Colloidal crystals have been used for creating stimuli‐responsive photonic materials. Here, macroporous hydrogels are designed, through a simple and reproducible protocol, that rapidly and reversibly ...switch between highly transparent and structurally colored states. The macroporous hydrogels are prepared by film‐casting photocurable dispersions of silica particles in hydrogel‐forming resins and selectively removing silica particles. The silica particles spontaneously form a nonclose‐packed array due to repulsive interparticle interaction, which form the regular array of cavities after removal. However, the cavities are randomly collapsed by drying, losing a long‐range order and rendering the materials highly transparent. When the hydrogels are swollen by either water, ethanol, or the mixture, the regular array is restored, which develops brilliant structural colors. This switching is completed in tens of seconds and repeatable without any hysteresis. The resonant wavelength depends on the composition of the water–ethanol mixture, where the dramatic shift occurs in one‐component‐rich mixtures due to the composition of the hydrogel. Micropatterns can be designed to have distinct domains of the macroporous hydrogels, which are transparent at the dried state and disclose encrypted graphics and unique reflectance spectra at the wet state. This class of solvent‐responsive photonic hydrogels is potentially useful for alcohol sensors and user‐interactive anti‐counterfeiting materials.
Macroporous photonic hydrogels are designed by selective removal of a nonclose‐packed array of silica particles from a hydrogel matrix. The macroporous hydrogels are transparent in a dried state, which drastically turns structurally colored in a wet state. The wavelength of structural resonance strongly depends on the composition of liquid, which renders the macroporous hydrogels useful as sensors and anti‐counterfeiting patterns.
Dendrites are ubiquitous crystals produced in supersaturated solutions and supercooled melts, but considerably less is known about their formation and growth kinetics. Here, the key factors are ...explored that dictate dendrite formation and growth, utilizing experimental colloidal models in which the particles act as molecules with Mie potential. Depletion attraction is employed to colloids and manipulate their strength to control supersaturation. Dendrites are predominantly produced under conditions of low supersaturation, where the separation between crystals is large due to slow nucleation. The dendrites do not emerge directly from nuclei. Instead, isotropic grains, initially produced from nuclei, morph into polygons. Arms then sprout from the vertices of these polygons, eventually giving rise to dendrites. Triggering this polygon‐to‐dendrite transformation requires a high diffusional flux. This necessitates a prolonged diffusion time to maintain a steep concentration gradient in the surrounding environment even after the transformation from circular grains to polygons.
Dendritic crystals are produced by depletion‐mediated spontaneous assembly of colloids at low supersaturation. The low supersaturation causes slow nucleation, which gives large separation between crystals, ensuring a prolonged diffusion time to maintain a steep concentration gradient. The crystals initially take isotropic grains, morph into polygons, and eventually giving rise to dendrites by sprouting arms from the vertices of the polygons.
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 droplets. The silica particles spontaneously form crystalline arrays in the entire volume of the droplets due to interparticle repulsion which is unperturbed by the diffusion of the surrounding oil whereas weakened for oil‐in‐water droplets. The crystalline arrays are permanently stabilized by photopolymerization of the precursor, forming elastic photonic microbeads. The microbeads are transferred into the refractive‐index‐matched biocompatible oil. The high crystallinity of colloidal arrays increases the reflectivity at stopband and the index matching reduces incoherent scattering at the surface of the microbeads, enhancing color saturation. The colors can be adjusted by mixing two distinctly colored microbeads. Also, low stiffness and high elasticity reduce foreign‐body sensation and enhance fluidity, potentially serving as pragmatic structural colorants for photonic inks.
Elastic photonic microbeads with high color saturation are produced by employing oil‐in‐oil emulsion droplets. As the continuous oil phase does not lessen the interparticle repulsion, particles form crystalline arrays in the entire volume. In addition, refractive‐index matching further reduces incoherent scattering. High color saturation and elasticity render the microbeads promising as structural colorants in cosmetics and inks.
