Metastable state

A metastable state persists for a finite time even though another accessible state has lower free energy. A barrier to transition can make it long-lived.

Free energy: A thermodynamic potential that measures a system’s capacity to do work under specified conditions. Its landscape identifies the lower-free-energy states that metastable states have not yet reached.
Thermodynamic equilibrium: A condition in which macroscopic properties remain constant and no net thermodynamic driving force remains. Metastability is persistence without full equilibrium.
Supercooling: Cooling a liquid below its freezing point without it becoming solid. The liquid remains metastable until crystals nucleate.
Global minimum: The lowest value of a function over its entire domain. A metastable state lies above this free-energy minimum.
Energy barrier: The energy required for a system to move between states or cross a reaction pathway. Crossing this barrier limits escape from a metastable state.
Thermodynamic stability: A system’s resistance to changes that would lower its thermodynamic potential. A metastable state is locally stable against small disturbances, but not globally stable.
Superheating: Heating a liquid above its boiling point without it immediately vaporizing. The liquid can persist because vapor bubbles have not nucleated.
Unstable equilibrium: An equilibrium that departs from its initial state after sufficiently small disturbances. Unlike a metastable state, it does not resist small perturbations.
Nucleation: The formation of small, stable clusters that can initiate a new phase or structure. A critical nucleus often enables the transition out of a metastable phase.
Phase transition: A transformation between distinct physical phases of matter as conditions change. Many metastable states occur when a phase transition is delayed.

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