We introduce a well-defined geometrical parameter, $\Lambda$, related to dynamically connected pore sizes in composite materials. $\Lambda$ 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 $\Lambda$ is also related to the dc permeability to flow of a viscous fluid.

• Received 12 June 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.2564