Slip effects at the vapor-liquid boundary

The paper addresses the microscopic description of the interaction of a liquid and gaseous phase through the common planar vapor-liquid interface, in the particular case of a pure shearing motion, in absence of net mass transfer between the two phases. A two-phase Couette flow is considered as test problem and studied by numerical solutions of the Enskog-Vlasov kinetic equation and molecular dynamics simulations of a Lennard-Jones fluid. It is shown that the velocity slip between the hydrodynamic regions of the two phases is due not only to the presence of the Knudsen layer in the dilute vapor, but also to a rapid acceleration of the velocity within the outermost half of the vapor-liquid interface. Both Enskog- Vlasov model and molecular dynamics simulations results suggest that effective liquid-vapor slip lengths are smaller than those predicted on the basis of purely thermal emission of atoms from the liquid surface into the vapor phase.