Colloid transport in saturated porous media under unfavorable conditions has been extensively studied in recent years. Deposition in the secondary energy minimum, straining and retention in rear stagnation regions are the major retention mechanisms proposed to date for unfavorable conditions. Theoretical torque analysis marks the key role of combining effects of hydrodynamics and colloids-surface interactions on colloid retention. However, the complete understanding of the processes and the factors controlling colloid retention is still not clear due to the lack of direct evidence. Using high speed confocal microscope, we experimentally track the three-dimensional colloid motions in both bulk solution and grain-to-grain contact regions. The first time constructed 3D colloid trajectories depict the complex colloid behaviors in porous media, especially in grain-to-grain contact regions, at both qualitative and quantitative basis.
For confocal experiments, fluorescent carboxylate-modified colloids with 1µm diameter are used as modeled colloids and rectangular flow cells packed with hydrophilic glass beads are employed as porous media. Three-dimensional colloid movement is captured in time series at a very fast speed and corresponding trajectories can be constructed by locating the coordinates of colloids at different time points. Preliminary results show that most colloids moving through grain-to-grain contact regions tend to follow the streamline to pass around grain-to-grain contact points. A small fraction of colloids can approach grain-to-grain contact points via diffusion, which could finally be strained by two adjacent surfaces. We also suggest the less importance of rolling and sliding motions to drive colloid movement to grain-to-grain contact region, since most colloids moving along the surface tend to return to bulk flow before they enter grain-to-grain contact regions. Ongoing work mainly focuses on the investigation of colloid trajectories in sand system. Combining with the results in glass beads system will allow us to describe colloid transport more quantitatively in both idealized and natural-like porous media.