Interfacial-energy difference from two contact angles (${\gamma }_{S{L}_1}-{\gamma }_{S{L}_2}={\gamma }_{L_{2}V}\cos {\theta }_2-{\gamma }_{L_{1}V}\cos {\theta }_1$)

Subtracting Young's equations for two liquids on the same solid eliminates the unknown solid–vapour interfacial energy. The result assumes equilibrium angles and an unchanged solid surface. Swelling, contamination or contact-angle hysteresis can invalidate this cancellation as a comparison of equilibrium material properties.