Using available volume to predict fluid diffusivity in random media.

We propose a simple equation for predicting self-diffusivity of fluids embedded in random matrices of identical, but dynamically frozen, particles (i.e., quenched-annealed systems). The only nontrivial input is the volume available to mobile particles, which also can be predicted for two common matrix types that reflect equilibrium and nonequilibrium fluid structures. The proposed equation can account for the large differences in mobility exhibited by quenched-annealed systems with indistinguishable static pair correlations, illustrating the key role that available volume plays in transport.