nukleation

Nukleation, commonly spelled nucleation, is the initial process by which a new thermodynamic phase or structure emerges within a parent phase. It involves the formation of small clusters, or nuclei, of the new phase that must reach a critical size before they are likely to grow rather than dissolve. Nukleation is a key first step in phase transitions such as condensation, crystallization, precipitation, and spinodal decomposition.

Nukleation can occur homogeneously, without preferential surfaces, or heterogeneously, on preexisting surfaces, impurities, or interfaces. Heterogeneous nucleation typically requires a smaller energy barrier and thus proceeds more readily, which is why containers, dust, or substrates influence crystallization and cloud formation.

The process is governed by thermodynamics and kinetics. A nucleus forms by fluctuations that create a region of the new phase; the total free energy includes a favorable volume term and an unfavorable surface term. An energy barrier must be overcome, defining a critical nucleus size. Classical nucleation theory provides relationships among supersaturation or undercooling, interfacial energy, temperature, and nucleation rate, but many systems exhibit non-classical pathways with multiple steps or preassembled clusters.

Nukleation controls material properties by setting crystal size, grain structure, and phase composition. It is also essential in precipitation hardening, polymer crystallization, metal refining, and atmospheric science. Experimental approaches measure induction times and nucleation rates, while simulations and advanced microscopy illuminate microscopic pathways.