(268e) Permeability in Fractal Colloidal Networks

In unstable colloidal gels, the viscous flow through the non-neutrally buoyant colloidal network determines the initial rate of collapse. The resistance to this flow through the network is characterized by the permeability. In fractal networks the permeability is generally accepted to be a power-law function of volume fraction, where the power is dependent on the fractal dimension of the network. To probe microscopically the influence of gel structure on permeability, we investigate this relation in structures generated by diffusion-limited cluster aggregation (DLCA), reaction-limited cluster aggregation (RLCA) and related simulation techniques. Permeabilities are determined using finite element solutions of Stokes equations for pressure-driven flow of Newtonian fluids through the networks. Geometric analyses are used to determine network pore size distributions, fractal dimensions, percolation characteristics and tortuosities, and the results used to correlate and interpret the permeability data, both numerically and through Kozeny-type theories. In addition to rigid networks, structures from DLCA-type simulations which incorporate intra-cluster relaxation are studied. Relaxation is shown to lead to larger pores and higher permeabilities. Networks of high aspect-ratio rod-shaped particles are also studied. At a given volume fraction these display smaller pore sizes and lower permeabilities than do networks of spherical particles, and only at extremely low volume fractions do their permeabilities exhibit the expected power-law dependence on volume fraction. Comparisons with available experimental data result in good quantitative agreement in some cases, but only agreement on power-law exponents in others.