Interfacial tensions of three fluid phases in equilibrium

Abstract

It is suggested that it may be a general thermodynamic law that when three mutually saturated fluid phases are in equilibrium, the three interfacial tensions σ satisfy the triangle inequality σ_max < σ_med + σ_min, where σ_max is the largest of the three, σ_med the median, and σ_min the smallest. An argument for the inequality is given, and systems in which to test it experimentally are suggested. The quantitative law by which the interfacial tensions vanish as the three fluid phases become identical at a tricritical point is derived by an adaptation of an earlier argument on the vanishing of the interfacial tension on approach to the tricritical point in phase−separated ³He−⁴He mixtures.