The accommodation coefficient and the evaporation coefficient of water

Abstract

Previous experiments have already indicated that the maximum rate of evaporation of water into a vacuum is not so great as would be expected theoretically. The ratio of the experimental to the theoretical rate is defined as the evaporation coefficient f and has been found to have a value of about 0⋅04 for pure water at temperatures about 0º C. This result would indicate that, of the vapour molecules striking the liquid surface, about 96% must return to the vapour without entering the liquid. It is therefore of interest to enquire whether these vapour molecules attain temperature equilibrium with the surface or rebound at once before this equilibrium can be established. In the present paper experiments are described in which vapour molecules are incident on a liquid surface which is at a temperature lower than that of the vapour itself and the itself and the energy transferred to the surface by the vapour molecules is measured. If α, the accommodation coefficient, is defined as usual as the ratio of the energy actually transferred to the maximum possible transfer, it is found that for water at 10º C— α= 1⋅0 f = 0⋅036 so that, while only a very small fraction of the vapour molecules enter the liquid, all of them reach temperature equilibrium with the surface before re-evaporating into the vapour. Method If a drop of water is allowed to form on a glass tip in a vessel maintained at a pressure ( p ) which is lower than the saturated vapour pressure corresponding to the temperature of the drop, steady evaporation takes place from the surface of the latter throughout the period of its formation. This evaporation cools the surface. When the drop is fully formed it falls from the tip and may be collected and the drop weight determined. The surface tension can be deduced therefrom and hence the surface temperature may be obtained. This data makes possible the direct calculation of f as follows.