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Photonic emulsion droplets are densely packed through rapid evaporation of continuous phase to create structural color coatings. The liquid droplets adaptively transform into hexagonal disks while redirecting the orientation of the colloidal crystals along the flattened surfaces, resulting in void-free, flat, and iridescent coatings.

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Capillary Wetting of Colloidal Monolayers

In article number 2208402, Jong Bin Kim and co-workers study capillary wetting of colloidal monolayers by a limited volume of molten polymer, which enables the production of complex wavy surfaces with controlled waviness. The directional metal deposition on the 3D structures develops both plasmonic and diffraction colors which are otherwise difficult to achieve with conventional pseudo-3D structures.

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Suspensions of colloidal particles experience spontaneous phase separation into particle-rich and particle-depleted domains in the presence of depletant when the enthalpy gain overwhelms entropic loss. The particle-rich solid phase has either crystalline or amorphous arrays depending on the rate of assembly.

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A primary building block of colloidal particles is self-assembled to crystallites along the inner wall of an elastic shell by osmotic compression. The elastic microcapsules are further assembled as a secondary building block to form macroscopic photonic surfaces and patterns.

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In article number 2100782, Shin-Hyun Kim and co-workers design thermo-responsive microcapsules with ultra-thin hydrogel membranes. The microcapsules show molecular-size-selective transmembrane permeation due to the consistent mesh size of the hydrogel membrane. More importantly, the cut-off threshold of permeation can be reversibly adjusted by temperature control. Therefore, the microcapsules provide molecule-selective encapsulation and controlled release of the encapsulants in a programmed manner.

In article number 2002166, Sang Seok Lee, Shin‐Hyun Kim, and Sihun Park design onion‐like photonic multishells with distinct photonic bandgaps. The multilayered structure is spontaneously created in emulsion drops by consecutive steps of phase separation between cholesteric liquid crystals (CLCs) and hydrophilic liquid, triggered by material exchange with the continuous phase. During the phase separation, a chiral dopant is unevenly partitioned in the CLC shells, resulting in multiple stopbands. The photonic multishells provide multi‐wavelength lasing and advanced optical barcoding.

In article 2001384, Shin‐Hyun Kim and co‐workers design plasmonic Janus microspheres using emulsion drops whose interface is decorated by particles. The emulsion drops made of acrylate resin are photopolymerized and the particles are removed. The directional deposition of the metal renders the top hemispheres as plasmonically colored and the position of the particles at the interface is precisely controlled by aging, which enables the tuning of the plasmonic property. The plasmonic colors can be switched on and off by controlling the orientation of the Janus microspheres with an external electric field.

In article number 1802520, Dong‐Ho Kim, Shin‐Hyun Kim, and co‐workers design charged microgels to encapsulate agglomerates of gold nanoparticles for the direct analysis of biological samples through Raman analysis. The charged microgels concentrate oppositely charged small molecules while excluding large proteins, increasing Raman intensity and preventing gold contamination. Using the microgels, fipronil sulfone, a metabolite of a toxic insecticide dissolved in eggs, is directly detected without any sample pretreatment.

In article number 1606894, structural colors are mixed by Yun Ho Kim, Shin-Hyun Kim, and co-workers to yield new colors by designing core–shell capsules that contain two distinct helical structures of cholesteric liquid crystals. The core reflects right-handed red light and the shell reflects left-handed green light, thereby mixing the two colors to develop yellow.

Omniphobic inverse opals are created by structurally and chemically modifying the surface of inverse opals through reactive ion etching. During the etching, void arrays of the inverse opal surface evolves to a triangular post array with re-entrant geometry. The elaborate structure can efficiently pin the air–liquid interface and retain air cavities against water and oil, thereby providing liquid-impermeable inverse opals with invariant photonic bandgap.

Robust photonic microcapsules are created by microfluidic encapsulation of cholesteric liquid crystals with a hydrogel membrane. The membrane encloses the cholesteric core without leakage in medium of water and the core exhibits pronounced structural colors. The photonic ink capsules which have precisely controlled bandgap position and size will provide new opportunity in colorimetric micro-thermometers and optoelectric applications.

We report a new class of bio-inspired nanotadpoles (NTPs) with component-specific functionalities. The plasmonic NTPs with a gold-coated head and a reactive ion etching-treated tail showed the tail length dependence of their cellular uptake, enabling the photothermal treatment of cancer cells with high efficacy.

