The Condensation of a Vapour at a Crystalline Surface

Abstract

The theory of self-nucleation in a supersaturated vapour has been developed with considerable success by Becker and Döring (1935), but little attention has hitherto been given to condensation at a solid surface. A theory is given for the nucleation of liquid condensates on a plane surface and at lines and corners formed by the intersection of plane surfaces. It is suggested that the line and corner sites are representative of typical features of real crystalline surfaces. The equilibrium properties and free energies of formation of embryos at these sites, which are required for the nucleation theory, have been given in the previous paper. The theory shows that the supersaturation for which nucleation proceeds at a just observable rate on a plane surface increases rapidly as the contact angle increases from zero. Unless an edge free energy is introduced, no supersaturation is necessary for nucleation at a line or corner of intersection of plane surfaces when the contact angle 1s less than a threshold value which depends on the angles between the planes. As the contact angle Increases above the threshold value the supersaturation required for observable condensation increases considerably. The effect of contamination of the substrate surfaces considered and the suggestions are made that, ~f a small amount of soluble contamination is present it is likely to become localized at surface steps and corners, sensitizing them as nucleation sites, and that microscopic observation of incipient condensation on solid surfaces may provide a tool for the investigation of surface structure.