Structural Color Palettes of Core-Shell Photonic Ink Capsules Containing Cholesteric Liquid Crystals

Self-positioning microdevices enable adaptable spatial displaying

Adaptable display with spatial imaging, fostering advancements in extended reality with unconventional form requirements, is indispensable in scientific research, telemedicine, rescue, and space exploration. The adjustable photon spin angular momentum derived from chiral optical materials offer applicative lights for binocular stereo imaging displays, thus allowing an unimaginable immersive experience while maintaining awareness of surroundings. However, current chiral illuminant struggles to obtain adequate electroluminescence asymmetry during power-on display. Here, we present a designed self-positioning strategy to build new flexible spatial displays, integrating numerous multilayered circularly polarized electroluminescent microdevices, for real-time depth information control on the screen. With the devices' luminescence asymmetry value of up to 1.0 under electro-excitation, we visualize third-dimensional information using our chiral material-integrated tablet. Afterward, combined with a robot, we realize a series of remote human-machine interaction operations based on extended reality conditions. Our adaptable spatial display bridges the gap between virtuality and reality, making pioneering explorations in chiral luminous fields for extended reality and beyond.

Arrays of Bowl-Shaped Janus Particle Film with Structured Colors

Structural colors generated via total internal reflection (TIR) using nanostructure-free micro-concave shapes have garnered increasing attention. However, the application of large micro-concave structures for structural coloration remains limited. Herein, a flexibly tunable structural color film fabricated by casting polydimethylsiloxane (PDMS) on an array of large poly(glycidyl methacrylate) (PGMA) bowl-shaped particles is reported. The resultant film exhibits tunable red to green structural colors with changing observation angles. Moreover, the color can be further tailored by altering the shape of the film itself. The incorporation of the PDMS layer not only facilitates a shift in the locus of TIR from the bottom surface to the top concave surface of the particles, thereby enabling the generation of structural color, but also confers enhanced flexibility to the film. Further decoration with silver nanoparticles imparts antimicrobial properties, yielding a novel antimicrobial coating material with structural colors. The simple and cost-effective strategy for the production of structural color films provides potential applications in antimicrobial coatings, enabling accessible and customizable structural coloration using big-size micro-concave particles.

Large-scale production of chiral nematic microspheres

The membrane emulsification technique enables the self-assembly of cellulose nanocrystals (CNCs) confined within a spherical geometry for large-scale production. The resulting solid microspheres show long-range ordering with chiral nematic structures, and this fascinating hierarchical architecture can even be transferred to mesoporous carbon or silica microparticles by a sacrificial template method.

Progress in Fabrication and Applications of Cholesteric Liquid Crystal Microcapsules

Liquid crystals (LCs) are well known for inherent responsiveness to external stimuli, such as light, thermal, magnetic, and electric fields. Cholesteric LCs are among the most fascinating, since they possess distinctive optical properties due to the helical molecular orientation. However, the good flow, easy contamination, and poor stability of small-molecule LCs limit their further applications, and microencapsulation as one of the most effective tools can evade these disadvantages. Microencapsulation can offer shell-core structure with LCs in the core can strengthen their stability, avoiding interference with the environment while maintaining the stimuli-responsiveness and optical properties. Here, we report recent progress in the fabrication and applications of cholesteric LC microcapsules (CLCMCs). We summarize general properties and basic principles, fabrication methods including interfacial polymerization, in-situ polymerization, complex coacervation, solvent evaporation, microfluidic and polymerization of reactive mesogens, and then give a comprehensive overview of their applications in various popular domains, including smart fabrics, smart sensor, smart displays, anti-counterfeiting, information encryption, biomedicine and actuators. Finally, we discuss the currently facing challenges and the potential development directions in this field.

Asymmetric Pairing of Cholesteric Liquid Crystal Droplets for Programmable Photonic Cross-Communication

The photonic cross-communication between photonic droplets has provided complex color patterns through multiple reflections, potentially serving as novel optical codes. However, the cross-communication is mostly restricted to symmetric pairs of identical droplets. Here, a design rule is reported for the asymmetric pairing of two distinct droplets to provide bright color patterns through strong cross-communication and enrich a variety of optical codes. Cholesteric liquid crystal (CLC) droplets with different stopband positions and sizes are paired. The brightness of corresponding color patterns is maximized when the pairs are selected to effectively guide light along the double reflection path by stopbands of two droplets. The experimental results are in good agreement with a geometric model where the blueshift of stopbands is better described by the angles of refraction rather than reflection. The model predicts the effectiveness of pairing quantitatively, which serves as a design rule for programming the asymmetric photonic cross-communication. Moreover, three distinct droplets can be paired in triangular arrays, where all three cross-communication paths yield bright color patterns when three droplets are selected to simultaneously satisfy the rule. It is believed that asymmetric pairing of distinct CLC droplets opens new opportunities for programmable optical encoding in security and anti-counterfeiting applications.

Liquid Crystal Emulsions: A Versatile Platform for Photonics, Sensing, and Active Matter

The self-assembly of liquid crystals (LCs) is a fascinating method for controlling the organization of discrete molecules into nanostructured functional materials. Although LCs are traditionally processed in thin films, their confinement within micrometre-sized droplets has recently revealed new properties and functions, paving the way for next-generation soft responsive materials. These recent findings have unlocked a wealth of unprecedented applications in photonics (e.g. reflectors, lasing materials), sensing (e.g. biomolecule and pathogen detection), soft robotics (e.g. micropumps, artificial muscles), and beyond. This Minireview focuses on recent developments in LC emulsion designs and highlights a variety of novel potential applications. Perspectives on the opportunities and new directions for implementing LC emulsions in future innovative technologies are also provided.

