Metastability

Metastability is the persistence of a state that has higher free energy than an accessible, more stable alternative. A barrier to transition makes the state long-lived without making it the thermodynamic equilibrium.

Free-energy landscape: A representation of a system’s free energy as a function of its possible configurations or state variables. A local minimum in this landscape can trap the system in a metastable state.
Thermodynamic equilibrium: A state in which macroscopic properties do not change and thermodynamic driving forces vanish. Metastability can look stationary while remaining short of equilibrium.
Supercooling: Cooling a liquid below its freezing point without causing it to solidify. A supercooled liquid remains liquid because crystal formation has not crossed its nucleation barrier.
Unstable equilibrium: An equilibrium state in which small disturbances grow rather than decay. Unlike a metastable state, an unstable state does not resist sufficiently small perturbations.
Activation energy: The energy required to reach a transition state during a chemical or physical transformation. Crossing an activation barrier is one way a system escapes metastability.
Gibbs free energy: A thermodynamic potential that determines equilibrium at constant temperature and pressure. Its minima identify stable equilibrium states and help rank metastable alternatives.
Superheating: Heating a liquid above its boiling point without causing it to vaporize. A superheated liquid persists until vapor bubbles nucleate and grow.
Kinetic trapping: Persistence in a state because transition pathways are slow or obstructed. Kinetic trapping describes the dynamical cause that often sustains metastability.
Nucleation: The formation of small, stable clusters that initiate a new phase or structure. A critical nucleus can overcome the barrier separating a metastable phase from a stable one.
Local minimum: A point whose value is lower than nearby values of a function, though not necessarily its lowest value. Metastable states correspond to local, rather than global, minima of free energy.

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