Integrated Grain and Pore Networks

Comparisons demonstrate that under equivalent conditions, deposition efficiencies are greater in porous media relative to flat surfaces (e.g. an impinging jet where flow is directed normal to the surface), by factors of 2-50 or more for a wide range of colloid sizes (100 nm to 2 pm) and fluid velocities. Much of this “excess" deposition is reversible with respect to IS. According to mean-field theory, the observed release of retained colloids upon IS reduction implicated the secondary energy minimum as the mechanism of association of the colloids with the grain surfaces since mean-field theory predicts an insurmountable barrier to detachment from the primary minimum.

More recently, incorporation of representative nanoscale heterogeneity into simulations challenges the above inference, since attached colloids (primary minimum) may be released upon IS reduction if the colloid-surface interaction becomes net repulsive as the zone of interaction expands relative to the heterodomains, as described earlier.

The source of excess deposition at the porous media scale may include wedging into grain-to-grain contacts, retention at rear flow stagnation zones, and also accumulation of colloids in the near surface fluid domain. The near surface fluid domain corresponds to significant colloid-collector interactions and, under unfavorable conditions, involves secondary minimum association with the surface outside the repulsive barrier until immobilization occurs via attractive interaction with heterodomains.

Because all colloids must necessarily pass through secondary minimum association prior to being immobilized or otherwise retained, accumulation of colloids in the near surface fluid domain drives the retention processes described before and is facilitated by transfer of colloids from upgradient to downgradient adjacent grains via their grain-to-grain contact. Therefore, the integrated grain and pore networks (alignment of grain-to-grain contacts with the local flow field) greatly influence the likelihood of accumulation of colloids in the near surface fluid domain.

Li et al. observed hyperexponential profiles of retained colloids in glass bead-packed columns (uniform 510 µm, 1 µm carboxylate-modified polystyrene latex colloids, 4md^-1 average pore water velocity, 3 pore volume injection, 7 pore volume elution, etc.), whereas Li et al. observed nonmonotonic retention profiles in quartz sand-packed columns of equivalent grain size under equivalent conditions. Tong et al. showed that nonmonotonic profiles may be transient, with the maximum in the retention profile migrating downgradient with increased elution. The number of grain-to-grain contacts per grain, and the length of grain-to-grain contacts, was greater in the quartz sand relative to the glass beads.

These observations suggest that hyperexponential retention profiles reflect conditions where colloid immobilization exceeds colloid retention without immobilization, as partly dictated by limited number and length of grain-to-grain contacts (greater colloid expulsion to bulk fluid from rear flow stagnation zones). Nonmonotonic retention profiles arise when colloid retention without immobilization in the near surface fluid domain exceeds colloid immobilization, as facilitated by accumulation of near surface colloids as facilitated by greater numbers of, and longer, grain-to-grain contacts. Recent simulations show that these behaviors emerge from pore-scale simulations incorporating representative nanoscale heterogeneity.