Sol-gel transition by evaporation in porous media

Historical monuments, outdoor stone sculptures, and artworks made of porous materials are exposed to chemical and physical degradations over time. Presently, the most promising route for consolidation of weakened porous materials is the injection of viscoelastic solutions of polymerizing compounds. Those compounds, after injection, undergo a sol-gel transition inside the porous media through evaporation of the solvent. Finding a suitable gelifying solution as a consolidant calls for understanding the drying kinetics of viscoelastic fluids in porous media. Here, we present a multiscale study of the drying kinetics of fluids during the sol-gel transition in porous materials using NMR and x-ray microtomography techniques. We find that from the early stage of the drying, a heterogeneous desaturation develops and advances from the free surface of evaporation towards the inner parts of the stone. We identify different drying periods, which appear to be dependent on the intrinsic properties of the porous medium influencing strongly the homogeneity of the final gel distribution within a treated stone. Our findings not only are relevant for the consolidation of porous artworks but also for civil and soil engineering processes where the fluids considered are generally more complex than water.