= Interfacial tension
{title2=$\sigma$}
Interfacial tension is the excess equilibrium <free energy> per unit area associated with an interface between phases, relative to the corresponding bulk phases. For a nearly flat interface of area $A$, its leading contribution is $F_{\rm int}=\sigma A$ with tension $\sigma$.
Increasing area therefore costs free energy, explaining why a droplet tends toward a compact shape when other forces are negligible. A curved interface also produces <capillary pressure>; for a spherical interface with isotropic tension, its magnitude is $2\sigma/R$. In <nucleation>, the area cost competes with the volume-dependent gain from forming the stable phase. In a <Landau-Ginzburg theory>, the tension is computed from the equilibrium order-parameter profile across the interface, including both gradient and local free-energy contributions.
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