Morpho butterflys show beautiful colors that arise from periodic nanostructures. Inspired by the butterfly, colloidal photonic crystals are designed to have a form of microdisks, which are then further rendered to be magnetoresponsive. S.-H. Kim and co-workers demonstrate that magnetic-field-controlled flipping of the photonic microdisks enables the switching of structural colors, thereby providing a photonic microdisk display.

Monodisperse microcapsules with ultra-thin membrane are microfluidically-designed to be highly sensitive to osmotic pressure, thereby providing a tool for direct measurement of osmotic strength. To make such osmocapsules, water-in-oil-in-water double-emulsion drops with ultra-thin shell are prepared as templates through emulsification of core-sheath biphasic flow in a capillary microfluidic device. By employing a set of distinct osmocapsules confining aqueous solutions with different osmotic strengths, the osmotic strength of unknown solutions can be estimatedthrough observation of the capsules which are selectively buckled. This approach provides the efficient measurement of osmotic strength with very small volume of liquid, thereby providing a useful alternative to other measurement methods which use complex setups. In addition, in-vivo measurement of the osmotic strength can be potentially accomplished by implanting these biocompatible osmocapsules into tissue, which is difficult to achieve in conventional methods.

Recent advances in droplet microfluidics have enabled the production of monodisperse emulsions that yield highly uniform microparticles, albeit only on a drop-by-drop basis. In addition, microfluidic devices have provided a variety of means for particle functionalization through shaping, compartmentalizing, and microstructuring. These functionalized particles have significant potential for practical applications as a new class of colloidal materials. This feature article describes the current state of the art in the microfluidic-based synthesis of monodisperse functional microparticles. The three main sections of this feature article discuss the formation of isotropic microparticles, engineered microparticles, and hybrid microparticles. The complexities of the shape, compartment, and microstructure of these microparticles increase systematically from the isotropic to the hybrid types. Each section discusses the key idea underlying the design of the particles, their functionalities, and their applications.significant potential for practical applications as a new class of colloidal materials. This feature article describes the current state of the art in the microfluidic-based synthesis of monodisperse functional microparticles. The three main sections of this feature article discuss the formation of isotropic microparticles, engineered microparticles, and hybrid microparticles. The complexities of the shape, compartment, and microstructure of these microparticles increase systematically from the isotropic to the hybrid types. Each section discusses the key idea underlying the design of the particles, their functionalities, and their applications.

Novel microcarriers for encapsulation of bioactive agents have been extensively investigated for therapeutic applications. Recent advances in microfluidics and other techniques have inspired the design of new microcarriers with precisely controlled size, shape, and function, which possibly allow for new medical and biological applications. Various types of novel microcarriers, including block-copolymer nanoparticles, cylindrical microparticles, dense-shell microcapsules, macroporous microcapsules, polymer vesicles, and foldable bilayer microparticles, are reviewed by Seung-Man Yang and co-workers on page 9.

Origami of hydrogel bilayers provides robust mircocapsules through anisotropic volume expansion. In their Communication on page 1420 ff., S.-M. Yang and co-workers show planar bilayer microparticles composed of active and passive layers that can transform into microcapsules with a closed compartment. The reversible transformation by folding and unfolding of microparticles enables in situ encapsulation and triggered release of micro- to nanoscopic encapsulants.

An on-chip spectrometer has been developed using chirped 3D photonic crystals mounted on a complementary metal-oxide-semiconductor sensor array. In their Communication on page 11 649 ff., S.-H. Kim, S.-M. Yang, and co-workers show that colloidal diffusion in a photocurable medium created gradual variations in the lattice constant of the colloidal crystals. The variations in the lattice constant resulted in a color gradient that spanned the entire visible range.

Multicompartment microcapsules with a unique configuration are presented by Shin-Hyun Kim, and Seung-Man Yang, and co-workers on page 1608. Photocurable densely confined core droplets within an oily shell droplet rearrange into a unique configuration that minimizes the interfacial energy. Photopolymerization of the shell phase results in microcapules that are capable of isolating active materials and releasing them in a controlled manner using well-defined nanohole arrays or photothermal nanoscopic silver architectures on thin membranes.