Evaporation effect on pore-scale concentration changes and interplay between the resident and invading liquids in micromodels

Evaporation and interplay between the resident (old) and the invading (new) water affect solute transport in the vadose zone. Evaporation can affect the pore-scale concentration changes and the new-old water interplay in porous media. In this study, we performed a series of wetting-drainage-evaporation-wetting experiments with the PDMS microfluidic devices. Two micromodels with regular and irregular patterns were applied. The working fluids were dyed alcohol (wetting phase) and air (non-wetting phase). The wetting phase fluid was drained by constant air injection rates corresponding to the capillary number 7.65E-6, 7.65E-5, and 7.65E-4. After drainage, the experiments separated into two group: (1) with and (2) without the evaporation process. For the group with evaporation, we controlled the duration of the evaporation process for different initial saturations, which were 0.1, 0.2, and 0.3, before the second wetting. We observed a non-uniform distribution of the dye concentration during evaporation. The high concentration area located at the position of air-liquid interface with higher curvature. Without the evaporation process, the invading new liquid completely displaced and mixed with old liquid after the second wetting. With the evaporation process, the old liquid remained in the micromodel without contacting with new liquid after the second wetting with the injection rate of the capillary number of 7.65E-5.