Salt crystallization at hydrophobic/hydrophilic interfaces in two- and three-dimensional model porous networks and its consequences on drying

Understanding the ability of salts to crystallize at the interface between two porous media with different material properties is of paramount importance to understanding the more complex real-world degradation mechanisms associated with salt crystallization in composite materials and the causes of the delamination of ceramics, tiles, and mural paintings. We present our results on salt crystallization at the interface between hydrophilic and hydrophobic porous media during the evaporation of aqueous salt solutions. For this purpose, we design artificial 2D monolayer porous media to study the fundamentals of crystallization at the interface of two porous materials with different wettability properties at the microscale, and we compare the results with 3D samples to show the scalability of the process. In the experiments, the drying kinetics and dynamics of salt precipitation (sodium chloride and sodium sulfate) are studied in real time using optical microscopy with image analysis or x-ray microtomography combined with mass-change monitoring during evaporation. Our study reveals how the interface between different wettability regions, combined with the interfacial properties of the precipitating crystals, impacts the drying kinetics and the crystallization dynamics of the salt solution. Depending on the salt, precipitated crystals can either block the interface and prevent the samples from drying or form subflorescence in the hydrophilic region, potentially causing further damage. Understanding the material dynamics in the presence of salt and hydrophobic treatments helps with the development of more adequate solutions for the conservation of multilayered composite artworks.