The reversible work of formation of unit area of new surface by cleavage. It has previously been called specific @S06193@, surface energy, surface tension or surface @F02515@. The symbol recommended is \(\gamma _{\pi }\) in preference to \(\sigma \) because of the confusion with @S06159@. The superficial work \(\gamma _{\pi }\) is a scalar quantity and consequently is @I03353@.