Nucleation and Growth

Nucleation is a physical reaction characterized by localized budding of a distinct thermodynamic phase by precipitation of components from a solution in the form of nuclei inducing more precipitation.  In general, a snapshot of the forming condensed phase at any time instant would consist of clusters of particles of varying sizes and numbers which might grow or shrink. Nucleation is said to take place if and only if such a cluster reaches a critical size (i.e. critical radius of nucleation) when it continues to grow rather than shrinking.

Nucleation occurs on nucleation sites contacting the vapor or liquid phases (homogeneous nucleation) or on suspended particles or bubbles (heterogeneous nucleation).

Homogeneous nucleation

Fig.5.1: Formation of vapor nucleus

In this section we are going to discuss formation of a liquid nucleus from its vapors. When vapors are in supersaturated state they condense to form a liquid nucleus which grows till it comes to equilibrium with the surrounding vapors. Consider a liquid nucleus surrounded by its vapors as a system with a volume .

We can assume the nucleus to be spherical in shape. For small nuclei sizes this assumption is valid as gravity effects are negligible in this case.

(component which suffers phase change)