Impact of mesogenic aromaticity and cyano termination on the alignment and stability of liquid crystal shells

We carry out a strategic and systematic variation of the molecular structure of liquid crystals (LCs) molded into spherical shells, surrounded by aqueous isotropic phases internally and externally. Contrary to common expectation, based on previous studies that have almost exclusively been carried out with cyanobiphenyl-based LCs, we find that the director field aligns normal to the LC-water interface when we use an LC molecule that is entirely non-aromatic. We propose to explain this by the inability of such an LC to participate in hydrogen bonding, rendering the normal configuration favorable as it minimizes the molecular cross section in contact with the water. We also find that cyano-terminated LC molecules contribute greatly to stabilizing the LC-water interface. This explains why shells made of cyanobiphenyl LCs are much more stable than shells of LCs with non-cyano-terminated molecules, even if the latter exhibit aromatic cores. Unstable LC shells can be stabilized very efficiently, however, through the addition of a low concentration of molecules that are cyano-terminated, preferably below the threshold for dimerization. Our study provides a much clarified understanding of how the molecular structure dictates the stability and alignment of LC shells, and it will enable a diversification of LC shell research and applications to systems where the use of non-cyanobiphenyl LCs is required.

Nanostructure-free crescent-shaped microparticles as full-color reflective pigments

Structural colors provide a promising visualization with high color saturation, iridescent characteristics, and fade resistance. However, pragmatic uses are frequently impeded by complex manufacturing processes for sophisticated nanostructures. Here, we report a facile emulsion-templating strategy to produce crescent-shaped microparticles as structural color pigments. The micro-crescents exhibit brilliant colors under directional light originating from total internal reflections and optical interferences in the absence of periodic nanostructures while being transparent under ambient light. The colors are finely tunable by adjusting the size of the micro-crescents, which can be further mixed to enrich the variety. Importantly, the pre-defined convex surface secures high stability of colors and enables structural coloration on target surfaces through direct deposition as inks. We anticipate this class of nanostructure-free structural colorants is pragmatic as invisible inks in particular for anti-counterfeiting patches and color cosmetics with distinctive impressions due to low-cost, scalable manufacturing, unique optical properties, and versatility.

Janus Microdroplets with Tunable Self-Recoverable and Switchable Reflective Structural Colors

Microdroplets made from chiral liquid crystals (CLCs) can display reflective structural colors. However, the small area of reflection and their isotropic shape limit their performance. Here, Janus microdroplets are synthesized through phase separation between CLCs and silicone oil. The as-synthesized Janus microdroplets show primary structural colors with ≈14 times larger area compared to their spherical counterparts at a specific orientation; the orientation and thus the colored/transparent states can be switched by applying a magnetic field. The color of the Janus microdroplets can be tuned ranging from red to violet by varying the concentration of the chiral dopant in the CLC phase. Due to the density difference between the two phases, the Janus microdroplets prefer to orientate the silicone oil side up vertically, enabling the self-recoverable structural color after distortion. The Janus microdroplets can be dispersed in aqueous media to track the configuration and speed of magnetic objects. They can also be patterned as multiplexed labels for data encryption. The magnetic field-responsive Janus CLC microdroplets presented here offer new insights to generate and switch reflective colors with high color saturation. It also paves the way for broader applications of CLCs, including anti-counterfeiting, data encryption, display, and untethered speed sensors.

Liquid crystal random lasers

The enthusiasm for research on liquid crystal random lasers (LCRLs) is driven by their unusual optical properties and promising potential for broad applications in manufacturing, communications, medicine and entertainment. From this perspective, we will summarize the most attractive advances in the development of LCRLs in the last decade and propose future prospects. This article will begin with a fundamental description of LCRLs, including the principle of laser generation and a description of LC substances. Then, we spend several chapters on the lasing performance control methods of LCRLs, including random lasing wavelength, threshold, and polarization properties. In addition, we analyze how the LC chiral agent structures, LC core-shell structures and new light-amplifying materials affect the design of LCRL devices. In the last chapter, we discuss the application of LCRLs in 3D displays, information encryption, biochemical sensing and other optoelectronics devices and finally end the perspective with LCRLs' likely directions in future research.

Elastic Instability of Cubic Blue Phase Nano Crystals in Curved Shells

Many crystallization processes, including biomineralization and ice-freezing, occur in small and curved volumes, where surface curvature can strain the crystal, leading to unusual configurations and defect formation. The role of curvature on crystallization, however, remains poorly understood. Here, we study the crystallization of blue phase (BP) liquid crystals under curved confinement, which provides insights into the mechanism by which BPs reconfigure their three-dimensional lattice structure to adapt to curvature. BPs are a three-dimensional assembly of high-chirality liquid crystal molecules arranged into body-centered (BPI) or simple cubic (BPII) symmetries. BPs with submicrometer cubic-crystalline lattices exhibit tunable Bragg reflection and submillisecond response time to external stimuli such as an electric field, making them attractive for advanced photonic materials. In this work, we have systematically studied BPs confined in spherical shells with well-defined curvature and boundary conditions. The optical behavior of shells has also been examined at room temperature, where the cholesteric structure forms. In the cholesteric phase, perpendicular anchoring generates focal conic domains on the shell's surface, which transition into stripe patterns as the degree of curvature increases. Our results demonstrate that both higher degrees of curvature and strong spatial confinement destabilize BPI and reconfigure that phase to adopt the structure and optical features of BPII. We also show that the coupling of curvature and confinement nucleates skyrmions at greater thicknesses than those observed for a flat geometry. These findings are particularly important for integrating BPs into miniaturized and curved/flexible devices, including flexible displays, wearable sensors, and smart fabrics.

