Nucleating

Nucleating refers to the initial stage of a phase transition in which small clusters, or nuclei, of a new phase form within a parent phase. This process is a prerequisite for crystallization, solidification, condensation, or phase separation.

Homogeneous nucleation occurs spontaneously in the interior of a uniform material, while heterogeneous nucleation proceeds at interfaces, defects, or impurities. The latter typically requires a smaller driving force because surfaces and interfaces lower the energy barrier.

In a supersaturated or undercooled system, fluctuations must overcome an energy barrier to form a stable nucleus. Nuclei smaller than a critical size tend to dissolve, while larger ones grow into a macroscopic phase.

The nucleation rate depends on temperature, degree of supersaturation or undercooling, interfacial energy, and the presence of foreign particles. Classical nucleation theory describes this balance between surface energy and volume free energy, yielding a rate that often varies exponentially with conditions.

Nucleation governs material microstructure and phase formation in metals and polymers, cloud and rain formation in the atmosphere, and the crystallization of proteins and other biomolecules. Nucleating agents, seeds, or surfaces are frequently used to promote or control the process.