Interfacial tension ($\sigma$)

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 $F_{\mathrm{int}}=\sigma A$ 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.