Experimental study of the effect of sodium chloride on drying of porous media: The crusty–patchy efflorescence transition

We present an experimental study of drying in the presence of dissolved sodium chloride. The process is characterized by the formation of a crystallized salt layer, referred to as efflorescence, at the evaporative surface of the porous medium. By varying the average size of the beads forming the porous medium, we show that the formation of the crystal layer does not affect significantly the drying process and can even enhance the drying rate when the beads are sufficiently large. By contrast, the crystal layer can greatly affect the drying process and even blocks or severely limit the evaporation process for sufficiently small beads. We therefore show the existence of two regimes, namely the blocking regime and the enhanced drying rate regime. It is shown that the two regimes correspond to two different types of efflorescence, referred to as crusty and patchy, respectively. Then by varying the initial salt concentration for a given bead size, we show that the interplay between drying and the efflorescence formation leads to a nonmonotonous variation of the drying rate with the initial salt concentration when the efflorescence is patchy but not when the efflorescence is crusty. The crusty–patchy transition is finally discussed from a simple model of capillary rise in the efflorescence

Porous medium coffee ring effect and other factors affecting the first crystallisation time of sodium chloride at the surface of a drying porous medium

We study the distribution of ions in a drying porous medium up to the formation of first crystals at the surface. The study is based on comparisons between numerical simulations and experiments with packings of glass beads. The experimental configuration, which is representative of many previous drying experiments, is characterized by the formation of an efflorescence fairy ring at the surface of the porous medium. The preferential formation of crystals at the periphery is explained by the combined effect of higher evaporation fluxes at the surface periphery, as in the classical coffee ring problem, and variations in the porosity near the wall bordering the packing. It is shown that both effects have a great impact on the time marking the occurrence of first crystals, which is referred to as the first crystallization time. The experiments indicate that the first crystallization time increases with a decreasing bead size for a given initial ion concentration. This is explained by the variation with bead size of the characteristic size of the near wall region where a preferential desaturation of the sample occurs as a result of the porosity increase near the wall. The study also reveals a significant salt supersaturation effect. This represents a noticeable fact in relation with salt weathering issues

Evaporation-driven growth of large crystallized salt structures in a porous medium

Subflorescence refers to crystallized salt structures that form inside a porous medium. We report a drying experiment revealing major development of subflorescence in the dry region of the porous medium away from the liquid zone. Using a combination of image analyses and numerical computations, we show that the growth is directly correlated to the evaporation flux distribution along the boundary of the growing salt structure. This indicates that the salt is transported into the domain occupied by the salt structure in the porous medium up to the structure periphery, where salt deposition takes place. This is confirmed when a growing salt structure encounters dry subflorescence formed earlier during the drying process. The dry subflorescence is reimbibed and resumes its growth. The analysis also suggests that the solution within the growing subflorescence is in equilibrium with the crystallized salt wall. These results shed light on the growth mechanisms of subflorescence, a phenomenon that can play a fundamental role in several important issues such as carbon dioxide sequestration or salt weathering