Characteristics of evaporation from partially wettable porous media

The evaporation rate from porous media often exhibits an abrupt transition from a high and nearly constant rate supplied by capillary-induced liquid flow (stage 1) to lower values supported by vapor diffusion. Evidence suggests that evaporation from hydrophobic porous media is suppressed relative to evaporation from similar hydrophilic media. The mechanism for evaporation suppression remains unclear; some implicate effects of partial wettability on liquid phase continuity. Here we examine potential effects of wettability on capillary driving forces required for sustaining liquid flows. Evaporation experiments from sand-filled columns with different fractions of hydrophobic grains enabled comparisons of evaporative mass loss rates and drying front depths. Results show a gradual reduction in drying front depth at the end of stage 1 (denoted as evaporation characteristic length) with an increasing fraction of hydrophobic grains. A model based on the simple averaging of partial wettability effects on capillary driving forces was in good agreement with the experimentally determined drying front depth. Experimental and modeling results suggest that liquid phase continuity was less important in suppressing the duration of stage 1 and reducing the evaporation characteristic length relative to capillarity effects, as is also confirmed in a 3-D percolation-based morphological pore network model. Nevertheless, partial wettability significantly modifies phase distribution above the drying front, as shown by spatially resolved observations using neutron radiography and by a pore-scale percolation model. New insights concerning the effects of partial wettability on evaporation may provide engineering solutions for reducing evaporative losses from porous surfaces.