Direct Determination of the Phase Diagram of a Depletion-Mediated Colloidal System

Depletion is one widely used potential to modulate colloidal interaction because it enables the production of a wide variety of crystalline and glassy phases of spherical and shape-tailored colloids. The attractive depletion potential gives rise to qualitatively new behavior. However, depletion-mediated phase behaviors have never been systematically investigated experimentally regarding pair potentials for aqueous suspensions. In this work, we implement three distinct phases of fluid, crystal, and glass by adjusting the concentrations of depletant and salt in the aqueous suspension of polystyrene particles. To define the phase boundary between the fluid and crystal, we calculate pair potential with a superposition of van der Waals, electrostatic, and depletion interactions. Two unknown parameters in the pair potentialthe concentration of ionic impurities and the ratio of the molar concentration of depletant to osmolarityare experimentally determined from sets of reflectance spectra. The interparticle spacing in the crystalline phase is extracted from the peak wavelength originating from Bragg diffraction, which corresponds to the interparticle separation at energy minimum in the pair potential. The boundary between the fluid and crystal is well defined with the depth of the energy well of 3kBT. By contrast, the onset of glass formation is better characterized by not the well depth but the assembly rate, which is estimated from the slope of the pair potential from force balance. Glasses are produced as the speed exceeds 300 μm/s. That is, crystals are produced by enthalpy gain overwhelming entropy loss, whereas glasses are kinetically produced due to fast jamming

Hydrogel-Encased Photonic Microspheres with Enhanced Color Saturation and High Suspension Stability

Regular arrays of colloidal particles can produce striking structural colors without the need for any chemical pigments. Regular arrays of colloidal particles can be processed into microparticles via emulsion templates for use as structural colorants. Photonic microparticles, however, suffer from intense incoherent scattering and lack of suspension stability. We propose a microfluidic technique to generate hydrogel-shelled photonic microspheres that display enhanced color saturation and suspension stability. We created these microspheres using oil-in-water-in-oil (O/W/O) double-emulsion droplets with well-defined dimensions with a capillary microfluidic device. The inner oil droplet contains silica particles in a photocurable monomer, while the middle water droplet carries the hydrogel precursor. Within the inner oil droplet, silica particles arrange into crystalline arrays due to solvation-layer-induced interparticle repulsion. UV irradiation solidifies the inner photonic core and the outer hydrogel shell. The hydrogel shell reduces white scattering and enhances the suspension stability in water. Notably, the hydrogel precursor in the water droplet aids in maintaining the solvation layer, resulting in enhanced crystallinity and richer colors compared with microspheres from O/W single-emulsion droplets. These hydrogel-encased photonic microspheres show promise as structural colorants in water-based inks and polymer composites

Colloidal Assembly in Leidenfrost Drops for Noniridescent Structural Color Pigments

Noniridescent structural color pigments have great potential as alternatives to conventional chemical color pigments in many coloration applications due to their nonbleaching and color-tunable properties. In this work, we report a novel method to create photonic microgranules composed of glassy packing of silica particles and small fraction of carbon black nanoparticles, which show pronounced structural colors with low angle-dependency. To prepare isotropic random packing in each microgranule, a Leidenfrost drop, which is a drop levitated by its own vapor on a hot surface, is employed as a template for fast consolidation of silica particles. The drop randomly migrates over the hot surface and rapidly shrinks, while maintaining its spherical shape, thereby consolidating silica particles to granular structures. Carbon black nanoparticles incorporated in the microgranules suppress incoherent multiple scattering, thereby providing improved color contrast. Therefore, photonic microgranules in a full visible range can be prepared by adjusting the size of silica particles with insignificant whitening

Colloidal Assembly in Leidenfrost Drops for Noniridescent Structural Color Pigments

