  The equation of motions and the conditions on surfaces and edges between
fluids and solids in presence of non-constant surface energies, as in the case
of surfactants attached to the fluid particles at the interfaces, are revisited
under the principle of virtual work. We point out that adequate behaviors of
surface concentrations may drastically modify the surface tension which
naturally appears in the Laplace and the Young-Dupr\'e equations. Thus, the
principle of virtual work points out a strong difference between the two
revisited concepts of surface energy and surface tension.
