A metastable phase is a local but nonglobal minimum of the thermodynamic potential. A finite nucleation barrier can delay conversion to the globally stable phase until fluctuations create a sufficiently large nucleus or the metastable minimum reaches a spinodal point.
A vapor whose partial pressure exceeds its vapor pressure at the local temperature; nucleation can permit condensation.
The fractional excess of water-vapor pressure above equilibrium saturation over a flat pure-water surface at the same temperature: . Positive can drive droplet growth, but activation of a particular aerosol particle requires exceeding its critical supersaturation. Cooling, condensation and mixing jointly determine parcel supersaturation.
Nucleation forms a sufficiently large region of a new phase within an old metastable phase. In classical nucleation theory, positive surface energy competes with bulk free-energy gain, producing a critical nucleus and a barrier to conversion. The Arrhenius nucleation time depends exponentially on this barrier; boundaries and impurities can alter it. This explains how metastability and hysteresis can persist despite a different globally stable phase.
Formation of most nuclei within an interval short compared with subsequent particle growth. A common birth period can help narrow a particle size distribution if mixing and later growth are sufficiently uniform and secondary nucleation is suppressed.
A schematic time history of growth-species concentration showing buildup, a short nucleation interval above an effective nucleation threshold, then growth above the equilibrium solubility but below substantial new nucleation. The concentration history suggests how to separate particle birth from growth; it does not establish that every particle synthesis follows that mechanism.
Formation of a critical nucleus at a pre-existing surface or interface. Favorable wetting can replace part of the cost of a free nucleus and lower its barrier; later deposition of a free particle on a support is a different process.
Ice forms from ice-supersaturated vapour at a suitable foreign surface. The relevant temperature and surface properties differ from liquid-droplet immersion freezing.
A suitable particle colliding with a supercooled droplet can trigger heterogeneous freezing. This is distinguished from nucleation around an inclusion already inside it.
Ice nucleates on an inclusion already immersed in supercooled water. The interfacial nucleation barrier, inclusion properties, droplet volume and observation time affect freezing probability.
Formation of a critical nucleus within the bulk parent phase without assistance from a pre-existing interface. Its interfacial cost competes with the bulk free-energy gain.
Ice embryos form without a foreign nucleating surface once thermal fluctuations can overcome the free-energy barrier on the relevant timescale. An apparent freezing threshold depends on droplet volume and measurement duration.
Dependence of a system's observed state on the path by which its control parameters were reached. In a first-order phase transition, a metastable local minimum may persist after another phase becomes globally stable, so increasing and decreasing the control parameter give different switching points. Nucleation and sweep rate determine actual switching; a mean-field spinodal point is a local-stability limit, not the equilibrium phase coexistence condition.
Magnetic hysteresis means that a material's magnetization depends on its field history as well as the present applied field. A closed quasistatic magnetic cycle dissipates energy per unit volume equal to its – loop area. It is relevant to remanence in permanent magnets and core losses in electrical transformers.
Magnetic coercivity is the magnitude of a reverse applied magnetic field intensity required to bring a previously magnetized specimen's magnetization to zero on a specified hysteresis branch. It depends on material, preparation and measurement history. It differs from remanent magnetization, which is the remaining magnetization at zero applied field.
Superheating is the metastable persistence of a low-temperature phase above its equilibrium transition temperature.
A liquid maintained above its equilibrium boiling temperature at the imposed pressure. A barrier to nucleation can delay its conversion into vapor, making this a metastable state.
A solid can locally exceed its solidus because a thin solute-diffusion layer changes composition faster than thermal conduction changes temperature. Near a melting phase boundary, a depleted solid can join the equilibrium solidus at the interface while the adjacent, more concentrated solid remains above its own lower solidus. The assumed single sharp interface is then thermodynamically unstable to additional melting and may be replaced by a mushy layer.
Supercooling is the metastable persistence of a high-temperature phase below its equilibrium transition temperature.