Noniridescent structural color pigments have great potential as alternatives to conventional chemical color pigments in many coloration applications due to their nonbleaching and color-tunable properties. In this work, we report a novel method to create photonic microgranules composed of glassy packing of silica particles and small fraction of carbon black nanoparticles, which show pronounced structural colors with low angle-dependency. To prepare isotropic random packing in each microgranule, a Leidenfrost drop, which is a drop levitated by its own vapor on a hot surface, is employed as a template for fast consolidation of silica particles. The drop randomly migrates over the hot surface and rapidly shrinks, while maintaining its spherical shape, thereby consolidating silica particles to granular structures. Carbon black nanoparticles incorporated in the microgranules suppress incoherent multiple scattering, thereby providing improved color contrast. Therefore, photonic microgranules in a full visible range can be prepared by adjusting the size of silica particles with insignificant whitening

Nonspherical Double Emulsions with Multiple Distinct Cores Enveloped by Ultrathin Shells

Microfluidics has provided means to control emulsification, enabling the production of highly monodisperse double-emulsion drops; they have served as useful templates for production of microcapsules. To provide new opportunities for double-emulsion templates, here, we report a new design of capillary microfluidic devices that create nonspherical double-emulsion drops with multiple distinct cores covered by ultrathin middle layer. To accomplish this, we parallelize capillary channels, each of which has a biphasic flow in a form of core–sheath stream; this is achieved by preferential wetting of oil to the hydrophobic wall. These core–sheath streams from the parallelized channels are concurrently emulsified into continuous phase, making paired double-emulsion drops composed of multiple cores and very thin middle shell. This microfluidic approach provides high degree of controllability and flexibility on size, shape, number, and composition of double-emulsion drops. Such double-emulsion drops are useful as templates to produce microcapsules with multicompartments which can encapsulate and deliver multiple distinct components, while avoiding their cross-contamination. In addition, nonspherical envelope exerts strong capillary force, leading to preferential coalescence between innermost drops; this is potentially useful for nanoliter-scale reactions and encapsulations of the reaction products

Nonspherical Double Emulsions with Multiple Distinct Cores Enveloped by Ultrathin Shells

Microfluidics has provided means to control emulsification, enabling the production of highly monodisperse double-emulsion drops; they have served as useful templates for production of microcapsules. To provide new opportunities for double-emulsion templates, here, we report a new design of capillary microfluidic devices that create nonspherical double-emulsion drops with multiple distinct cores covered by ultrathin middle layer. To accomplish this, we parallelize capillary channels, each of which has a biphasic flow in a form of core–sheath stream; this is achieved by preferential wetting of oil to the hydrophobic wall. These core–sheath streams from the parallelized channels are concurrently emulsified into continuous phase, making paired double-emulsion drops composed of multiple cores and very thin middle shell. This microfluidic approach provides high degree of controllability and flexibility on size, shape, number, and composition of double-emulsion drops. Such double-emulsion drops are useful as templates to produce microcapsules with multicompartments which can encapsulate and deliver multiple distinct components, while avoiding their cross-contamination. In addition, nonspherical envelope exerts strong capillary force, leading to preferential coalescence between innermost drops; this is potentially useful for nanoliter-scale reactions and encapsulations of the reaction products

Nonspherical Double Emulsions with Multiple Distinct Cores Enveloped by Ultrathin Shells

Microfluidics has provided means to control emulsification, enabling the production of highly monodisperse double-emulsion drops; they have served as useful templates for production of microcapsules. To provide new opportunities for double-emulsion templates, here, we report a new design of capillary microfluidic devices that create nonspherical double-emulsion drops with multiple distinct cores covered by ultrathin middle layer. To accomplish this, we parallelize capillary channels, each of which has a biphasic flow in a form of core–sheath stream; this is achieved by preferential wetting of oil to the hydrophobic wall. These core–sheath streams from the parallelized channels are concurrently emulsified into continuous phase, making paired double-emulsion drops composed of multiple cores and very thin middle shell. This microfluidic approach provides high degree of controllability and flexibility on size, shape, number, and composition of double-emulsion drops. Such double-emulsion drops are useful as templates to produce microcapsules with multicompartments which can encapsulate and deliver multiple distinct components, while avoiding their cross-contamination. In addition, nonspherical envelope exerts strong capillary force, leading to preferential coalescence between innermost drops; this is potentially useful for nanoliter-scale reactions and encapsulations of the reaction products

