= Supercooling
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Supercooling is persistence of a phase below the equilibrium temperature where a lower-temperature phase becomes favorable. A liquid remaining unfrozen below its equilibrium melting temperature is the standard example.
Creating a crystal nucleus gains bulk free energy but costs interfacial energy. Small embryos can disappear, while one exceeding the critical size can grow; this <nucleation barrier> permits a <metastable> liquid. A surface or suspended inclusion can lower the barrier through <heterogeneous nucleation>, whereas <homogeneous nucleation> occurs within the bulk. Measured freezing onset consequently depends on droplet size, purity and waiting time. Growth releases <latent heat> and can temporarily warm the remaining material toward the coexistence temperature, separating the onset of nucleation from the later heat-removal problem.
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