Designing photonic microparticles with droplet microfluidics

Photonic materials with a periodic change of refractive index show unique optical properties through wavelength-selective diffraction and modulation of the optical density of state, which is promising for various optical applications. In particular, photonic structures have been produced in the format of microparticles using emulsion templates to achieve advanced properties and applications beyond those of a conventional film format. Photonic microparticles can be used as a building block to construct macroscopic photonic materials, and the individual microparticles can serve as miniaturized photonic devices. Droplet microfluidics enables the production of emulsion drops with a controlled size, composition, and configuration that serve as the optimal confining geometry for designing photonic microparticles. This feature article reviews the recent progress and current state of the art in the field of photonic microparticles, covering all aspects of microfluidic production methods, microparticle geometries, optical properties, and applications. Two distinct bottom-up approaches based on colloidal assembly and liquid crystals are, respectively, discussed and compared.

One-Pot Self-Assembly of Dual-Color Domes Using Mono-Sized Silica Nanoparticles

Spots with dual structural colors on the skin of some organisms in nature are of tremendous interest due to the unique function of their dye-free colors. However, imitation of them requires complicated manufacturing processes, expensive equipment, and multiple predesigned building blocks. In this work, a one-pot strategy based on the phase-separation-assisted nonuniform self-assembly of monosized silica nanoparticles is developed to construct domes with dual structural colors. In drying poly(ethylene glycol)-dextran-based (PEG-DEX) droplets, monosized nanoparticles distribute nonuniformly in two compartments due to the droplet inner flow and different nanoparticle compatibility with the two phases. The dome colors are derived from the self-assembled nanoparticles and are programmable by regulating the assembly conditions. The one-pot strategy enables the preparation of multicolor using only one type of building block. With the dual-color domes, encrypted patterns with a high volume of contents are designed, showing promising applications in information delivery.

Recent advances in the microfluidic production of functional microcapsules by multiple-emulsion templating

Multiple-emulsion drops serve 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 with a controlled composition, order, and dimensions, elevating the value of multiple-emulsion templates. Moreover, recent advances in the microfluidic control of the emulsification and parallelization of drop-making junctions significantly enhance the production throughput for practical use. Metastable multiple-emulsion drops are converted into stable microcapsules through the solidification of selected phases, among which solid shells are designed to function in a programmed manner. Functional microcapsules are used for the storage and release of active materials as drug carriers. Beyond their conventional uses, microcapsules can serve as microcompartments responsible for transmembrane communication, which is promising for their application in advanced microreactors, artificial cells, and microsensors. Given that post-processing provides additional control over the composition and construction of multiple-emulsion drops, they are excellent confining geometries to study the self-assembly of colloids and liquid crystals and produce miniaturized photonic devices. This review article presents the recent progress and current state of the art in the microfluidic production of multiple-emulsion drops, functionalization of solid shells, and applications of microcapsules.

Stable and tunable single-mode lasers based on cholesteric liquid crystal microdroplets

Although many studies on cholesteric liquid crystal (CLC) microdroplet single-mode lasers are available, it has been shown that the stability and tunability of such microdroplets are difficult to achieve simultaneously. In this paper, a new, to the best of our knowledge, method is proposed for the mass and rapid preparation of stable and tunable monodisperse CLC microdroplet single-mode lasers. This is based on the formation of polymer networks on the surface of the microdroplet via interfacial polymerization and a disruption of the orderliness of the polymer networks by increasing the temperature during polymerization, which results in a single pitch inside the microdroplets. This approach enables CLC microdroplet single-mode lasers to achieve improved environmental robustness, while maintaining the same temperature tunability as the unpolymerized sample. Our method has promising future applications in integrated optics, flexible devices, and sensors.

Applications of Microfluidics in Liquid Crystal-Based Biosensors

Liquid crystals (LCs) with stimuli-responsive configuration transition and optical anisotropic properties have attracted enormous interest in the development of simple and label-free biosensors. The combination of microfluidics and the LCs offers great advantages over traditional LC-based biosensors including small sample consumption, fast analysis and low cost. Moreover, microfluidic techniques provide a promising tool to fabricate uniform and reproducible LC-based sensing platforms. In this review, we emphasize the recent development of microfluidics in the fabrication and integration of LC-based biosensors, including LC planar sensing platforms and LC droplets. Fabrication and integration of LC-based planar platforms with microfluidics for biosensing applications are first introduced. The generation and entrapment of monodisperse LC droplets with different microfluidic structures, as well as their applications in the detection of chemical and biological species, are then summarized. Finally, the challenges and future perspectives of the development of LC-based microfluidic biosensors are proposed. This review will promote the understanding of microfluidic techniques in LC-based biosensors and facilitate the development of LC-based microfluidic biosensing devices with high performance.

