Retardation of diffusion in porous media with stagnation zones

The problem of diffusion in a porous medium with stagnation zones, which reduce the rate of transport of particles, is considered. A method based on the concept of entropy barriers associated with the small size of inlets into the stagnation zone is used to solve the diffusion problem. This method made it possible to reduce the diffusion problem in 3D structures to a set of one-dimensional equations that can be solved analytically. The calculations yielded an exact value of the effective diffusion coefficient, which is reduced due to random delays in the stagnation zones, and the transition time required to attain this reduction.