Old and new wetting liquids separation in grain-based pattern micromodel during wetting cycles

In soils, old (residual) water can persist despite new (invading) water infiltration, potentially due to trapped air bubbles isolating old water pockets. However, the mechanisms behind air bubble formation and liquid separation remain unclear. This study aims to investigate the interaction between new and old wetting liquids and the mechanism that traps air bubbles, isolating old water. Using a grain-based pattern micromodel, we examined these processes during a repeated wetting cycle (wetting-drainage-evaporation-wetting). To enhance visualization and evaporation, we used dyed alcohol solutions as the wetting phase, with air as the non-wetting phase. Results indicate that a liquid film on grains plays a crucial role in old liquid entrapment, influenced more by soil wettability than initial liquid content. Strong wettability resulted in significant film development, allowing old and new liquids to connect and potentially mix. In contrast, weak wettability led to air bubble entrapment, isolating old water and preventing its displacement. The findings highlight that soil wettability and wetting film development are key factors in the interaction between new and old wetting liquids.