Nucleation

Nucleation is the initial process by which a new thermodynamic phase or structure begins to form within a parent phase that is metastable. It involves the appearance of small clusters, or nuclei, that may either grow into a macroscopic phase or dissolve back into the original phase.

There are two main types of nucleation. Homogeneous nucleation occurs within a uniform medium without preferential surfaces, whereas heterogeneous nucleation takes place at interfaces or on impurities, container walls, or other foreign surfaces. The latter typically requires a lower energy barrier to form stable nuclei because of favorable interfacial energies.

In nucleation, a nucleus must reach a critical size to become stable and able to grow. Clusters smaller than this critical radius tend to dissolve due to the dominance of surface energy, while larger clusters can lower the system’s free energy by forming the new phase. The rate of nucleation depends on factors such as supersaturation or undercooling, temperature, interfacial energy, and the presence of catalysts or defects.

Classical nucleation theory describes the balance between the bulk free-energy gain of forming the new phase and the surface energy cost of creating an interface, yielding a nucleation rate that is highly sensitive to conditions. Beyond classical descriptions, non-classical pathways such as two-step nucleation, prenucleation clusters, and intermediate metastable states have been observed in some systems.

Nucleation is central to processes including condensation, crystallization from melts or solutions, polymer crystallization, mineral and metal solidification, and cloud formation. It also plays a role in biological contexts, notably protein crystallization for structural biology. Controlling nucleation involves adjusting supersaturation, temperature, impurities, seeding, and surface chemistry, and is studied via microscopy, light scattering, calorimetry, diffraction, and modeling.