Bounds for the complex dielectric constant of a two-phase composite

We study the effective dielectric constant of a two-phase composite for the case where the dielectric constants of the two components are complex. Milton has derived a sequence of narrowing bounds in the complex plane limiting the possible values of the effective dielectric constant, assuming that certain geometrical coefficients characterizing the medium are known. We cast Milton's bounds in a different form, making use of auxiliary functions introduced by Bergman. We derive explicit expressions for the bounds up to third order and compare values for the corresponding geometrical coefficient derived for different geometries.