Colloids are ubiquitous in the natural environment, playing an important role in facilitating the transport of absorbed contaminants. However, due to the complexities arising from two-phase flow and difficulties in three-dimensional observations, the detailed mechanisms of colloid transport and retention under two-phase flow are still not well understood. In this work, we visualize the colloid transport and retention during immiscible two-phase flow based on confocal microscopy. We find that the colloid transport and retention behaviors depend strongly on the flow rate and pore/grain size. At low levels of saturation (high flow rate) with the wetting liquid mainly present as pendular rings, the colloids can aggregate at the liquid filaments in small-grain packings and are uniformly distributed in large-grain packings. Through theoretical analysis of the pendular ring geometry, we elucidate the mechanism responsible for the strong dependence of colloid clogging behavior on solid grain size. Our results further demonstrate that even at dilute concentrations, colloids can alter the flow paths and the wetting fluid topology, suggesting a strong two-way coupling dynamics between immiscible two-phase flow and colloid transport and calling for improved predictive models to incorporate the overlooked clogging behavior.
Nan Xu,Xinxing Huangfu,Zuling Li,Zhengying Wu,Duo Li,Mo Zhang
Due to a wide range of applications in industrial fields, engineered nanomaterials (ENMs) have a high potential to enter the soil. The soil's major component of clay likely dictates the fate and transport of ENMs in the subsurface. Currently, few studies are available on the fate and transport of nanoparticle silica (nSiO 2 ) in the presence of clay particles. Therefore, the sedimentation and transport of nSiO 2 with two representative clays (montmorillonite (M) and kaolin (K)) in porous media were investigated in monovalent (Na + ) and divalent (Ca 2+ ) ion solutions with multiple characterizations including SEM/TEM-EDX, zeta potentials , particle sizes and colloid transport modeling. It was shown that nSiO 2 -nSiO 2 homoaggregates and nSiO 2 -K (or M) heteroaggregates dominated in the nSiO 2 -clay nanoaggregate suspension. A distinct decrease in the stability and transport of nSiO 2 -M (or K) in NaCl solution and an increase in CaCl 2 occurred when M or K was added to the nSiO 2 suspension at pH 6.0. This was attributed to the faster settlement of the individual M or K in NaCl vs. the better stability in CaCl 2 (compared to nSiO 2 alone). Particularly, more negative individual M platelets occurred in the high NaCl solution until extensive flocculated structures built up, which contributed to the faster deposition of nSiO 2 -M compared to nSiO 2 -K, even though the nSiO 2 -M was more negatively charged. Comparably, the effect of M and K on the fate and transport of nSiO 2 almost disappeared at pH 9.0. The values of the first-order attachment/detachment rate coefficients (k 1 /k 1d ) and first-order straining coefficient (k 2 ) obtained from two-site kinetic attachment model fitting are responsible for the deposition of nSiO 2 -clay nanoaggregates in sand. This study suggests potential groundwater contamination due to the clay-facilitated transport of ENMs in calcareous soil .
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