Evaporation from thin porous media with mixed intermediately-wet and hydrophobic networks

Evaporation from porous media with mixed wettability is still unclear, and different results have been reported experimentally. To address this issue, a pore network (PN) model with the capillary valve effect is developed to shed light on evaporation from thin porous media with intermediately-wet and hydrophobic networks. The PN is composed cubic pore bodies connected by cylindrical pore throats. Gas invasion in the PN during evaporation is dominated by capillary forces and shows the same pattern in the purely hydrophilic and hydrophobic PNs. Effects of the hydrophilic fraction on evaporation are trivial if hydrophilic and hydrophobic pore bodies have the similar threshold pressure. When hydrophobic pores have a lower threshold pressure than hydrophilic pore bodies, gas invasion during evaporation can be divided into two stages; and the evaporation rate at the constant rate period increases first and then reduces with the increase of the hydrophilic fraction. Furthermore, we also discuss effects of pore wettability and structure on evaporation in mixed wet PNs with super-hydrophilic and hydrophobic pores, and reveal three limiting cases for gas invasion. We find that effects of the hydrophilic fraction on evaporation in porous media depend on not only the pore wettability but also the pore structure.