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.

Multiple-emulsion drops have served as versatile templates to design functional microcapsules due to their core-shell geometry and multiple compartments. Microfluidics has been used for the elaborate production of multiple-emulsion drops...

Microcapsules with regulated transmembrane transport are of great importance for various applications. The membranes with a tunable cut‐off threshold of permeation provide advanced functionality. Here, thermo‐responsive microcapsules are designed, whose hydrogel membrane shows a tunable cut‐off threshold of permeation with temperature. To produce the microcapsules, water‐in‐oil‐in‐water (W/O/W) double‐emulsion droplets are microfluidically produced, whose oil shell contains oil‐soluble hydrogel precursor of poly(N, N‐diethylacrylamide) copolymerized with benzophenone (PDEAM‐BP). The PDEAM hydrogels, crosslinked by BP, show volume‐phase transition around 34 °C, which makes the microcapsules with the PDEAM hydrogel membrane thermo‐responsive. The microcapsules show temperature‐dependent changes in radius and membrane thickness. More importantly, the cut‐off threshold of permeation can be reversibly adjusted by temperature control as the degree of swelling decreases with temperature. This enables the molecule‐selective encapsulation and the controlled release of the encapsulants in a programmed manner by adjusting the temperature. The microcapsules can be rendered to be photo‐responsive by encapsulating photothermal polydopamine nanoparticles during the microfluidic operation, which allows the control of the degree of swelling with near‐infrared (NIR) irradiation. The thermo‐ and photo‐responsive microcapsules with a tunable cut‐off threshold are appealing as a new platform for drug carriers, microreactors, and microsensors.

Microgels, microparticles made of hydrogels, show fast diffusion kinetics and high reconfigurability while maintaining the advantages of hydrogels, being useful for various applications. Here, presented is a new microfluidic strategy for producing polymer‐graphene oxide (GO) composite microgels without chemical cues or a temperature swing for gelation. As a main component of microgels, polymers that are able to form hydrogen bonds, such as polyvinyl alcohol (PVA), are used. In the mixture of PVA and GO, GO is tethered by PVA through hydrogen bonding. When the mixture is rapidly concentrated in the core of double‐emulsion drops by osmotic‐pressure‐driven water pumping, PVA‐tethered GO sheets form a nematic phase with a planar alignment. In addition, the GO sheets are linked by additional hydrogen bonds, leading to a sol–gel transition. Therefore, the PVA–GO composite remains undissolved when it is directly exposed to water by oil‐shell rupture. These composite microgels can be also produced using poly(ethylene oxide) or poly(acrylic acid), instead of PVA. In addition, the microgels can be functionalized by incorporating other polymers in the presence of the hydrogel‐forming polymers. It is shown that the multicomponent microgels made from a mixture of polyacrylamide, PVA, and GO show an excellent adsorption capacity for impurities.

Colloidal crystals have been used to develop structural colors. However, incoherent scattering causes the colors to turn whitish, reducing the color saturation. To overcome the problem, light‐absorbing additives have been incorporated. Although various additives have been used, most of them are not compatible with a direct co‐assembly with common colloids in aqueous suspensions. Here, the authors suggest eumelanin nanoparticles as a new additive to enhance the color chroma. Eumelanin nanoparticles are synthesized to have diameters of several nanometers by oxidative polymerization of precursors in basic solutions. The nanoparticles carry negative charges and do not weaken the electrostatic repulsion among same‐charged polystyrene particles when they are added to aqueous suspensions. To prove the effectiveness of eumelanin as a saturation enhancer, the authors produce photonic balls through direct co‐assembly of polystyrene and eumelanin using water‐in‐oil emulsion droplets, while varying the weight ratio of eumelanin to polystyrene. The high crystallinity of colloidal crystals is preserved for the ratio up to at least 1/50 as the eumelanin does not perturb the crystallization. The eumelanin effectively suppresses incoherent scattering while maintaining the strength of structural resonance at an optimum ratio, improving color chroma without compromising brightness.

Phase behaviors of metals have been modeled with colloidal systems. Inspired by the thermal treatment of metals, we design colloidal counterparts confined in microcapsules that show cooling rate-dependent crystallization behavior and optical properties. Thermoresponsive colloidal particles are highly concentrated in microcapsules to make colloidal crystals. The crystals melt upon heating and particles recrystallize upon cooling due to reversible change of the volume fraction. The crystallization behavior strongly depends on the cooling rate, similarly to metals. For slow cooling, large single-crystalline grains are grown from the inner wall of microcapsules by heterogeneous nucleation and merging. By contrast, fast cooling causes both homogeneous and heterogeneous nucleation, resulting in small crystallites with random orientations in the inner volume. This structural variation changes optical properties originated from Bragg diffraction. The slowly cooled microcapsules display structural colors from the entire projection that are bright and orientation-dependent. The fast-cooled microcapsules show axisymmetric, rotation-independent color patterns in the projection. As the thickness of crystalline grains that align parallel to the inner wall decreases along with the cooling rate, so does the color brightness and reflectance.

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.