Tunable multi-mode laser based on robust cholesteric liquid crystal microdroplet

To date, various studies have been dedicated to the development of cholesteric liquid crystal (CLC) microdroplet omnidirectional lasers. In this work, a stable and tunable multi-mode laser emission is achieved by designing a dye-doping CLC microdroplet. In such a structure, the polymer network only exists on the surface, maintaining stability while providing tunability, and due to the uneven distribution of the pitch, it leads to multi-mode laser emission. A large number of microdroplets are produced quickly via a new method based on ultrasonic separation. During the reaction, we introduce interfacial polymerization where monomers and photoinitiator are respectively distributed inside and outside the microdroplets through mutual diffusion, which enables one to make the polymer network exist on the surface instead of the interior. The obtained microdroplet-based multi-mode laser is shown to possess stability and tunability, demonstrating a great potential for flexible devices and 3D displays.

Cylindrical Confinement of Nanocolloidal Cholesteric Liquid Crystal

The organization of nanocolloidal liquid crystals in constrained geometries has fundamental and practical importance, since under confinement, liquid crystals contain stable topological defects that can serve as templates for nanoparticle organization. Three-dimensional confinement of cholesteric (Ch) liquid crystals formed by cellulose nanocrystals (CNCs) have been extensively studied; however, their two-dimensional confinement remains under-investigated. Here, we report the results of systematic experimental studies of two-dimensional confinement of Ch-CNC liquid crystal in cylindrical capillaries with varying inner diameters. Confinement resulted in phase separation of the Ch-CNC liquid crystal into a Ch shell formed by concentric CNC pseudolayers with the helicoidal axis perpendicular to the inner surface of the capillary walls, and a micrometer-diameter isotropic core thread running parallel to the long axis of the capillary. The morphology of the confined Ch-CNC liquid crystal varied when progressively increasing the degree confinement. Finally, we show that phase separation of the Ch-CNC liquid crystal into a Ch shell and an isotropic core is preserved in flexible capillary tubing, suggesting the applicability of this system for the fabrication of flexible optical waveguides.

Liquid Crystals in Curved Confined Geometries: Microfluidics Bring New Capabilities for Photonic Applications and Beyond

The quest for interesting properties and phenomena in liquid crystals toward their employment in nondisplay application is an intense and vibrant endeavor. Remarkable progress has recently been achieved with regard to liquid crystals in curved confined geometries, typically represented as enclosed spherical geometries and cylindrical geometries with an infinitely extended axial-symmetrical space. Liquid-crystal emulsion droplets and fibers are intriguing examples from these fields and have attracted considerable attention. It is especially noteworthy that the rapid development of microfluidics brings about new capabilities to generate complex soft microstructures composed of both thermotropic and lyotropic liquid crystals. This review attempts to outline the recent developments related to the liquid crystals in curved confined geometries by focusing on microfluidics-mediated approaches. We highlight a wealth of novel photonic applications and beyond and also offer perspectives on the challenges, opportunities, and new directions for future development in this emerging research area.

3D-Printed Biomimetic Systems with Synergetic Color and Shape Responses Based on Oblate Cholesteric Liquid Crystal Droplets

Living organisms in nature have amazing control over their color, shape, and morphology in response to environmental stimuli for camouflage, communication, or reproduction. Inspired by the camouflage of the octopus via the elongation or contraction of its pigment cells, oblate cholesteric liquid crystal droplets are dispersed in a polymer matrix, serving as the role of pigment cells and showing structural color due to selective Bragg reflection by their periodic helical structure. The color of 3D-printed biomimetic systems can be tuned by changing the helical pitch via the chiral dopant concentration or temperature. When the oblate liquid crystal droplets are heated up to isotropic, the opaque and colored biomimetic systems become transparent and colorless. Meanwhile, the isotropic liquid crystal droplets tend to become spherical, causing volume contraction along the film plane and volume dilation in the perpendicular direction. The internal strain combined with the gradient distribution of the oblate isotropic liquid crystal droplets result in corresponding shape transformations. The camouflage of a biomimetic octopus and the blossom of a biomimetic flower, both of which show synergetic color and shape responses, are demonstrated to inspire the design of functional materials and intelligent devices.

Bioresorbable Microdroplet Lasers as Injectable Systems for Transient Thermal Sensing and Modulation

Minimally invasive methods for temperature sensing and thermal modulation in living tissues have extensive applications in biological research and clinical care. As alternatives to bioelectronic devices for this purpose, functional nanomaterials that self-assemble into optically active microstructures offer important features in remote sensing, injectability, and compact size. This paper introduces a transient, or bioresorbable, system based on injectable slurries of well-defined microparticles that serve as photopumped lasers with temperature-sensitive emission wavelengths (>4-300 nm °C^-1). The resulting platforms can act as tissue-embedded thermal sensors and, simultaneously, as distributed vehicles for thermal modulation. Each particle consists of a spherical resonator formed by self-organized cholesteric liquid crystal molecules doped with fluorophores as gain media, encapsulated in thin shells of soft hydrogels that offer adjustable rates of bioresorption through chemical modification. Detailed studies highlight fundamental aspects of these systems including particle sensitivity, lasing threshold, and size. Additional experiments explore functionality as photothermal agents with active temperature feedback (ΔT = 1 °C) and potential routes in remote evaluation of thermal transport properties. Cytotoxicity evaluations support their biocompatibility, and ex vivo demonstrations in Casper fish illustrate their ability to measure temperature within biological tissues with resolution of 0.01 °C. This collective set of results demonstrates a range of multifunctional capabilities in thermal sensing and modulation.

