids are linked. The increase in hydrodynamic slip for liquid–liquid interfaces is attributed to the enhancement of fluid diffusion, which reduces friction.