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

Photonic supraballs have been designed by evaporation-inducedcrystallizationof colloidal particles confined in emulsion droplets for various applications.However, it has been a challenge to exclusively produce a single-crystallinestructure in a short consolidation time due to droplet-by-dropletvariation in the evaporation rate. Here, we suggest a pragmatic osmoticextraction of water from emulsion droplets to achieve a consistentrate of consolidation and improve the selectivity of crystalline structures.Two distinct droplets particle-laden droplets and salt-dissolveddroplets are randomly mixed to induce an osmotic flow of waterfrom the particle droplets to salt droplets. The particle dropletsgradually shrink whereas the salt droplets expand, which causes theenrichment and crystallization of particles. Importantly, the rateof enrichment has a relatively small deviation as most particle dropletsare exposed to a similar number of neighboring salt droplets for therandom mixture. Moreover, the rate of enrichment is controlled byadjusting the concentration of salt. The low salt concentration providesslow enrichment, enabling dominant production of a single-crystallinestructure in a relatively short consolidation time. By contrast, highsalt concentration causes fast enrichment, yielding an isotropic polycrystallinestructure. The single-crystalline photonic supraballs are structurallyanisotropic and display orientation-dependent colors, potentiallyserving as twinkling structural colorants and active color pixels.

Automated summary

A method for producing single-crystalline photonic supraballs is proposed by controlling the crystallization process through osmotic extraction in emulsion droplets. The method allows for a consistent rate of consolidation and improved selectivity of crystalline structures. By mixing particle-laden droplets and salt-dissolved droplets, the particle droplets shrink and crystallize while the salt droplets expand, resulting in the enrichment and crystallization of particles. The rate of enrichment is controlled by adjusting the salt concentration, enabling dominant production of single-crystalline structures in a relatively short consolidation time.