Effects of hydrophobic layers on evaporation from porous media

Early stages of evaporation from initially-saturated porous media are marked by relatively high and constant evaporation rate ("stage 1") supplied by capillary flow from the receding drying front to evaporating surface. At a characteristic front depth defined by pore size distribution, gravity exceeds capillary-drive and interrupts liquid flow. Subsequently, evaporation falls to rates supported by vapor diffusion ("stage 2"). We investigated the role of thin hydrophobic layers on liquid phase continuity and on evaporative losses. Results confirm interruption of capillary flow by hydrophobic layers and overall reduction in evaporation mass loss. Persistence of capillary flow to the bottom of hydrophobic layer was evident by accumulation of dye tracer indicating formation of vaporization plane. Dye was not deposited on hydrophobic surfaces due to contact line configuration that promote evaporation away from pore walls. Evaporation flux across the hydrophobic layer was purely diffusive and proportional to diffusion length to the surface. Copyright 2008 by the American Geophysical Union.