Quantifying thick liquid films and their role in evaporative drying of porous media

Evaporation rate from porous media is determined by a combination of driving forces and external conditions interacting with liquid and vapor within complex pore spaces. The evaporation rate during first stage of a drying process is controlled primarily by atmospheric demand and generally is not limited by medium transport properties. Liquid connections between the evaporation surface and the receding drying front sustain sufficient water supply to maintain a constant evaporation rate. When hydraulic connections are disrupted, water transport to the surface becomes limited to rates supported by vapor diffusion. To improve our understanding of the properties of stage-one supporting liquid connections, we delineated pore geometry and liquid configuration in sand samples imaged using synchrotron X-rays tomography. As air invades large pores, remaining liquid in crevices and grain contacts form a network of thick films. We compared evaporation rates with liquid phase configuration above the drying front in an attempt to relate the end of the first stage of drying with liquid films connectivity. The dependency of high evaporation rates on residual liquid continuity implies sensitivity to surface wettability properties. These effects were examined using different mixtures of hydrophobic and hydrophilic particles and their impact on drying rates and liquid phase distribution. Improved understanding of relationships between pore scale effects, liquid configuration and evaporation processes enhances predictability of drying rates and may enable alterations or design of porous media with prescribed drying behavior.