Photonic Microbeads Templated by Oil‐in‐Oil Emulsion Droplets for High Saturation of Structural Colors

Abstract

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...

Designing photonic microparticles with droplet microfluidics

“…The microbeads are produced by capturing a non-close-packed array of silica particles dispersed in an elastomer-forming resin using either oil-in-water (O/W) or oil-in-oil emulsion drops. 51,52 Each microbead shows mechanochromic properties as the colloidal lattice also deforms upon elastic deformation of the matrix, as shown in Fig. 7d.…”

“…To overcome this problem, a resin-immiscible oil phase, such as a mineral oil, is used as a continuous phase. 52 As the non-polar oil does not perturb the formation of the silica–resin solvation layer, photonic microbeads with an enhanced crystallinity and color saturation are produced.…”

Tailor-made urethane-linked alkyl-celluloses: a promising stabilizer for oil-in-oil Pickering emulsions

“…Substantial efforts have been devoted to the design and application of nonaqueous emulsions, and tremendous application potential has emerged, [5][6][7][8][9][10][11] especially for the polymerization of water-sensitive monomers/catalysts [12][13][14][15][16][17][18] and the encapsulation of hygroscopic materials. [19][20][21][22][23] Even so, nonaqueous emulsions are far less developed because of the difficulty of finding or designing suitable surfactant/stabilizers.…”

Injectable Granular Hydrogels as Colloidal Assembly Microreactors for Customized Structural Colored Objects

“…[1][2][3] Also, it is highly tunable: simply by varying the structural dimensions, colors can be tuned within a single set of materials. Together with its potentially low toxicity, these unique features are highly valued in areas such as sensors, [4][5][6][7] optical encryption, [8][9][10] displays, [11,12] paints, [13][14][15] etc. Despite progress made in the past decades in engineered structural coloration, it has been limited to thin films or bulk materials.…”