Geminate labels programmed by two-tone microdroplets combining structural and fluorescent color

Creating a security label that carries entirely distinct information in reflective and fluorescent states would enhance anti-counterfeiting levels to deter counterfeits ranging from currencies to pharmaceuticals, but has proven extremely challenging. Efforts to tune the reflection color of luminescent materials by modifying inherent chemical structures remain outweighed by substantial trade-offs in fluorescence properties, and vice versa, which destroys the information integrity of labels in either reflection or fluorescent color. Here, a strategy is reported to design geminate labels by programming fluorescent cholesteric liquid crystal microdroplets (two-tone inks), where the luminescent material is 'coated' with the structural color from helical superstructures. These structurally defined microdroplets fabricated by a capillary microfluidic technique contribute to different but intact messages of both reflective and fluorescent patterns in the geminate labels. Such two-tone inks have enormous potential to provide a platform for encryption and protection of valuable authentic information in anti-counterfeiting technology.

Temperature-induced liquid crystal microdroplet formation in a partially miscible liquid mixture

Liquid-in-liquid droplets are typically generated by the partitioning of immiscible fluids, e.g. by mechanical shearing with macroscopic homogenisers or microfluidic flow focussing. In contrast, partially miscible liquids with a critical solution temperature display a temperature-dependent mixing behaviour. In this work, we demonstrate how, for a blend of methanol (MeOH) and the thermotropic liquid crystal (LC) 4-Cyano-4'-pentylbiphenyl (5CB), cooling from a miscible to an immiscible state allows the controlled formation of microdroplets. A near-room-temperature-induced phase separation leads to nucleation, growth and coalescence of mesogen-rich droplets. The size and number of the droplets is tunable on the microscopic scale by variation of temperature quench depth and cooling rate. Further cooling induces a phase transition to nematic droplets with radial configuration, well-defined sizes and stability over the course of an hour. This temperature-induced approach offers a scalable and reversible alternative to droplet formation with relevance in diagnostics, optoelectronics, materials templating and extraction processes.

Responsive Photonic Crystal Microcapsules of Block Copolymers with Enhanced Monochromaticity

Self-assembly of block copolymers (BCPs) has been developed as a promising approach for constructing photonic crystal (PC) microspheres for dynamic optical modulation. However, high curvature in the center of microspheres usually distorts the periodic core structure, leading to an inconsistency of photonic bandgap and poor monochromaticity of structural color. Herein, we report a simple yet robust strategy for fabricating responsive PC microcapsules of polystyrene-b-poly(2-vinylpyridine) through self-emulsification strategy. Interestingly, the microcapsules exhibit bright structural color with significantly enhanced monochromaticity, compared to their solid counterpart, since the microcapsules have no irregular cores. The structural colors of the PC microcapsules not only exhibit a variability through binary mixing of BCPs but also show a responsiveness to pH value. As a colored microcarrier, the PC microcapsules show a potential for visualizing the pH-dependent release behavior of encapsulated hydrophilic cargos on account of pH-responsive structural color.

Inorganic Photonic Microspheres with Localized Concentric Ordering for Deep Pattern Encoding and Triple Sensory Microsensor

Photonic microspheres offer building units with unique topological structures and specific optical functions for diverse applications. Here, a new class of inorganic photonic microspheres with superior robustness, optical and electrical properties is reported by introducing a unique localized concentric ordering architecture and chemical interaction, which further serve as building blocks for deep pattern encoding and multiple sensory optoelectronic devices. Benefiting from localized concentric ordering architecture, the resultant photonic microspheres demonstrate orientation- and angle-independent structural colors. Notably, the formation of well-combined lamellae inorganic layers by chemical interaction grants the microspheres superior mechanical robustness, excellent solvent resistance, thermal stability, and multiple optoelectronic properties simultaneously, rarely seen in previous reports. Owing to these merits, such microspheres are used to construct diverse encoded photonic patterns for anti-counterfeiting applications. Interestingly, cross-communication among neighboring microspheres creates complex photonic sub-patterns, which provide "fingerprint information" with deep encryption security. Moreover, a single photonic microsphere-based optoelectronic microsensor is demonstrated for the first time, which achieves appealing application for real-time health monitoring and safety warning toward triple environmental stimuli. This work not only provides a new kind of robust, multifunctional photonic material, but also opens a new avenue for their uses as complexed pattern encoding and multi-parametric sensing platforms.

Nature-Inspired Emerging Chiral Liquid Crystal Nanostructures: From Molecular Self-Assembly to DNA Mesophase and Nanocolloids

Liquid crystals (LCs) are omnipresent in living matter, whose chirality is an elegant and distinct feature in certain plant tissues, the cuticles of crabs, beetles, arthropods, and beyond. Taking inspiration from nature, researchers have recently devoted extensive efforts toward developing chiral liquid crystalline materials with self-organized nanostructures and exploring their potential applications in diverse fields ranging from dynamic photonics to energy and safety issues. In this review, an account on the state of the art of emerging chiral liquid crystalline nanostructured materials and their technological applications is provided. First, an overview on the significance of chiral liquid crystalline architectures in various living systems is given. Then, the recent significant progress in different chiral liquid crystalline systems including thermotropic LCs (cholesteric LCs, cubic blue phases, achiral bent-core LCs, etc.) and lyotropic LCs (DNA LCs, nanocellulose LCs, and graphene oxide LCs) is showcased. The review concludes with a perspective on the future scope, opportunities, and challenges in these truly advanced functional soft materials and their promising applications.

Photonic Multishells Composed of Cholesteric Liquid Crystals Designed by Controlled Phase Separation in Emulsion Drops

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.

