Nucleationen

Nucleation is the primary step in a phase transition in which a new phase first appears as small aggregates of molecules, called nuclei, within a parent phase. In crystallization from a liquid or solution, solid crystals emerge from molecular clusters; in condensation, droplets form from supersaturated vapor.

There are two main pathways: homogeneous nucleation, which occurs uniformly within the bulk without preferential sites; and heterogeneous nucleation, which proceeds at interfaces, impurities, container walls or defects and requires lower energy.

Key prerequisites include supersaturation or undercooling: the parent phase must be in a metastable state where the new phase is thermodynamically favored but has to overcome a free-energy barrier to form a stable nucleus. The process is governed by the balance between the volume free-energy gain from forming the new phase and the surface energy cost of creating an interface. The result is a critical nucleus: a cluster of a certain size that, if formed, will tend to grow, whereas smaller clusters tend to dissolve. The rate of nucleation depends on temperature, degree of supersaturation, interfacial tension, and molecular properties; in mathematical terms, a higher energy barrier or lower driving force reduces the rate.

Classical nucleation theory provides a framework to estimate nucleation rates and critical sizes, though real systems can deviate due to non-classical pathways, impurities, or complex molecular interactions.

Nucleation is central to natural phenomena such as cloud formation and frost, and to technological processes including crystallization of pharmaceuticals, metal solidification, and polymer crystallization. It is often studied through induction time, light scattering, microscopy, or calorimetry.