Evaporation-induced crystal self-assembly (EICSA) of salt drops regulated by trace of polyacrylamide

The anisotropic growth and the different interface properties of ionic crystal at microscale make it difficult to control and obtain the ordered crystals at macroscale, resulting in rare researches on self-assembly of ionic crystal. The method of evaporation-induced crystal self-assembly (EICSA) has been demonstrated for the construction of highly ordered self-assembly configurations in the sodium chloride (NaCl) drops regulated by trace of water-soluble organic polymer (polyacrylamide, PAM) on the hydrophobic substrate. According to the dynamic behaviors of self-assembly configurations, the concentration phase diagram of configurations is given, showing that EICSA is a process from disorder to order with the increase of the amount of PAM. We interpret this phenomenon as a result of the addition of PAM promotes the nucleation and inhibits the growth, producing a large number of NaCl with small size. Meanwhile, the adhesion of PAM makes NaCl orderly assembled together. Particularly, the mechanisms of the spontaneous lifting of crystals are directly revealed through our experiments. Moreover, we develop the lattice model to explain the physical mechanisms behind the evolution of selfassembly configurations. Furthermore, the dependence of the evaporating rate from regions (III) to (I) on the mixing free energy change is found to be consistent with the Arrhenius behavior. Our results have shown a further understanding of construction and control in self-assembly of ionic crystal, which has great applications in protection of outdoor electronics, conservation of cultural relics and chemical production.

Automated summary

The anisotropic growth and different interface properties of ionic crystal at microscale pose challenges to controlling and obtaining ordered crystals at macroscale. This has led to limited research on self-assembly of ionic crystal. This study demonstrates the use of evaporation-induced crystal self-assembly (EICSA) method to regulate sodium chloride (NaCl) drops on a hydrophobic substrate using trace amounts of water-soluble organic polymer (polyacrylamide, PAM) to construct highly ordered self-assembly configurations. The addition of PAM promotes nucleation and inhibits growth, resulting in the formation of numerous small-sized NaCl crystals, which are then assembled together in an orderly manner with the adhesion of PAM. Additionally, the spontaneous lifting of crystals is directly revealed through experiments, and a lattice model is developed to explain the physical mechanisms behind the evolution of the self-assembly configurations. The research findings provide a deeper understanding of the construction and control of self-assembly in ionic crystal, with significant applications in outdoor electronics protection, cultural relics conservation, and chemical production.