Unraveling the Formation Mechanism of Salt Flowers

Abstract

Crystals in nature and industry typically form simple morphologies, yet complex hierarchical structures from multicomponent solutions remain challenging to predict and control. Here, we report the spontaneous formation of biomimetic “salt flowers” through the vacuum evaporation of mixed NaCl and Na₂CO₃ aqueous solutions on the silicon surface. These structures consist of hexagonal star-like NaCl crystals atop dendritic Na₂CO₃, resembling natural flowers at macroscales. Experiments show that evaporation under vacuum induces coffee ring effect, where Na₂CO₃ nucleates and forms dendrites first at the edges of the droplet due to lower solubility. NaCl subsequently crystallizes at the center of the droplet into hexagonal star-like crystals. Selective ion adsorption bidirectionally regulates crystal habit, converting cubic NaCl into truncated polyhedral morphology and promoting anisotropic branching in Na₂CO₃. Evaporation flux modeling and capillary transport elucidate the spatial and temporal segregation and non-equilibrium self-assembly. Owing to high specific surface area, synergistic hygroscopic and ionic conduction properties, flower-like crystals exhibit significantly enhanced humidity response compared with cubic NaCl. This work reveals how simple environmental control and mutual ion interactions in multicomponent systems generate complex microstructures, offering a scalable, additive-free strategy for regulating structure and properties of salt-based materials, with potential applications in sensing, catalysis, and optics.