Direct investigation of colloid/micro-particle behavior near grain to grain contacts

Transport of micron-sized particles and microbes through porous media is typically modeled with colloid filtration theory (CFT), based on the advective-dispersive equation. Larger colloids and micro-particles, however, are consistently observed to travel through porous media at velocities greater than the average fluid velocity, with larger particles traveling faster than smaller particles. It is commonly posited that the particles' size causes them to be physically excluded from narrow pores or streamlines near pore walls. We suggest, however, that the reduced diffusivity of the particles and pore-scale flow field heterogeneities porous media conspire to keep particles on faster-moving streamlines. To test this hypothesis, we used epi- fluorescence microscopy and digital image analysis to investigate the 3-dimensional trajectories of 5 micron latex microspheres as they were transported at various flow rates (40~350μ ms-1) through a saturated micro-model consisting of pair of contacting 1mm glass beads. At all flow rates, particles appeared to be preferentially excluded from a given volume surrounding grain-to-grain contacts. At lower flow rates some microspheres were observed to enter this low-flow zone. At all flow rates, the average velocity of particles passing through the bead pair was slightly greater than predicted. In addition, after exiting the bead pair, the average velocity of particles did not immediately return to the pre-bead velocity. We discuss the potential of these observations to explain the continuum-scale enhanced velocity of particles.