Approximate symmetry laws for percolation in complex systems: Percolation in polydisperse composites.

The concept of so-called global symmetry of percolation models is discussed and extended to multicolored models. An integral equation is obtained, which determines the partial percolation probabilities P(a) for sites of color a. This equation is applied to a polydisperse particulate composite: a mixture of conducting (of relative fraction x(m)) and nonconducting spheres with distributions of sizes n(m)(R) and n(i)(R), respectively. We find the probability P(R) for a conducting particle of radius R to belong to the percolation cluster as a function of x(m) and a functional of n(m)(R') and n(i)(R'). The percolation threshold x is shown to decrease with increasing dispersion Delta of particle sizes. A simple law x=1/(3[1+(Delta/4)]) is obtained in the range of moderate dispersions. The theory is applicable also to a mixture of electronic and ionic conductors.