Heterogeneous nucleation at macroscopic steps

Abstract

Macroscopic steps on a substrate surface are known to affect condensation characteristics of a substance from its vapor. A simple model is presented, for the case of isotropic interfacial free energy, to explain the kinetics of nucleation on a macroscopic step in terms of capillarity theory. A critical contact angle θc is shown to exist above which a macroscopic step on a substrate will have no effect on nucleation. The model also predicts a contact angle of 45° below which the free energy barrier for nucleation at a step vanishes. Between 50° and θc nucleation occurs preferentially at steps. Such nucleation is thought to give rise to a characteristic decoration pattern in electron-microscope replicas of the substrate surface. Experiments in which cadmium and gold were condensed from the vapor onto cleaved sodium chloride and lithium fluoride crystals tend to support the theory. However one notes a reservation in regard to application of the present thermodynamic theory to these small nuclei which are of the order of only several atoms. © 1964.