Nonspherical Double Emulsions with Multiple Distinct Cores Enveloped by Ultrathin Shells

Microfluidics has provided means to control emulsification, enabling the production of highly monodisperse double-emulsion drops; they have served as useful templates for production of microcapsules. To provide new opportunities for double-emulsion templates, here, we report a new design of capillary microfluidic devices that create nonspherical double-emulsion drops with multiple distinct cores covered by ultrathin middle layer. To accomplish this, we parallelize capillary channels, each of which has a biphasic flow in a form of core–sheath stream; this is achieved by preferential wetting of oil to the hydrophobic wall. These core–sheath streams from the parallelized channels are concurrently emulsified into continuous phase, making paired double-emulsion drops composed of multiple cores and very thin middle shell. This microfluidic approach provides high degree of controllability and flexibility on size, shape, number, and composition of double-emulsion drops. Such double-emulsion drops are useful as templates to produce microcapsules with multicompartments which can encapsulate and deliver multiple distinct components, while avoiding their cross-contamination. In addition, nonspherical envelope exerts strong capillary force, leading to preferential coalescence between innermost drops; this is potentially useful for nanoliter-scale reactions and encapsulations of the reaction products

Photothermal Control of Membrane Permeability of Microcapsules for On-Demand Release

We report the use of a simple microfluidic device for producing microcapsules with reversible membrane permeability that can be remotely controlled by application of near-infrared (NIR) light. Water-in-oil-in-water (W/O/W) double-emulsion drops were prepared to serve as templates for the production of mechanically stable microcapsules with a core–shell structure and highly uniform size distribution. A biocompatible ethyl cellulose shell was formed, containing densely packed thermoresponsive poly­(<i>N</i>-isopropylacrylamide) (pNIPAAm) particles in which gold nanorods were embedded. Irradiation with a NIR laser resulted in heating of the hydrogel particles due to the photothermal effect of the gold nanorods, which absorb at that wavelength. This localized heating resulted in shrinkage of the particles and formation of macrogaps between them and the matrix of the membrane. Large encapsulated molecules could then pass through these gaps into the surrounding fluid. As the phase transition behavior of pNIPAAm is highly reversible, this light-triggered permeability could be repeatedly switched on and off by removing the laser irradiation for sufficient time to allow the gold nanorods to cool. This reversible and remote control of permeability enabled the programmed release of encapsulants, with the time and period of the open valve state able to be controlled by adjusting the laser exposure. This system thus has the potential for spatiotemporal release of encapsulated drugs

Polymeric Inverse Glasses for Development of Noniridescent Structural Colors in Full Visible Range

Amorphous colloidal array with short-range order displays noniridescent structural colors due to the isotropic nature of the colloidal arrangement. The low angle dependence renders the colloidal glasses, which is promising for various coloration applications. Nevertheless, the colloidal glasses are difficult to develop red structural color due to strong cavity-like resonance from individual particles in the blue region. To suppress the cavity mode and develop the colors in the full visible range, we prepare inverse glasses composed of amorphous array of air cavities with short-range order. To produce the structures in a simple and reproducible manner, monodisperse silica particles are dispersed in a photocurable resin of poly­(ethylene glycol) dimethacrylate (PEGDMA) at a volume fraction of 0.3. The particles spontaneously form the amorphous array with short-range order, which is rapidly captured in polymeric films by photopolymerization of the resin. Selective removal of silica particles from the polymerized resin leaves behind amorphous array of air cavities. The inverse glasses display structural colors with negligible backscattering in blue due to short optical path and low index in each cavity. Therefore, the colors can be tuned in full visible range by simply controlling the cavity size. The photocurable suspensions of silica particles can be patterned by photolithography, which enables the production of freestanding films containing patterned inverse glasses with noniridescent structural colors