  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.
