Interface structure

Phase-separated fluids are analyzed from several viewpoints: local thermodynamic, in which all density correlations are ignored; mean field, which correctly treats those at medium range; and a modified Kac-Siegert form which only coarse-grains those at short range. The last form appears as an ensemble average over tail potential fields, and is treated variationally with a Gaussian ansatz. Application to a planar interface shows the anticipated surface softening as any confining field is removed, and in statistically homogeneous condensation, a form of the familiar level set description assumes relevance. Suggestions are made as to removing the Gaussian assumption.