Evaporation—Condensation Coefficient for Small Droplets

Abstract

The evaporation—condensation coefficient for very small droplets is derived in the form α(a) =δ·φ(a), where δ is the free-angle ratio and φ(a) is the size coefficient which is expressed in terms of the droplet radius a, the absolute temperature, and other characteristic variables of the liquid phase and the gas—liquid interface. With the inclusion of α(a) into the rate equations of Fuchs and of Monchick and Reiss, the rate of evaporation per unit area increases with decreasing droplet radius, reaches a maximum at droplet radii ranging between 1 and 0.01 μ, then decreases to zero as the radius approaches zero. Numerical results were obtained for H2O, He, and Hg at their respective freezing and boiling points.