Static light scattering resolves colloid structure in index-matched porous media

The fractal dimension of colloid aggregates within porous media is resolved by static light scattering in optical cells containing index-matched granular Nafion. [Display omitted] ► A new technique probes the spatial and temporal scales of colloidal phenomena. ► Measured fractal dimensions are identical inside and outside porous media. ► The technique quantifies colloid aggregate restructuring by fluid shear. Colloidal phenomena play an important role in natural porous media, where they influence soil structuring, contaminant migration, filtration, and clogging. Several methods are available to measure pore space geometry within porous media, but these methods have limited applicability when the relevant physical, chemical, or biological processes are dominated by dynamic colloidal phenomena. Here we report a new technique to quantify colloid aggregate structure as a fractal dimension using static light scattering within index-matched porous media (granular Nafion). We validate the method by obtaining consistent results for scattering in suspensions and in porous media, and verify that multiple scattering at environmentally relevant colloid concentrations does not affect the determination of fractal dimension. We also observe restructuring of aggregates during homogenization in the porous media, indicated by an apparent increase in fractal dimension, which can be explained by an analysis of the fluid shear stress caused by repeated inversions of test tubes either containing or not containing granular media. This technique will permit progress in obtaining fundamental descriptions of colloidal phenomena in porous media.