The work of adhesion per unit area, w A α β δ, is the work done on the system when two condensed phases α and β, forming an interface of unit area are separated reversibly to form unit areas of each of the αδ- and βδ- interfaces. \[w_{\text{A}}^{\unicode[Times]{x3B1} \unicode[Times]{x3B2} \unicode[Times]{x3B4} }=\gamma ^{\unicode[Times]{x3B1} \unicode[Times]{x3B4} }+\gamma ^{\unicode[Times]{x3B2} \unicode[Times]{x3B4} }- \gamma ^{\unicode[Times]{x3B1} \unicode[Times]{x3B2} }\] where γ α β, γ α δ and γ β δ are the surface tensions between two bulk phases α, β; α, δ and β, δ respectively. The work of adhesion as defined above, and traditionally used, may be called the work of separation.