Prolate and oblate chiral liquid crystal spheroids

Liquid crystals are known to exhibit intriguing textures and color patterns, with applications in display and optical technologies. This work focuses on chiral materials and examines the palette of morphologies that arises when microdroplets are deformed into nonspherical shapes in a controllable manner. Specifically, geometrical confinement and mechanical strain are used to manipulate orientational order, phase transitions, and topological defects that arise in chiral liquid crystal droplets. Inspired by processes encountered in nature, where insects and animals often rely on strain and temperature to alter the optical appearance of dispersed liquid crystalline elements, chiral droplets are dispersed in polymer films and deformation induced by uniaxial or biaxial stretching. Our measurements are interpreted by resorting to simulations of the corresponding systems, thereby providing an in-depth understanding of the morphologies that arise in these materials. The reported structures and assemblies offer potential for applications in smart coatings, smart fabrics, and wearable sensors.

Microfluidic Generation of Nanomaterials for Biomedical Applications

As nanomaterials (NMs) possess attractive physicochemical properties that are strongly related to their specific sizes and morphologies, they are becoming one of the most desirable components in the fields of drug delivery, biosensing, bioimaging, and tissue engineering. By choosing an appropriate methodology that allows for accurate control over the reaction conditions, not only can NMs with high quality and rapid production rate be generated, but also designing composite and efficient products for therapy and diagnosis in nanomedicine can be realized. Recent evidence implies that microfluidic technology offers a promising platform for the synthesis of NMs by easy manipulation of fluids in microscale channels. In this Review, a comprehensive set of developments in the field of microfluidics for generating two main classes of NMs, including nanoparticles and nanofibers, and their various potentials in biomedical applications are summarized. Furthermore, the major challenges in this area and opinions on its future developments are proposed.

Textile materials inspired by structural colour in nature

The concept of mimicking structural colour in nature as an alternative to traditional textile coloration techniques would reduce dependency on dyes, pigments and vast quantities of water in the textile supply chain.

Periodic nanostructure-induced change of director profiles and variable stop bands of photonic crystals infiltrated by nematic liquid crystals

Photonic crystals with periodic nanostructures infiltrated by nematic liquid crystals were investigated based on Landau-de Gennes theory. We studied the fine structures of the system within different amplitudes on the sinusoidal boundaries. When modulating the amplitude, the location of the defects will change. Two new director profiles occurred, and the state observed in Appl. Phys. Lett.87, 241108 (2005)APPLAB0003-695110.1063/1.2139846 also appeared. The transmittance curves show a redshift of ${\sim} {0.1}\,\,\unicode{x00B5}{\rm m}$∼0.1µm in the mid-infrared spectra when increasing the amplitude. The location change of defect rings will induce a shift of ${\sim} 22.4\,\,{\rm nm}$∼22.4nm. Variations in sinusoidal boundaries will have an effect on the transmittance spectrum. Elastic anisotropic will also induce a small shift when the structure is fixed. Results could be useful in designing new types of photonic crystal devices or sensors.

Monodisperse Liquid Crystal Network Particles Synthesized via Precipitation Polymerization

The production of liquid crystalline (LC) polymer particles with a narrow size distribution on a large scale remains a challenge. Here, we report the preparation of monodisperse, cross-linked liquid crystalline particles via precipitation polymerization. This versatile and scalable method yields polymer particles with a smectic liquid crystal order. Although the LC monomers are randomly dissolved in solution, the oligomers self-align and LC order is induced. For the polymerization, a smectic LC monomer mixture consisting of cross-linkers and benzoic acid hydrogen-bonded dimers is used. The average diameter of the particles increases at higher polymerization temperatures and in better solvents, whereas the monomer and initiator concentration have only minor impact on the particle size. After deprotonating of the benzoic acid groups, the particles show rapid absorption of a common cationic dye, methylene blue. The methylene blue in the particles can be subsequently released with the addition of Ca²⁺, while monovalent ions fail to trigger the release. These results reveal that precipitation polymerization is an attractive method to prepare functional LC polymer particles of a narrow size distribution and on a large scale.

Photocontrolled Healable Structural Color Hydrogels

Structural color hydrogels with healable capability are of great significance in many fields, however the controllability of these materials still needs optimizing. Thus, this work presents a healable structural color hydrogel with photocontrolling properties. The component parts of the hydrogel are a graphene oxide (GO) integrated inverse opal hydrogel scaffold and a hydrogel filler with reversible phase transition. The inverse opal scaffold provides stable photonic crystal structure and the hydrogel filler is the foundation of healing. Taking advantage of the prominent photothermal conversion efficiency of GO, the healable structural color material is imparted with photocontrolled properties. It is found that the structural color hydrogel shaped in complex patterns can heal under near-infrared (NIR) irradiation. These features indicate that the optical controllable healable structural color hydrogel can be employed in various applications, such as constructing complex objects, repairing tissues, and so on.

Isotropic-isotropic phase separation and spinodal decomposition in liquid crystal-solvent mixtures

Phase separation in mixtures forming liquid crystal (LC) phases is an important yet under-appreciated phenomenon that can drastically influence the behaviour of a multi-component LC. Here we demonstrate, using polarising microscopy with active cooling as well as differential scanning calorimetry, that the phase diagram for mixtures of the LC-forming compound 4'-n-pentylbiphenyl-4-carbonitrile (5CB) with ethanol is surprisingly complex. Binary mixtures reveal a broad miscibility gap that leads to phase separation between two distinct isotropic phases via spinodal decomposition or nucleation and growth. On further cooling the nematic phase enters on the 5CB-rich side, adding to the complexity. Significantly, water contamination dramatically raises the temperature range of the miscibility gap, bringing up the critical temperature for spinodal decomposition from ∼ 2 °C for the anhydrous case to >50 °C if just 3 vol% water is added to the ethanol. We support the experiments with a theoretical treatment that qualitatively reproduces the phase diagrams as well as the transition dynamics, with and without water. Our study highlights the impact of phase separation in LC-forming mixtures, spanning from equilibrium coexistence of multiple liquid phases to non-equilibrium effects due to persistent spatial concentration gradients.

