Nukleations

Nukleations is the process by which a new phase begins to form within a material, typically appearing as small clusters or nuclei that can grow into a macroscopic region of the new phase. It is central to phase transitions such as crystal formation from a supersaturated solution, condensation of vapor into droplets, and solidification from a melt.

Nukleations can occur homogeneously, within the bulk of a uniform medium, or heterogeneously, on surfaces, impurities, or interfaces that lower the energy barrier to forming a stable nucleus.

In classical nucleation theory, there is a competition between the bulk free-energy gain from forming the new phase and the interfacial energy cost of creating a boundary. This competition yields a critical nucleus size: clusters smaller than this size are thermodynamically unstable and tend to dissolve, while larger clusters tend to grow. The nucleation rate depends on temperature, the degree of supersaturation or undercooling, interfacial tension, and kinetic factors governing molecular attachment.

Spinodal decomposition is another pathway to phase separation that does not involve a barrier and is distinct from nucleation; it occurs when a system becomes unstable to infinitesimal fluctuations.

Nukleations is widely relevant in materials science and engineering, governing crystallization in metals and polymers, ice formation in the atmosphere, protein crystallization in biochemistry, and the production of pharmaceuticals. In practice, nucleation can be influenced by additives, surfaces (seeding or heterogeneous nucleants), cooling or saturation rates, and pressure.