Electroosmosis in Non-homogeneously Charged Microporous Media

Abstract:

Understanding electroosmosis in microporous media is important for species separation in electrochromatography, flooding control in fuel cells, contaminants removal in soil and electrokinetic displacement in oil extraction. Because of the complexity of transport and randomness of porous media, investigation of electroosmosis in porous media is mostly based on the assumption that the solid surface is homogeneously charged. However, based on the surface complexation model, the surface charge and zeta potential are much dependent on the local chemical environment of the solution. Therefore, variations of concentration, pH and temperature and so on will lead to a non-homogeneous charge distribution in the porous media. In addition, since the characteristic length of the pore is comparable to the characteristic length of the double layer in the microporous media, overlap of the double layer occurs and this will further regulate the surface charge distribution. In this work, we propose a 3-D lattice Poisson-Boltzmann model for electroosmosis in porous media with non-homogeneous charge for above reasons. We employ a generation-growth method to reproduce the 3-D random microstructures of natural porous media and solve the Navier-Stokes equation and Poisson-Boltzmann equation using lattice Boltzmann method. To determine the surface charge and zeta potential, we adopt the Basic-Stern model to describe the electrical double layer. Electroosmotic permeability is investigated for various cases with non-homogeneous charge and compared to the results with homogeneous charge to determine the validity of the homogeneous charge assumption.