Solute spreading enhancement by drainage-imbibition cycles in unsaturated porous media

Transport of solute species under unsaturated conditions, where multiple immiscible fluids coexist, is a common occurrence in various environmental and engineering applications within subsurface porous media. In this study, we integrate microfluidic experiments and direct simulation to analyze the effect of successive drainage-imbibition cycles on solute transport, a process that is not yet well understood in the context of unsaturated porous media. The spatial distribution of water and air is found by experiments, and the transport process is modeled by high-fidelity direct numerical simulation, remarkably reducing computational costs and enabling individual investigation of injection cycles. We show that cycles of non-wetting and wetting phases increase the rate of solute spreading non-monotonically by altering the volume and tortuosity of the percolating pathways of the carrier fluid (where transport occurs). Drainage-imbibition cycles reduce the saturation of the carrier fluid by entrapping a higher volume of the non-wetting phase, thereby decreasing the magnitude of mobile pathways. Simultaneously, cyclic injection increases the length of the pathways that solute species must travel through the hysteresis phenomenon. Through the analysis of mobile and immobile pathways, we demonstrate that the effect of drainage-imbibition cycles on the mixing of solute species becomes negligible after 1-2 cycles. These results advance our understanding of the complex dynamics of unsaturated transport, providing new insights into the impacts of cyclic variations in the soil water content on contaminants and nutrients transport.