The work done on a system when the process of immersional wetting involving unit area of phase β is carried out reversibly: \[w_{\text{W}}^{\unicode[Times]{x3B1} \unicode[Times]{x3B2} \unicode[Times]{x3B4} }=\gamma ^{\unicode[Times]{x3B2} \unicode[Times]{x3B4} }- \gamma ^{\unicode[Times]{x3B1} \unicode[Times]{x3B2} }\] where γ α β and γ β δ are the surface tensions between two bulk phases α, β and β, δ, respectively.