Fluid mass transport through a porous solid was studied by carrying out non-equilibrium molecular-dynamics simulations for a simple model. It was found that through the porous solid, the steady state density distribution was not linear, even in the case of a quite small current. Hence, Fick's law could not be applied simply. Nevertheless, the density distribution was piecewise linear. An adequate interpretation could be made by applying Fick's law piecewise with different diffusion coefficients in different regions. Thus a distinct diffusion coefficient could be unambiguously defined for the interfacial region, which could be seven times smaller than the diffusion coefficient in the interior of the porous solid. So the surface crossing was a limiting step in the overall process of mass transport through a porous solid.

Fluid Diffusion through a Porous Solid: Nonequilibrium Molecular-Dynamics Simulation. Dong, W., Luo, H.: Physical Review E, 1995, 52[1], 801-4