Effect of Efflorescence Formation on Drying Kinetics of Porous Media

We study experimentally the evaporation of an aqueous NaCl solution with efflorescence formation in hydrophilic or hydrophobic two-dimensional model porous media. The efflorescence formation only marginally affects the invasion patterns but greatly modifies the drying kinetics compared to pure water. Two mechanisms are identified for explaining the impact of efflorescence formation on drying kinetics. It is shown that the efflorescence contributes to increase the evaporation rate compared to drying with pure water, a surprising result since the water activity is reduced in the presence of dissolved salt. This effect is explained by the efflorescence liquid capillary pumping effect associated with the porous nature of the efflorescence. Then, we identify a second phase where the efflorescence dries out and acts as a vapour diffusion barrier, leading to a dramatic reduction of evaporation rate.