  The concept of a non-equilibrium interfacial tension, defined via the work
required to deform the system such that the interfacial area is changed while
the volume is conserved, is investigated theoretically in the context of the
relaxation of an initial perturbation of a colloidal fluid towards the
equilibrium state. The corresponding general formalism is derived for systems
with planar symmetry and applied to fluid models of colloidal suspensions and
polymer solutions. It is shown that the non-equilibrium interfacial tension is
not necessarily positive, that negative non-equilibrium interfacial tensions
are consistent with strictly positive equilibrium interfacial tensions, and
that the sign of the interfacial tension can influence the morphology of
density perturbations during relaxation.
