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

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.