Stress-strain relations in nonuniform equilibrium fluids

The local stress tensor is defined as the field thermodynamically conjugate to the local strain tensor. Any free-energy change resulting from a strain can then be expressed either in terms of the stresses or in terms of the equivalent single-particle external force field. Using this equivalence, one obtains a well-defined expression for the local stress tensor in terms of this force field. On this basis we obtain unambiguous expressions for the surface tension and for the location of the surface of tension of a planar interface.