The work of adhesion per unit area, , 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. where , and 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.