FLUID DIFFUSION THROUGH A POROUS SOLID - NONEQUILIBRIUM MOLECULAR-DYNAMICS SIMULATION

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