Gibbs film elasticity

Pertains to a film element of a soap film changing in area at constant mass and is the differential change of its surface tension with relative change in area $A$ ,

$E = A ( ∂ σ ∂ A ) T , p , n i$

where $T$ is the thermodynamic temperature, $p$ is the pressure, and $n i$ is the amount of substance of the species $i$ .

Source:

PAC, 1972, 31, 577 (Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry) on page 615