Heterogeneous Nucleation

Nucleation that occurs naturally is almost always heterogeneous as it occurs at nonuniformities in the system. Nucleation can be catalyzed in this way by planar, linear, or point heterogeneities. For nucleation on planar interfaces, the classical model is that of a spherical-cap-shaped embryo. The spherical-cap embryo is bounded by an α–β interface of uniform curvature analogous to that of a spherical embryo in a simple model for homogeneous nucleation. The basis of preferred nucleation on a planar substrate is an elimination of the part of the interface between the substrate and the original α phase, lowering the work of embryo formation. However, this effect is augmented when a structure can act as a template for growth of the new phase. The structure may be that of the substrate itself, or it may be that of an adsorbed or wetting layer on the substrate. The critical work is lower for heterogeneous than for homogeneous nucleation, but this factor is offset by the lower number of molecules capable of participating in heterogeneous nucleation.