Note on Evaporation

A simple kinetic model is employed to investigate nonequilibrium evaporation from a liquid. Molecules are assumed to evaporate into a (one-sided) Maxwellian velocity distribution at the liquid surface. Molecules reaching the surface from the vapor are assumed to form part of an ellipsoidal velocity distribution. Of the molecules approaching the liquid at the interface only the fraction α condenses there; the remainder is taken to undergo specular reflection back into the vapor. By equating physical conditions at the surface with conditions in the vapor, one can relate the vapor pressure to that which would prevail at equilibrium, as a function of the surface accommodation coefficient α and the mean velocity u of the vapor relative to the interface.