A porous medium is a solid with voids distributed more or less uniformly throughout the bulk of the body.
The basic characteristic of this medium is porosity. The bulk porosity Π of a material is defined as the ratio of void volume Vv to body volume Vo, Π = Vv/Vo. Since the remaining portion Vs of the total volume of the material is in the form of a solid “skeleton”, then
For example, the porosity of porous materials with the skeleton formed by spherical particles with diameter dp can be found from the relation
where Np is the number of particles per unit volume. These spheres can be arranged in various ways (Figures 1a and b). The cubic arrangement of spheres of the same diameter is characterized by a porosity of 0.476, while at a denser, rhombic, packing the porosity reduces to 0.259 (theoretically, this is the minimum porosity of packing of uniform spheres without deformation of the solid). The real porosity generally is estimated using its relation to density ρΣ = ρ0(1−Π) or Π = 1(ρΣ/ρ0), where ρΣ and ρ0 are the densities of the medium and of the solid material forming its skeleton, respectively.
Permeability (or gas permeability) is the property which gives a measure of the gas flow through a porous medium exposed to a pressure difference. The superficial velocity V of fluid flow depends on permeability and pressure gradient in accordance with a modified Darcy equation