New pore-size parameter characterizing transport in porous media.

We introduce a well-defined geometrical parameter, $\ensuremath{\Lambda}$, related to dynamically connected pore sizes in composite materials. $\ensuremath{\Lambda}$ describes the effects of an internal boundary layer on a variety of processes including electrical surface conduction, high-frequency viscous damping of acoustic waves, and healing length effects in fourth sound. We argue that $\ensuremath{\Lambda}$ is also related to the dc permeability to flow of a viscous fluid.