(357e) A Microscopic Analog of Saturated Soil for the Study of Colloid-Facilitated Transport of Contaminants

Authors: 
Guo, Y., Colorado School of Mines
Wu, N., Colorado School of Mines
Neeves, K. B., Colorado School of Mines
Yin, X., Colorado School of Mines

Although many contaminants have low solubility in water, they can adsorb on mobile colloids and travel with underground water flow over 80 m/year. Therefore, understanding colloid-facilitated transport is important for predicting the spatial and temporal distribution of contaminants. However, the detailed flow pathway and transport mechanism of those colloids are still elusive due to the conventional non-transparent experimental methods (e.g. soil cores) and the physical and chemical heterogeneity of the pore space. Here, we describe a microfluidic device to construct ananalog of saturated soil. Soft lithography was used to fabricate a device consisting of a rhombus chamber, inlet, and outlet channels in polydimethylsiloxane. The chamber dimensions are 1 mm for each side, 20 μm in height and 80° for the apex angle. The cross section of the inlet and outlet are 100μm×20μm and 1286μm×10μm, respectively. Due to the height difference between the chamber and outlet channel, ~4800 polystyrene spheres (15 µm) weretrapped and packed one-by-one to form the porous medium within the chamber.The physical and chemical heterogeneity in the bead-packed arrays can be decoupledby using different sized beads with varying surface properties. The linear velocity of flow at inlet was 1μm/s to 25 mm/s, yielding Peclet numbers of that range from 10 to 105. We also used the Lattice Boltzmann (LB) method to simulate the flow field and colloidal transport in the media. The computational domain was faithfully re-constructed from the experimental porous media using a custom image processing routine. We used the Canny algorithm to detect the outline of each bead and obtained its x-y coordinate based on the circularity of each outline. The z-position was estimated by adjusting focal plane to achieve the sharpest contrast of the bead edge. Using this approach, we measured and simulated colloidal transport trajectories and extracted key parameters such as the breakthrough curve, effective retention, and entrapped colloids distribution. We will discuss the comparison between our experiments and LB simulations results, which reveal the rate-limiting transport mechanismunder different flow conditions.