Modeling microscale flow and colloid transport in saturated porous media Hui Gao a , Jie Han b , Yan Jin b , and Lian-Ping Wang a,* a Department of Mechanical Engineering, University of Delaware, Newark, DE 19716-3140, USA b Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19716-3140, USA Abstract The microscale flow of water in natural soil porous media affects the transport of colloids and other con- taminants contained in groundwater. In this study, two completely different computational approaches are applied to simulate pore-scale viscous flows in saturated porous media. The first is the lattice Boltzmann method based on the mesoscopic lattice Boltzmann equation. The second method, referred to as Physalis by its developers, is a hybrid representation in which a numerical solution based on discretized Navier-Stokes equation is coupled with analytical Stokes flow solutions valid locally near the surface of porous-medium grains. The porous medium is represented by a channel partially filled with circular (in 2D) or spherical (in 3D) particles. We demonstrate that the two methods can produce almost identical viscous flow at the pore scale, providing a rigorous cross-validation for each approach. A Lagrangian particle-tracking approach is then used to study the transport of colloids in these flows, considering hydrodynamic forces, Brownian force, and electro-chemical surface-interaction forces acting on each colloid. Due to the competing effects of hy- drodynamic transport and electro-chemical interactions, it is shown that enhanced removal of colloids from the fluid by solid surfaces occurs when the residence time of colloids in a given flow passage is increased, in qualitative agreement with pore-scale visualization experiments using confocal microscopy. Key words: Porous medium; saturated soil; lattice Boltzmann equation; colloid; deposition. 1 Introduction Understanding the mechanisms of colloid retention and transport in soil porous media is of importance to the management of groundwater contamination by contaminants that could sorb to and migrate with mobile colloids or by pathogenic microorganisms. Even for the relatively simple case of saturated soil and aquifer, the transport of colloids and their attachment to solid surfaces are governed by a multitude of physical processes: transport by low-speed microscale water flows, Brownian motion due to random thermal fluctuation, and a variety of electro- chemical interactions between colloids and solid surfaces [8]. These physical processes together encompass a large range of length scales from millimeter scale to nanometer scale, with each possibly dominating the motion of a colloid depending on the colloid’s relative location within a pore-scale passage. A quantitative modeling tool * Corresponding author. Email address: lwang@udel.edu (Lian-Ping Wang). Preprint submitted to Elsevier Science 15 February 2008