Pore-scale processes that control dispersion of colloids in saturated porous media

Colloidal dispersion in porous media is a consequence of the different paths and velocities experienced by the colloids. We examined at the pore scale the effect of particle and pore size on colloid dispersion using water-saturated micromodels. The micromodels were produced with polydimethylsiloxane (PDMS), using a soft photolithography technique that allows creating transparent patterns that have dimensions in the range of those existing at the pore space. Four sizes of colloids were transported at several total pressure differences, and image analysis was used to determine particle trajectories, residence times, and dispersion coefficients through the micromodels. The magnitude of the dispersion at any given flow rate was found to be controlled by the pore-space geometry and the relative size of colloids with regards to pore channels. Dispersion coefficient and dispersivity decrease with increasing colloid size. Dispersivity is thus not just a function of pore geometry but depends on colloid characteristics. Because of their size, larger colloids travel in the center streamlines, leading to faster velocities, less detours, and thus lower range of transit times. These findings emphasize the role of particle and pore size on colloidal dispersion and have significant implications for predicting the movement of colloids through saturated porous media.