Heterogeneous nucleation of water vapor on high and low energy surfaces

Abstract

The rate of nucleation of water vapor condensing to drops on polycrystalline substrates having low and high surface energy was found to be strongly dependent upon the degree of supercooling. For each surface, a reproducible critical degree of supercooling was observed at which the nucleation rate changed rapidly. A plot of the critical degree of supercooling versus substrate surface energy gave a continuous curve with a minimum. The lowest degrees of supercooling were achieved by the silver halides. Materials having higher and lower surface energies than these halides gave greater critical degrees of super-cooling. For substrates of relatively low surface energy, the observed critical degree of super-cooling was generally smaller than the theoretical prediction of the Volmer-Becker-Döring heterogeneous nucleation theory indicating that the active nucleation sites must have a higher energy than the surface average. On high energy surfaces of cadmium and mercuric sulfide, where the contact angle was zero, the theory completely breaks down, and consequently predictions considerably underestimate the critical degree of supercooling. Variables not included in the theory such as the surface diffusion and adsorption free energies may account for these discrepancies.