Self-similarity in creeping salt crystallization

TL;DR

This paper explores the self-similar, fractal-like structures formed by salt crystallization in efflorescence deposits, revealing hierarchical growth patterns and the primary influence of initial salt mass on deposit size.

Contribution

It uncovers the self-similar, fractal nature of salt crystallization structures and introduces a hierarchical growth model emphasizing initial salt mass as the key factor.

Findings

Salt deposits exhibit fractal geometries with self-similarity across scales.

Crystallization occurs hierarchically, forming nested microstructures.

Deposit height is mainly determined by initial salt mass.

Abstract

The self-amplifying creeping of salts can produce striking macroscopic structures, such as desert roses in arid regions and salt pillars near saline lakes. While these formations are visually remarkable, salt crystallization, often seen as efflorescence on surfaces, also poses significant challenges for cultural heritage conservation, materials science, and soil management. In this study, we investigate the mechanisms underlying self-organized crystallization within efflorescence deposits. Our findings reveal that these porous salt deposits exhibit pronounced self-similarity, with the crystallization process recurring at multiple length scales. This results in smaller replicas of the overall structure nested within larger ones, creating fractal geometries similar to those found in cauliflower and broccoli. By performing controlled evaporation experiments and microscale analysis using advanced imaging techniques combined with fractal dimension analysis, we uncover the hierarchical and size-controlled precipitation of cubic microcrystals within the porous efflorescence. Furthermore, we develop a hierarchical growth model demonstrating that the ultimate height of the macroscopic salt deposit is primarily determined by the initial mass of salt, rather than by the interplay of capillary and viscous forces when salt solution flows within the porous salt structure.