The fact that the effective permeability has dimensions of area raises the question whether has an interpretation as a length scale. The traditional answer to this question is provided by hydraulic radius theory which uses the approximate result (5.61) for the capillary tube model to postulate more generally the relation
where the length scale is still given by the hydraulic radius, and the geometrical tortuosity was replaced by the electrical tortuosity defined as . Because the length scale is still given by the hydraulic radius this theory is still faced with the objection that the hydraulic radius contains contributions from the dead ends which do not contribute to the transport.
An alternative was proposed in [318, 43]. It postulates where is a length scale related to the breakthrough pressure in mercury injection experiments. The length scale is well defined for network models with a broad distribution of cylindrical pores. A dynamical interpretation of was proposed in [319, 320, 328] as
where is the unknown exact solution of the microscopic dielectric problem. This “electrical length” is expected to measure, somehow, the “dynamically connected pore size [319, 328, 4]. The interpretation of within local porosity theory is obtained by eliminating between the result (5.48) for the conductivity, and equation (5.95) for the permeability. This yields in general
where and are the local electrical conductivity and the local permeability. Thus involves macroscopic geometrical information through and and microscopic dynamical and geometrical information through the local transport coefficients. If one assumes the hydraulic radius expressions and locally and the expression valid for large measurment cells, then it follows that becomes the local hydraulic radius . This expression is no longer proportional to the total internal surface but only to the average local internal surface, and thus the argument against hydraulic radius theories no longer apply.