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

TL;DR

This study reveals a novel tensile stress generation mechanism during salt crystallization in pores, caused by capillary effects, leading to pore collapse and other phenomena, challenging the traditional view of compressive stress.

Contribution

It introduces direct optical evidence of tensile stress generation due to salt crystallization, contrasting with the classical compressive stress model.

Findings

Tensile stress occurs next to compressive regions during salt crystallization.

Pore collapse results from tensile stress caused by capillary effects.

Observed phenomena include hyperslow drying and asymmetric crystal growth.

Abstract

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.