Experimental investigation of water distribution in two-phase zone during gravity-dominated evaporation

TL;DR

This study investigates how wettability, particle size, and gravity influence water distribution and saturation in the two-phase zone during evaporation in porous media, revealing the roles of connected films and disconnected pockets.

Contribution

It provides experimental and numerical insights into water distribution in the PSZ, highlighting the effects of wettability and particle size on saturation and water loss during evaporation.

Findings

Saturation decreases with increasing Bond number.

PSZ extent and saturation increase with wettability.

Connected liquid films are more prevalent in hydrophilic systems.

Abstract

We characterize the water repartition within the partially saturated (two-phase) zone (PSZ) during evaporation out of mixed wettable porous media by controlling the wettability of glass beads, their sizes, and as well the surrounding relative humidity. Here, Capillary numbers are low and under these conditions, the percolating front is stabilized by gravity. Using experimental and numerical analyses, we find that the PSZ saturation decreases with the Bond number, where packing of smaller particles have higher saturation values than packing made of larger particles. Results also reveal that the extent (height) of the PSZ, as well as water saturation in the PSZ, both increase with wettability. We also numerically calculate the saturation exclusively contained in connected liquid films and results show that values are less than the expected PSZ saturation. These results strongly reflect that the two-phase zone is not solely made up of connected capillary networks, but also made of disconnected water clusters or pockets. Moreover, we also find that global saturation (PSZ + full wet zone) decreases with wettability, confirming that greater quantity of water is lost via evaporation with increasing hydrophilicity. These results show that connected liquid films are favored in more hydrophilic systems while disconnected water pockets are favored in less hydrophilic systems.