Diffusiophoresis of colloids in partially-saturated porous media

It is known that structural heterogeneity induced by the distribution of the water and air phases creates complex flow patterns with a broad distribution of flow velocities, which in turn control key aspects of transport including arrival and residence times, dispersion and spatial distributions of dissolved salts and suspended colloidal particles. Stagnation zones serve as hot spots where colloidal particles can get entrapped, providing a challenging task in controlling their transport. Recent investigations in simple confined geometries suggest diffusiophoresis, the colloid migration driven by local salt gradients, to be an efficient mechanism to control colloidal migration. However, despite its potential, diffusiophoresis in complex porous media remains poorly understood. We use detailed numerical simulations to unravel the effects of diffusiophoresis occurring at pore-scale on the macroscopic dispersion of colloids in partially-saturated porous media with different water-saturation degrees. Diffusiophoresis can promote particle retention or removal, depending on the diffusiophoretic mobility. For fully-saturated media, the pore-scale dynamics due to diffusiophoresis are manifested in the long-time tailing of the breakthrough curves. For partially-saturated media as the degree of water-saturation decreases and flow heterogeneity increases, we observe accumulation and depletion effects in the colloid breakthrough curves which can be traced back to trapping and release in dead-end zones. Finally, our results suggest that colloid mobilisation and retention due to diffusiophoresis can be controlled by the flow rate of the injected salt solution.