Colloidal crystals display photonic stopbands that generate reflective structural colors. While micropatterning offers significant value for various applications, the resolution is somewhat limited ...for conventional top‐down approaches. In this work, a simple, single‐step bottom‐up approach is introduced to produce photonic micropatterns through depletion‐mediated regioselective growth of colloidal crystals. Lithographically‐featured micropatterns with planar surfaces and nano‐needle arrays as substrates are employed. Heterogeneous nucleation is drastically suppressed on nano‐needle arrays due to minimal particle‐to‐needles overlap of excluded volumes, while it is promoted on planar surfaces with large particle‐to‐plane volume overlap, enabling regioselective growth of colloidal crystals. This strategy allows high‐resolution micropatterning of colloidal photonic crystals, with a minimum feature size as small as 10 µm. Stopband positions, or structural colors, are controllable through concentration and depletant and salt, as well as particle size. Notably, secondary colors can be created through structural color mixing by simultaneously crystallizing two different particle sizes into their own crystal grains, resulting in two distinct reflectance peaks at controlled wavelengths. The simple and highly reproducible method for regioselective colloidal crystallization provides a general route for designing elaborate photonic micropatterns suitable for various applications.
Photonic micropatterns are created through single‐step growth of colloidal crystals on lithographically prepatterned substrates with planar surfaces and nano‐needle arrays. When the colloidal assembly is mediated by depletion attraction, the planar surfaces effectively cause heterogeneous nucleation and crystal growth, whereas the nano‐needle arrays inhibit it. This results in regioselective crystallization on the planar areas, yielding high‐resolution structural‐color graphics.
Cholesteric liquid crystals (CLCs), also known as chiral nematic LCs, show a photonic stopband, which is promising for various optical applications. In particular, CLCs confined in microcompartments ...are useful for sensing, lasing, and optical barcoding at the microscale. The integration of distinct CLCs into single microstructures can provide advanced functionality. In this work, CLC multishells with multiple stopbands are created by liquid–liquid phase separation (LLPS) in a simple yet highly controlled manner. A homogeneous ternary mixture of LC, hydrophilic liquid, and co‐solvent is microfluidically emulsified to form uniform oil‐in‐water drops, which undergo LLPS to form onion‐like drops composed of alternating CLC‐rich and CLC‐depleted layers. The multiplicity is controlled from one to five by adjusting the initial composition of the ternary mixture, which dictates the number of consecutive steps of LLPS. Interestingly, the concentration of the chiral dopant becomes reduced from the outermost to the innermost CLC drop due to uneven partitioning during LLPS, which results in multiple stopbands. Therefore, the photonic multishells show multiple structural colors. In addition, dye‐doped multishells provide band‐edge lasing at two different wavelengths. This new class of photonic multishells will provide new opportunities for advanced optical applications.
Photonic multishells are created by controlled phase separation of cholesteric liquid crystals and hydrophilic liquid confined in emulsion drops. The material exchange between the dispersed and continuous phases triggers the consecutive steps of phase separation, through which onion‐like drops are formed. Multiple photonic stopbands are spontaneously developed by uneven partitioning of the chiral dopant, which provides advanced photonic functionality.
Colloidal crystals are designed as photonic microparticles for various applications. However, conventional microparticles generally have only one stopband from a single lattice constant, which ...restricts the range of colors and optical codes available. Here, photonic microcapsules are created that contain two or three distinct crystalline grains, resulting in dual or triple stopbands that offer a wider range of colors through structural color mixing. To produce distinct colloidal crystallites from binary or ternary colloidal mixtures, the interparticle interaction is manipulated using depletion forces in double-emulsion droplets. Aqueous dispersions of binary or ternary colloidal mixtures in the innermost droplet are gently concentrated in the presence of a depletant and salt by imposing hypertonic conditions. Different-sized particles crystallize into their own crystals rather than forming random glassy alloys to minimize free energy. The average size of the crystalline grains can be adjusted with osmotic pressure, and the relative ratio of distinct grains can be controlled with the mixing ratio of particles. The resulting microcapsules with small grains and high surface coverage are almost optically isotropic and exhibit highly-saturated mixed structural colors and multiple reflectance peaks. The mixed color and reflectance spectrum are controllable with the selection of particle sizes and mixing ratios.
