Direct observation of pore collapse and tensile stress generation on pore wall due to salt crystallization

The generation of stress in a pore due to salt crystallization is generally analysed as a compressive stress generation mechanism using the concept of crystallization pressure. We report on a completely different stress generation mechanism. In contrast with the classical picture where the crystal pushes the pore wall, the crystal growth leads to the generation of a local tensile stress. This tensile stress occurs next to a region where a compressive stress is generated, thus inducing also shear stresses. The tensile stress generation is attributed to capillary effects in the thin film confined between the crystal and the pore wall. These findings are obtained from direct optical observations in model pores where the tensile stress generation results in the collapse of the pore region located between the crystal and the pore dead-end. The experiments also reveal other interesting phenomena, such as the hyperslow drying in PDMS channels or the asymmetrical growth of the crystal during the collapse.