Nucleation

Nucleation is the initial formation of a distinct phase from a metastable parent phase, when a cluster becomes large enough to grow rather than dissolve.

Critical nucleus: The smallest cluster of a new phase that is more likely to grow than to dissolve. Crossing this size threshold turns a transient fluctuation into a growing phase.
Phase transition: A transformation between distinct states of matter or phases as conditions such as temperature or pressure change. Nucleation is one route by which many first-order phase transitions begin.
Homogeneous nucleation: Nucleation that occurs within a uniform parent phase without a preferential surface or substrate. It is the baseline pathway when no foreign interface assists cluster formation.
Ice nucleation: The formation of initial ice crystals from supercooled water or water vapor. It shows how rare nuclei can trigger freezing in a metastable liquid.
Classical nucleation theory: A model that estimates nucleation barriers by balancing the bulk free-energy gain against the surface-energy cost of a cluster. It quantifies why forming a stable nucleus requires overcoming an energy barrier.
Metastability: A state that is locally stable but can transform into a more stable state after a sufficiently large disturbance. A metastable parent phase can persist until a viable nucleus appears.
Heterogeneous nucleation: Nucleation promoted by a surface, impurity, or other pre-existing interface. It contrasts with homogeneous nucleation by reducing the interfacial cost of a cluster.
Cloud condensation nuclei: Aerosol particles on which water vapor condenses to form cloud droplets. These particles provide surfaces that facilitate atmospheric droplet nucleation.
Supersaturation: A condition in which a solution or vapor contains more dissolved or condensed material than equilibrium permits. Greater supersaturation increases the thermodynamic drive for clusters to grow.
Surface tension: The energetic cost per unit area of creating an interface between phases. Creating a nucleus costs interfacial energy, especially when the cluster is small.

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