Diffusiophoresis of colloids in partially saturated three-dimensional porous media

The term diffusiophoresis refers to the phenomenon by which colloids migrate following salt concentration gradients. The strength of this effect actually follows a logarithmic scaling, and thus the diffusiophoretic drift can be very pronounced over small salt concentration variations when those concentrations are low. Therefore, this phenomenon could potentially be engineered for colloid manipulation. In the context of porous media, the emerging colloid transport behaviors that can arise from diffusiophoresis are still not fully understood. Some recent two-dimensional experimental and simulation results have demonstrated upscaled effects of diffusiophoresis in porous media. Depending on the sign of both the diffusiophoretic coefficient and the concentration gradients, diffusiophoresis can lead to either increased retention or increased flushing of colloids in the medium. This is magnified in the presence of pronounced small-scale medium heterogeneities, which are able to support concentration gradients for relatively longer times. Because water velocity fields are a lot more heterogeneous in the presence of air, unsaturated conditions enhance the accumulation or depletion of colloids. How these two-dimensional findings apply to natural three-dimensional porous media is still unclear, since recent work has shown that, compared to their two-dimensional counterparts, water velocity fields in unsaturated three-dimensional media tend to display a markedly lower occurrence of very low velocity regions, and a distinct non-monotonic behavior of mixing as a function of saturation. These features affect solute gradients and are therefore expected to have an impact on diffusiophoretic drift. Hence, the goal of the work presented here is to elucidate the effective behavior of diffusiophoresis in three-dimensional unsaturated porous media. We perform high-performance computing pore-scale simulations of Stokes flow at various saturation degrees and solute–colloid transport with diffusiophoretic drift. We identify the upscaled behaviors and their dependence on parameter configurations, and we compare them to the two-dimensional case. Our results provide new insight into how diffusiophoretic mechanisms operate under realistic three-dimensional unsaturated flow conditions, revealing scenarios in which diffusiophoresis can substantially influence colloid mobility and retention in porous media.