The Emergence of Photonic Crystalline Order and Time-Series Dynamics in NaCl Droplet Deposition

TL;DR

This study investigates the crystallization process of NaCl droplets on germanium substrates, revealing how anisotropic diffusion influences the formation of photonic nanostructures and providing a framework for predicting droplet crystallization behavior.

Contribution

It introduces the NaCl equivalent height metric and demonstrates the dominant role of diffusion anisotropy in structure size, advancing understanding of droplet crystallization mechanisms.

Findings

Diverse microstructures including 1D photonic crystal nanostructures were identified.

Diffusion anisotropy primarily governs the maximum size of crystalline structures.

Time-series analysis revealed the spatiotemporal evolution of the crystallization front.

Abstract

Crystallization during droplet evaporation gives rise to complex, self-organized structures, yet the mechanisms underlying the emergence of ordered functional phases remain poorly understood. In this study, we present a comprehensive, multi-scale investigation into the crystallization dynamics of NaCl during droplet evaporation on a germanium (001) substrate, relevant for its IR applications. Through systematic microscopic characterization, we identify the formation of diverse microstructures, including 1D photonic crystal nanostructures formed within hybrid crystal-glass photonic system. To enable quantitative comparison across experimental conditions, we introduce the NaCl equivalent height as a unified metric to describe and classify the evolution of crystalline morphology. Our results reveal that diffusion anisotropy, rather than growth kinetics, primarily governs the maximal attainable structure size. Quantitative thin film interference analysis demonstrates the presence of discrete thickness layers in the film. Controlled evaporation experiments yield homogeneous crystallization patterns across the entire droplet area, facilitating the emergence of ordered photonic structures. Time-series dynamics analysis of height profiles uncovered the spatiotemporal evolution of the crystallization front, providing insights into the details of underlying physical mechanisms. Together, these results establish a robust experimental framework for understanding and predicting crystallization behavior in evaporating droplets, with potential applications in materials synthesis, photonics, and microscale pattern formation.