Hydrophobic Poly(tert-butyl acrylate) Photonic Crystals towards Robust Energy-Saving Performance

Photonic crystals (PCs) have been widely applied in optical, energy, and biological fields owing to their periodic crystal structure. However, the major challenges are easy cracking and poor structural color, seriously hindering their practical applications. Now, hydrophobic poly(tert-butyl acrylate) (P(t-BA)) PCs have been developed with relatively lower glass transition temperature (T_g ), large crack-free area, excellent hydrophobic properties, and brilliant structure color. This method based on hydrophobic groups (tertiary butyl groups) provides a reference for designing new kinds of PCs via the monomers with relatively lower T_g . Moreover, the P(t-BA) PCs film were applied as the photoluminescence (PL) enhanced film to enhance the PL intensity of CdSe@ZnS QDs by 10-fold in a liquid-crystal display (LCD) device. The new-type hydrophobic force assembled PCs may open an innovative avenue toward new-generation energy-saving devices.

Super-Elastic Magnetic Structural Color Hydrogels

Structural color hydrogels are promising candidates as scaffold materials for tissue engineering and for matrix cell culture and manipulation, while their super-elastic features are still lacking due to the irreconcilable interfere of the precursor and the self-assembly unit. This hinders many of their practical biomedical applications where elasticity is required. Herein, hydrophilic and size-controllable Fe₃ O₄ @poly(4-styrenesulfonic acid-co-maleic acid) (PSSMA)@SiO₂ magnetic response photonic crystals are fabricated as the assembly units of the structural color hydrogels by orderly packing of core-shell colloidal nanocrystal clusters via a two-step facile synthesis approach. These units are capable of responding instantaneously to an external magnetic field with resistance to interference of ions, thus, by integrating super-elastic hydrogels, super-elastic magnetic structural color hydrogels can be achieved. The structural color arises from the dynamic ordering of the magnetic nanoparticles through the contactless control of external magnetic field, allowing regional polymerization of hydrogels via changing orientation and strength of external magnetic field. These regionally polymerized super-elastic magnetic structural color hydrogels can work as anti-counterfeiting labels with super-elastic identification, which may be widely used in the future.

Hollow Colloid Assembled Photonic Crystal Clusters as Suspension Barcodes for Multiplex Bioassays

Barcode particles have a demonstrated value for multiplexed high-throughput bioassays. Here, a novel photonic crystal (PhC) barcode is presented that consists of hollow colloidal nanospheres assembled through microfluidic droplet templates. Due to their gas-filled core, the resultant barcode particles not only show increased refractive index contrast, but also remain in suspension by adjusting the overall density of the PhC to match that of a detection solution. In addition, magnetic nanoparticles can be integrated to give the barcodes a magnetically controllable motion ability. The encoding ability of the barcodes is demonstrated in microRNA detection with high specificity and sensitivity, and the excellent features of the barcodes make them potentially very useful for biomedical applications.

Separation-cooperated assembly of liquid photonic crystals from polydisperse particles

Easy and cost-effective production of high-quality photonic crystals (PCs) remains challenging but attractive, not just because they are a type of gemstone but more for their scientific applications (e.g., serving as lossless waveguides, visual sensors, novel pigments and novel separation media). Herein presented is a separation-cooperated assembly (SCA) strategy able to organize cheap polydisperse particles into PCs. Its feasibility was validated through sink-induced SCA of poorly disperse (size variation up to 56%) particles into iridescent liquid PCs in 3 days or more. Strikingly, with a sharp photonic band gap down to 10 nm (ca. 1/7 of the reported 66 nm), the liquid PCs are able to cyclically recover their iridescent color in ca 20 s after agitation, and keep their structural order after dryness, making them practicable to write and paint directly. Also significant is that SCA yielded uniform particles with size variation down to 0.7%. It is thus an easy way to isolate homogeneous particles from disperse ones.

Hierarchically porous composite microparticles from microfluidics for controllable drug delivery

Abdominal wall defect repair remains a major clinical need, and a particle-based controllable drug delivery system offers a solution to this problem. Here, we present a new type of hierarchically porous microparticles (HPMs) composed of poly(lactic-co-glycolic acid) (PLGA) and hollow mesoporous silica nanoparticles (HMSNs) for the delivery. The HPMs are generated by drying microfluidic emulsion templates of HMSNs-dispersed PLGA solution. The resultant HPMs have tailorable porous structures, that provide a three-hierarchy architecture for the controlled release of actives. The first hierarchy is formed for controlling the drug release via physical absorption as a result of the presence of the HMSNs in the HPMs. The second hierarchy channels with small pores scattered throughout the surface of the HPMs are formed during evaporation of the solvent. The third hierarchy with openings on the surface of the HPMs is formed as a result of the inner droplets leaking out of the double emulsion templates during the PLGA solidification. Thus, by manipulating the flow of solutions during the microfluidic emulsification, the porous structures of HPMs can be easily and precisely adjusted, and the loaded drugs are delivered at the required rate. These features of the HPMs make them ideal for repairing abdominal wall defects.

