Bulk properties of two-phase disordered media. I. Cluster expansion for the effective dielectric constant of dispersions of penetrable spheres

We derive a cluster expansion for the effective dielectric constant €* of a dispersion of equal-sized spheres distributed with arbitrary degree of impenetrability. The degree of impenetrability is characterized by some parameter A whose value varies between zero (in the case of randomly centered spheres, i.e., fully penetrable spheres) and unity (in the instance of totally impenetrable spheres). This generalizes the results of Felderhof, Ford, and Cohen who obtain a cluster expansion for €* for the specific case of a dispersion of totally impenetrable spheres, i.e., the instance A = 1. We describe the physical significance of the contributions to the average polarization of the two-phase system which arise from inclusion-overlap effects. Using these results, we obtain a density expansion for €*, which is exact through second order in the number density p, and give the physical interpretations of all of the cluster integrals that arise here. The use of a certain family of equilibrium sphere distributions is suggested in order to systematically study the effects of details of the microstructure on €* through second order inp. We show, furthermore, that the second-order term can be written as a sum of the contribution from a reference system of totally impenetrable spheres and an excess contribution, which only involves effects due to overlap of pairs of inclusions. We also obtain an expansion for €* which is exact through second order in <P2' where <P2 is the sphere volume fraction. We evaluate, for concreteness, some of the integrals that arise in this study, for arbitrary A, in the permeable-sphere model and in the penetrable concentric-shell model introduced in this study.