Icosahedral colloidal clusters are a new class of spherical colloidal crystals. This cluster allows for potentially superior optical properties in comparison to conventional onion-like colloidal supraballs because of the quasi-crystal structure. However, the characterization of the cluster as an optical material has until now not been achieved. Here we successfully produce icosahedral clusters by assembling silica particles using bulk water-in-oil emulsion droplets and systematically characterize their optical properties. We exploit a water-saturated oil phase to control droplet drying, thereby preparing clusters at room temperature. In comparison to conventional onion-like supraballs with a similar size, the icosahedral clusters exhibit relatively strong structural colors with weak nonresonant scattering. Simulations prove that the crystalline array inside the icosahedral cluster strengthens the collective specular diffraction. To further improve color saturation, the silica particles constituting the cluster are coated with a thin-film carbon shell. The carbon shell acts as a broad-band absorber and reduces incoherent scattering with long optical paths, resulting in vibrant blue, green, and red colors comparable to inorganic chemical pigments.

cosmetics, as an alternative to chemical pigments. [9][10][11][12][13][14][15] To formulate structuralcolor inks, colloidal crystals have been tailored in a format of microbeads or microcapsules using emulsion templating. [5,[16][17][18][19][20] For example, colloidal particles are confined in emulsion droplets and enriched to form a close-packed array in a supraball by depleting the suspension medium through evaporation. [21][22][23][24] The supraballs contain either an onion-like arrangement, icosahedral structure, or single-crystalline structure depending on the rate of enrichment and relative size of supraballs to the particles, [25][26][27][28][29][30] which show pronounced structural colors. The color saturation has been enhanced by employing lightabsorbing additives. [31,32] However, evaporation-induced self-assembly requires a long time of consolidation and delicate conditions. Furthermore, the supraballs have limited mechanical stability due to a lack of interparticle adhesives. To avoid the use of the evaporation process and improve mechanical stability, colloidal particles are carefully dispersed in a photocurable medium to have repulsive interparticle potential, which is emulsified in water to form oil-in-water (O/W) droplets. [9,[33][34][35][36] The interparticle repulsion causes the spontaneous crystallization of particles in the absence of evaporation and the colloidal arrays are permanently stabilized by photopolymerizing the media of emulsion droplets. Therefore, photonic balls with high mechanical stability can be produced without the evaporation process. Nevertheless, color saturation and brightness are insufficient to directly use the photonic balls as colorants in photonic inks or cosmetic products, which is attributed to low crystallinity of colloidal arrays and incoherent scattering at the interface between the balls and suspension medium.Here, we suggest the evaporation-free production of photonic balls with enhanced color saturation and brightness using oil-in-oil (O/O) emulsion templates. With capillary microfluidic devices, monodisperse O/O emulsion droplets are prepared by emulsifying suspension of silica-in-resin in mineral oil containing surfactant. As the resin, poly(ethylene glycol) phenyl ether acrylate (PEGPEA) is selected to form elastic photonic balls, in which silica particles are dispersed at the volume fraction of 33%. Each particle dispersed in Photonic microbeads containing crystalline colloidal arrays are promising as a key component of structural-color inks for various applications including printings, paintings, and cosmetics. However, structural colors from microbeads usually have low color saturation and the production of the beads requires delicate and time-consuming protocols. Herein, elastic photonic microbeads are designed with enhanced color saturation through facile photocuring of oil-in-oil emulsion droplets. Dispersions of highly-concentrated silica particles in elastomer precursors are microfluidically emulsified into immiscible oil to produce monodisperse dr...

Colloidal crystals develop structural colors through wavelength-selective diffraction. Recently, a granular format of colloidal crystals has emerged as building blocks to construct macroscopic photonic surfaces or architectures with high reconfigurability through the secondary assembly. Here, we design elastic photonic microcapsules containing colloidal crystallites along the inner wall as a building block. Water-in-oil-in-water double-emulsion templates are microfluidically prepared to have an aqueous dispersion of polystyrene particles in the inner droplet and polydimethylsiloxane prepolymers in the shell. Colloidal particles are enriched in the presence of depletant and salt by osmotic compression, with the crystallization at the inner interface by depletion attraction. The number of nucleation sites depends on the rate of the enrichment, which enables control over the size and surface coverage of the crystallites with osmotic conditions. The enrichment is ceased by transferring the droplets into an isotonic solution, and the oil shell is cured to form an elastic membrane. As the elastic microcapsules have a large void in the core, they are deformable without structural damage in the crystallites. Therefore, the microcapsules can be closely packed to form macroscopic surfaces while achieving a high quality of structural colors with a collection of crystallites aligned along the flattened membrane.

Colloidal assembly in emulsion drops provides fundamental tools for studying optimum particle arrangement under spherical confinement and practical means for producing photonic microparticles. Recent progresses have found that energetically-favored cluster...