Funneling of flow into grain-to-grain contacts drives colloid-colloid aggregation in the presence of an energy barrier.

Deposition behaviors of carboxylate-modified polystyrene latex microspheres (five sizes ranging from 0.1 to 2.0 microm) were examined in packed porous media, impinging jet, and porous media-packed flow chamber systems under a variety of environmentally relevant ionic strength and flow conditions in the presence of an energy barrier to deposition. Temporally constant deposition rate coefficients were observed for all microsphere sizes under baseline conditions, whereas temporal increases in colloid deposition rate coefficients (ripening) occurred for all microsphere sizes in response to slight increases in solution ionic strength and slight decreases in fluid velocity. This transition from "clean bed" deposition to ripening was triggered by relatively subtle changes in solution chemistry and fluid velocity. Direct observation of colloid deposition in a flow chamber packed with porous media revealed that colloidal aggregates formed at grain-to-grain contacts in the porous media. The absence of ripening in an impinging jet system (unbounded flat surface) examined under equivalent conditions to the packed porous media further indicated that colloid aggregation was driven by the funneling of fluid into the grain-to-grain contacts. Comparison of colloid breakthrough in porous media comprised of smooth-spherical versus angular grains demonstrated that the propensity to trigger ripening increased with the number and length of grain-to-grain contacts.