Pore‐scale processes that control dispersion of colloids in saturated porous media

Abstract

[1] Colloidal dispersion in porous media is a consequence of the different paths and velocities experienced by the colloids. We examined at the pore scale the effect of particle and pore size on colloid dispersion using water-saturated micromodels. The micromodels were produced with polydimethylsiloxane (PDMS), using a soft photolithography technique that allows creating transparent patterns that have dimensions in the range of those existing at the pore space. Four sizes of colloids were transported at several total pressure differences, and image analysis was used to determine particle trajectories, residence times, and dispersion coefficients through the micromodels. The magnitude of the dispersion at any given flow rate was found to be controlled by the pore-space geometry and the relative size of colloids with regards to pore channels. Dispersion coefficient and dispersivity decrease with increasing colloid size. Dispersivity is thus not just a function of pore geometry but depends on colloid characteristics. Because of their size, larger colloids travel in the center streamlines, leading to faster velocities, less detours, and thus lower range of transit times. These findings emphasize the role of particle and pore size on colloidal dispersion and have significant implications for predicting the movement of colloids through saturated porous media.

Colloid particle size‐dependent dispersivity

“…These contradictory findings are probably due to the very limited number of experimental data used, and the experimental conditions that led to significant mass retention caused by straining and/or attachment. The velocity enhancement of colloid particles, which is often observed in studies of colloid and biocolloid transport in fractures and micromodels, is commonly attributed to colloid exclusion from the lower-velocity regions [James and Chrysikopoulos, 2003;Auset and Keller, 2004]. However, in porous media, in addition to colloid exclusion from the lower-velocity regions [Scheibe and Wood, 2003], colloid particles experience a substantial reduction of the effective porosity [Morley et al, 1998], as illustrated in Figure 5.…”

Direct relations between morphology and transport in Boolean models

“…The outer channels act as references to calibrate the relationship between particle velocity and fluid velocity. As shown in previous experiments [32][33][34], the average particle speed within thin rectangular channels typically deviates significantly from the average fluid velocity, depending on the particle diameter, the height of the channel and the particles' gravitational height [35,36]. However, the mean fluid velocityv and mean particle velocityū in a channel are proportional [37], so thatv = c dū .…”

Measurement of permeability of microfluidic porous media with finite-sized colloidal tracers

“…This leads to faster transport of larger colloids through constricted regions since their centre of mass is further away from the matrix surfaces (Auset and Keller 2004). Additionally, hard core repulsion leads to the displacement of particles perpendicular to streamlines (streamline crossing), which breaks the time-reversibility of particle trajectories even in the Stokes limit (Loutherback et al 2009).…”