Three-Dimensional Visualization and Modeling of Colloid Transport in Saturated Porous Media Under Unfavorable Conditions.

Transport of colloids in saturated porous media has been extensively studied in recent years.  Under unfavorable conditions, secondary energy minimum deposition and straining have been proposed as the main mechanisms of colloid retention. Despite recent intensive investigation and progresses, a complete understanding of the processes of colloid retention and the factors that control them is still lacking.  Using high-speed confocal microscope and micromodels, we experimentally track the 3-D motions of colloids and visualize their attachment, straining, and release at the pore scale in real time.  Employment of novel 3-D numerical methods provides parallel theoretical analysis of pore-scale flow fields and their effect on the colloid retention processes.

For confocal experiments we use fluorescent carboxyl-modified spherical colloids and square glass capillary micromodels packed with hydrophilic glass beads. A special focus of this study is visualization and analysis of colloid behavior at grain-to-grain contact regions by constructing 3-D images as a function of time. Preliminary results show that under unfavorable deposition conditions, colloids tend to move along the collector surface due to translation or rotation and be retained in flow stagnation regions. Such retention is weak and retained colloids are easily released to bulk suspensions by increasing flow rate. On-going work includes investigating the coupling effects of hydrodynamics and solution chemistry on different colloid retention mechanisms by employing various flow rate and solution ionic strength. Effects of colloid size are also examined. Computational efforts consist of pore-scale flow simulation using a mesoscopic lattice Boltzmann method (LBM) and direct Lagrangian tracking of individual colloid.  The LBM approach can handle both regular and random packing of grains. The numerical colloid tracking takes into account relevant forces and torques exerted on the colloids. Combining the experimental observation with modeling will allow us to distinguish and quantify different mechanisms of colloid retention at each interface or location.