Microscale Numerical Analyses of Charged Porous Materials

Estimation of effective diffusion coefficients is essential to be able to describe the diffusive transport of solutes in porous media. To estimate the apparent diffusion coefficients, the values of tortuosity and porosity should be known first. Estimation of tortuosity is intricate, particularly with increasing microgeometry complexity in porous media. For many engineering materials, estimation of effective diffusion coefficients have been mostly based on phenomenological equations with no link to underlying microscale properties of these charged materials. In this research a numerical method based on a recently proposed up-scaled Poisson-Nernst-Planck type of equation (PNP) and its microscale counterpart is employed to estimate the tortuosity and thus the effective and apparent diffusion coefficients in thin charged membranes. Beside this, a new mathematical approach for estimation of tortuosity is applied and validated. Steady-state diffusion studies dominate this report; however, the last section of this book briefly introduces transient diffusion through bentonite.