Enhanced dispersion in shear-thinning fluid flow through porous media

Mixing and dispersion in random porous materials are complex processes governed by the interplay of local velocity fields and pore morphology randomness. For purely advective transport in Newtonian fluid flow within porous media at macro scale, a linear relationship is established between the dispersion coefficient and average interstitial velocity, characterized by a coefficient known as dispersivity. Previous studies highlight that dispersivity remains constant and is solely dependent on the properties of the porous medium. However, whether dispersivity is constant when considering non-Newtonian fluids, which exhibit spatially variable viscosity, is questionable. For example, for a shear-thinning fluid, viscosity decreases as the shear rate increases. In non-Newtonian shear-thinning fluids, viscosity varies across pore spaces, influenced by local shear rates, making pore size distribution and fluid pathway connectivity critical factors that impact solute transport dynamics. In this study, we present experimental insights into solute transport for both non-Newtonian and Newtonian fluid flows within porous media using glass micromodels and optical microscopy. Xanthan gum solution, representing a shear-thinning fluid, and water, as a Newtonian control, were used to evaluate the differences in transport behavior. Results indicate an enhanced dispersion effect for shear-thinning fluids in porous media. While dispersivity remains constant in Newtonian fluid flow, our experiments reveal a nonmonotonic relationship between dispersivity and injection rate for shear-thinning fluids. Unlike in Newtonian fluids, dispersivity in non-Newtonian fluids is influenced not only by the properties of the porous medium but also by the fluid's rheology. We propose a mathematical model to account for this variability, offering a framework that aligns with our experimental findings and provides insights into the mechanisms governing solute transport in non-Newtonian fluid flow within porous media.