Cholesteric Liquid Crystal Shells as Enabling Material for Information-Rich Design and Architecture

The responsive and dynamic character of liquid crystals (LCs), arising from their ability to self-organize into long-range ordered structures while maintaining fluidity, has given them a role as key enabling materials in the information technology that surrounds us today. Ongoing research hints at future LC-based technologies of entirely different types, for instance by taking advantage of the peculiar behavior of cholesteric liquid crystals (CLCs) subject to curvature. Spherical shells of CLC reflect light omnidirectionally with specific polarization and wavelength, tunable from the UV to the infrared (IR) range, with complex patterns arising when many of them are brought together. Here, these properties are analyzed and explained, and future application opportunities from an interdisciplinary standpoint are discussed. By incorporating arrangements of CLC shells in smart facades or vehicle coatings, or in objects of high value subject to counterfeiting, game-changing future uses might arise in fields spanning information security, design, and architecture. The focus here is on the challenges of a digitized and information-rich future society where humans increasingly rely on technology and share their space with autonomous vehicles, drones, and robots.

Wavelength-tunable and shape-reconfigurable photonic capsule resonators containing cholesteric liquid crystals

Cholesteric liquid crystals (CLCs) have a photonic bandgap due to the periodic change of refractive index along their helical axes. The CLCs containing optical gain have served as band-edge lasing resonators. In particular, CLCs in a granular format provide omnidirectional lasing, which are promising as a point light source. However, there is no platform that simultaneously achieves high stability in air and wavelength tunability. We encapsulate CLCs with double shells to design a capsule-type laser resonator. The fluidic CLCs are fully enclosed by an aqueous inner shell that promotes the planar alignment of LC molecules along the interface. The outer shell made of silicone elastomer protects the CLC core and the inner shell from the surroundings. Therefore, the helical axes of the CLCs are radially oriented within the capsules, which provide a stable omnidirectional lasing in the air. At the same time, the fluidic CLCs enable the fine-tuning of lasing wavelength with temperature. The capsules retain their double-shell structure during the dynamic deformation. Therefore, the CLCs in the core maintain the planar alignment along the deformed interface, and a lasing direction can be varied from omnidirectional to bi- or multidirectional, depending on the shape of deformed capsules.

Responsive Block Copolymer Photonic Microspheres

Responsive photonic crystals (PCs) have attracted much attention due to their broad applications in the field of chemical and physical sensing through varying optical properties when exposed to external stimuli. In particular, assembly of block copolymers (BCPs) has proven to be a robust platform for constructing PCs in the form of films or bulk. Here, the generation of BCPs photonic microspheres is presented with 3D periodical concentric lamellar structures through confined self-assembly. The structural color of the spherical PCs can be tuned by selective swelling of one block, yielding large change of optical property through varying both layer thickness and refraction index of the domains. The as-formed spherical PCs demonstrate large reflection wavelength shift (≈400-700 nm) under organic solvent permeation and pH adjustment. Spherical shape and structural symmetry endow the formed spherical PCs with rotation independence and monochrome, which is potentially useful in the fields of displays, sensing, and diagnostics.

Photochromism into nanosystems: towards lighting up the future nanoworld

The ability to manipulate the structure and function of promising nanosystems via energy input and external stimuli is emerging as an attractive paradigm for developing reconfigurable and programmable nanomaterials and multifunctional devices. Light stimulus manifestly represents a preferred external physical and chemical tool for in situ remote command of the functional attributes of nanomaterials and nanosystems due to its unique advantages of high spatial and temporal resolution and digital controllability. Photochromic moieties are known to undergo reversible photochemical transformations between different states with distinct properties, which have been extensively introduced into various functional nanosystems such as nanomachines, nanoparticles, nanoelectronics, supramolecular nanoassemblies, and biological nanosystems. The integration of photochromism into these nanosystems has endowed the resultant nanostructures or advanced materials with intriguing photoresponsive behaviors and more sophisticated functions. In this Review, we provide an account of the recent advancements in reversible photocontrol of the structures and functions of photochromic nanosystems and their applications. The important design concepts of such truly advanced materials are discussed, their fabrication methods are emphasized, and their applications are highlighted. The Review is concluded by briefly outlining the challenges that need to be addressed and the opportunities that can be tapped into. We hope that the review of the flourishing and vibrant topic with myriad possibilities would shine light on exploring the future nanoworld by encouraging and opening the windows to meaningful multidisciplinary cooperation of engineers from different backgrounds and scientists from the fields such as chemistry, physics, engineering, biology, nanotechnology and materials science.

Core-Shell SiO2-PS Colloids with Controlled Eccentric Ratio

Precisely controlling microstructure of colloidal particles is crucial for their applications. Core-shell colloids have been extensively synthesized and used in past decades. However, controlling the location of cores in core-shell particles remains a challenge. To address this problem we explored the synthesis of SiO₂-PS core-shell colloids by using a simple system containing only core particles, monomer, initiator, and water/ethanol and found the increase of ethanol/water ratio can induce a structure transition sequence from eccentric to concentric to eccentric to concentric to eccentric. Furthermore, we illustrate that the eccentric ratios of SiO₂-PS core-shell colloids, that is, the location of SiO₂ cores in the whole particles, can be precisely controlled by a two-step polymerization procedure. It is anticipated that our results can widen the application of core-shell colloids, especially after the introduction of functionality for core or shell materials.