A CONTINUUM ANALYSIS OF SURFACE-TENSION IN NONEQUILIBRIUM SYSTEMS

The intermolecular forces that cause surface tension in multiphase sys tems at equilibrium give rise to pressure gradients in nonequilibrium systems. The present paper treats such systems within a framework of t hermodynamics and continuum mechanics and uses a generalization of the conceptual experiment of Rowlinson and Widom which applies to systems with curved interfaces and to nonequilibrium situations where the pha se interfaces are not fully developed. The pressure tensor has a compo nent p(n) acting in the direction normal to the concentration gradient and a component p(t) acting in the plane tangent to the surface of co nstant concentration where p(n) - p(t) = kappa(delc)2 The concentratio n gradient causes an additional volumetric force not present in homoge neous fluids: dF/dV = - kappa(delc)2 (1/R1 + 1/R2) n + del(s)kappa(del c)2 Here, kappa is the gradient energy parameter appearing in the Land au-Ginzburg functional, c is the concentration of the key component, R 1 and R2 are the principal radii of curvature for the surface of const ant constration, n is a unit vector normal to that surface, and del(s) is the gradient along the surface. The volumetric force generates pre ssure gradients in systems with curved interfaces at equilibrium or ca n drive flow in nonequilibrium situations.