Hydrogel‐shelled microcapsules allow communication with the surrounding through molecule‐selective exchanges, serving as microcarriers for cells, drugs, catalysts, and nanoparticle sensors. However, ...the low mechanical stability of hydrogels has restricted their use in strong shear flows or compressive fields. Here, microcapsules are designed composed of water core and double‐network (DN) hydrogel shell using water‐in‐oil‐in‐water‐in‐oil triple‐emulsion templates to secure high mechanical stability as well as molecular size‐selective permeation. Monodisperse triple‐emulsion droplets are prepared using microfluidic devices. The core water contains divalent ions and the outer water layer contains poly(ethylene glycol)diacrylate (PEGDA) and sodium alginate. The PEGDA is first photocrosslinked by ultraviolet irradiation and the alginate is second ionically crosslinked with an infusion of divalent ions from the core by rupturing the middle oil layer. The resulting DN shells show excellent mechanical stability in comparison with single‐network (SN) shells made of PEGDA only. Enhanced elasticity of the DN enables the microcapsules to elastically deform and fully recover the original state for wide ranges of strain and strain rate. Moreover, the DN shells show a threshold force for the shell rupture almost one order of magnitude larger than the SN and maintain their integrity under violent shear flows.
Microcapsules whose shell is made of double‐network hydrogel are produced by employing two independent gelation reactions in triple‐emulsion droplets. The double‐network shells are elastically deformable for wide ranges of strain and strain rate, which secures high mechanical stability. Moreover, the shells provide a clear cut‐off threshold and high permeability, serving as promising carriers of microencapsulation.
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.
Eumelanin nanoparticles are synthesized as novel additives of color‐saturation enhancers for colloidal crystals. As the eumelanin nanoparticles are sufficiently small and negatively charged, they are highly compatible with the self‐assembly of common water‐borne colloids with the same charge. The nanoparticles co‐assembled with latex particles effectively suppresses incoherent scattering while maintaining the strength of structural resonance, improving color chroma.
Chiral mesogens spontaneously form a cholesteric phase and show structural colors through wavelength‐selective reflection. Although chiral mesogens are pragmatic materials to build cholesteric phase ...due to their low toxicity, low cost, and robustness, an elaborate encapsulation technique is not reported yet. In this work, cholesteryl esters are encapsulated with a calcium‐alginate hydrogel shell using oil‐in‐water‐in‐oil double‐emulsion droplets. With capillary microfluidic devices, monodisperse double‐emulsion droplets are prepared to have a mixture of cholesteryl ester and toluene in the innermost droplets and an aqueous solution of sodium alginate and calcium‐carrying complex in the outer droplets. As toluene diffuses out from the core, cholesteryl ester forms the cholesteric phase. At the same time, calcium ions are dissociated from the complex upon chemical cues, which causes ionic crosslinking of alginate. The microcapsules show the thermochromic property as the structural color of the cholesteric core blue‐shifts along with the temperature. Therefore, the microcapsules report local temperatures with colors or reflectance spectra. As the range and sensitivity of the colorimetric temperature measurement are controllable by adjusting the composition of cholesteryl esters, a wide range of temperature can be covered by employing a proper set of compositions while maintaining high sensitivity.
Photonic microcapsules are designed by enclosing a core of cholesteryl esters with alginate shell using double‐emulsion templates. The microcapsules show high suspension stability and thermochromic property. The microcapsules report local temperatures in microenvironments through structural colors. A wide range of temperature measurement can be achieved by employing a proper set of various compositions while securing high sensitivity.
Chameleons use a non-close-packed array of guanine nanocrystals in iridophores to develop and tune skin colors in the full visible range. Inspired by the biological process uncovered in panther ...chameleons, we designed photonic films containing a non-close-packed face-centered-cubic array of silica particles embedded in an elastomer. The non-close-packed array is formed by interparticle repulsion exerted by solvation layers on the particle surface, which is rapidly captured in the elastomer by photocuring of the dispersion medium. The artificial skin exhibits a structural color that shifts from red to blue under stretching or compression. The separation between inelastic particles enables tuning without experiencing significant rearrangement of particles, providing elastic deformation and reversible color change, as chameleons do. The simple fabrication procedure consists of film casting and UV irradiation, potentially enabling the continuous high-throughput production. The mechanochromic property of the photonic films enables the visualization of deformation or stress with colors, which is potentially beneficial for various applications, including mechanical sensors, sound–vision transformers, and color display.
