Evaporation into vacuum: Mass flux from momentum fluxand the Hertz—Knudsen relation revisited

摘要

We performed molecular dynamics simulations of liquid film evaporation into vacuum for twocases: free evaporation without external supply of energy and evaporation at constant average liquidtemperature. In both cases we found that the pressure inside a liquid film was constant, whiletemperature decreased and density increased as a function of distance from the middle of the film.The momentum flux in the vapor far from the liquid was equal to the liquid pressure in theevaporating film. Moreover the pseudopressure (stagnation pressure) was found to be constant in theevaporating vapor and equal to the liquid pressure. The momentum flux and its relation to thepressure determined the number of evaporating molecules per unit time and as a consequence themass evaporation flux. We found a simple formula for the evaporation flux, which much betterdescribes simulation results than the commonly used Hertz–Knudsen relation