Production of Photonic Supraballs Composed of Single-Crystalline Colloidal Arrays through Osmosis-Induced Consolidation

Abstract

Photonic supraballs have been designed by evaporation-induced crystallization of colloidal particles confined in emulsion droplets for various applications. However, it has been a challenge to exclusively produce a single-crystalline structure in a short consolidation time due to droplet-by-droplet variation in the evaporation rate. Here, we suggest a pragmatic osmotic extraction of water from emulsion droplets to achieve a consistent rate of consolidation and improve the selectivity of crystalline structures. Two distinct droplets�particle-laden droplets and salt-dissolved droplets�are randomly mixed to induce an osmotic flow of water from the particle droplets to salt droplets. The particle droplets gradually shrink whereas the salt droplets expand, which causes the enrichment and crystallization of particles. Importantly, the rate of enrichment has a relatively small deviation as most particle droplets are exposed to a similar number of neighboring salt droplets for the random mixture. Moreover, the rate of enrichment is controlled by adjusting the concentration of salt. The low salt concentration provides slow enrichment, enabling dominant production of a single-crystalline structure in a relatively short consolidation time. By contrast, high salt concentration causes fast enrichment, yielding an isotropic polycrystalline structure. The single-crystalline photonic supraballs are structurally anisotropic and display orientation-dependent colors, potentially serving as twinkling structural colorants and active color pixels.

Structural and Optical Characterization of Decahedral-Type Spherical Colloidal Clusters

“…We optically examined this type of cluster with a diameter of 260 nm, as shown in Figure a, and confirmed the presence of parallel orange lines. These striped reflections have been observed in some literature. , These line reflections originate from the {101̅0} plane of the hcp lattice, which includes the (011̅0) and (11̅00) planes related by 60° rotation with a rotation axis in ⟨0001⟩ direction. Thus, when the colloidal cluster shown in Figure a is rotated with the rotation axis normal to the orange lines, the reflection intensity is expected to increase and decrease with a period of 60° at the rotation angle.…”

“…Recent studies have shown that spherical colloidal clusters have different particle arrangements, such as fcc and icosahedral-type structures, − and the structural differences can be attributed to the experimental conditions applied during the preparation processes. When a water droplet rapidly shrinks upon heating, clusters with onion-like structures are formed.…”

Structural Color Inks Containing Photonic Microbeads for Direct Writing

“…Colloidal arrays have been produced in a microbead format using emulsion droplets, offering structural colorants in photonic inks, which are potentially useful for structural color printing. − For example, colloidal particles self-assemble within the droplets through enrichment by evaporation or osmotic extraction. − Although these photonic microspheres, composed of close-packed arrays, exhibit structural colors, the colloidal assembly requires a long time for crystallization, and the microspheres have low mechanical stability due to the absence of particle binders, thereby restricting their applications. To overcome these limitations, photonic microspheres with non-close-packed arrays in a solid matrix have been developed. ,,− Silica particles dispersed in polymerizable resins spontaneously form non-close-packed arrays by interparticle repulsion, which are captured in the polymeric matrix by in situ polymerization, reducing production time and securing high mechanical stability.…”