Effect of Hydrodynamics on Colloid Retention Investigated at the Pore Scale.

Understanding of colloid transport and retention mechanisms is necessary for the ability to predict colloid and colloid-facilitated transport of contaminants in soil. Traditionally, colloid transport is studied by means of column experiments, which provide valuable information on transport and retention parameters, but still carry considerable uncertainty in distinguishing colloid retention mechanisms. Pore-scale experiments serve as a complementary technique in resolving some of the uncertainties. In unsaturated porous media (a representation of unsaturated soil), colloids can be potentially retained at solid-water interface (SWI), air-water interface (AWI), and contact line. Also, straining of colloids can be distinguished as another retention process. In pore-scale experiments, colloid behavior near the sites of potential retention can be visualized and examined. In addition to physicochemical parameters (ionic strength, surface potential, contact angle, etc.), hydrodynamic parameters have been recently demonstrated to play an important role in colloid retention. This study presents a pore-scale experimental investigation of colloid retention employing a microfluidic channel and a confocal microscope with the emphasis on colloid behavior in the vicinity of AWI and contact line under dynamic conditions. The dynamic regime in the experiments is produced with the moving fronts i.e. receding and advancing air-water interfaces in the channel. It has been confirmed that hydrodynamic conditions have a considerable effect on colloid retention at AWI and contact line. In addition to experiments, numerical modeling of viscous flow and colloid transport near the AWI and moving contact line has been performed. Both lattice Boltzmann and Navier-Stokes methods have been applied to simulate the interfacial flow. Computational results will be compared with the experimental observations.