Hydraulic radius and mesh size of gels

For materials with high porosity, the hydraulic radius (r_h) is not proportional to the characteristic dimension of the pore space. Techniques (e.g., NMR) that measure r_hwill give much larger "pore sizes" than techniques (e.g., thermoporometry) that measure pore entry radii. In this note, estimates of the relation between hydraulic radius and pore size of gel networks are obtained from cell models. The "constant" in the Kozeny equation, which relates permeability to r_h, is strongly density-dependent at low densities. For gels it is suggested that the permeability is more directly related to the mesh size of the network than to the hydraulic radius. The mesh size is also critical for immobilization of dyes or enzymes in a gel, and r_his a misleading measure of the ability of a network to entrap a large molecule.