nucléation

Nucléation is the initial process that leads to the formation of a new phase or structure within a material or at an interface. It marks the onset of phase transformation, such as crystal formation from a supersaturated solution, solidification from a melt, or condensation from vapor.

Two main routes are recognized. Homogeneous nucléation occurs uniformly in the bulk without preferential surfaces, while heterogeneous nucléation takes place at interfaces, impurities, or defects and typically requires a smaller energy barrier. In both cases, the new phase must form a cluster that reaches a critical size. Clusters smaller than this critical radius tend to dissolve, whereas larger ones become stable and can grow. The energy barrier and the critical size are governed by thermodynamics: there is a competition between the favorable volume free energy change and the unfavorable surface energy.

Kinetics are often described by classical nucléation theory, which relates the nucleation rate to temperature, supersaturation, and interfacial tension. In practice, factors such as impurities, surface energies, and confinement can strongly affect nucléation. Growth follows once nuclei are established, leading to the development of the new phase, which may then coarsen or ripen over time.

Nucléation is central to many natural and industrial processes, including crystallization from solution, solidification of metals and alloys, cloud droplet formation, boiling and bubble formation, and protein crystallization for structural studies. Understanding and controlling nucléation is important for material properties, product quality, and process efficiency.