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Construction of biomimetic models for structural color evolution not only gives new photonic phenomena but also provide cues for biological morphogenesis. Here, a novel confined self-assembly method is proposed for the generation of hydroxypropyl cellulose (HPC)-based cholesteric liquid crystals (CLCs) microbubbles. The assembly process relies on the combination of droplet microfluidics, solvent extraction, and a volume confined environment. The as-prepared HPC structural color microbubbles have a transparent shell, an orderly arranged cholesteric liquid crystal (CLC) middle layer, and an innermost bubble core. The size of the microbubble, shell thickness, and the color of the CLC layer can be adjusted by altering the microfluidic parameters. Intriguingly, benefited from the compartmentalization effect provided by droplet microfluidics, microbubbles with multiple cores of different color combinations are generated under precise control. The self-assembled CLCs microbubbles have bright structural color, suspending ability, and good temperature-sensitive characteristics, making them ideal underwater sensors. The present confined assembly approach will shed light on creating novel photonic structures and the HPC microbubble will find widespread applications in multifunctional sensing, optical display, and other related fields are believed.

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The photonic crystals (PhCs) have a bright structural color, but their angular dependence and naked-eye observation subjectivity only apply for qualitative analysis. The HSB color space is a three-channel color analysis technology based on hue (H)-saturation (S)-brightness (B). We use the HSB color space to analyze the structural color of the AM/NIPAM PhCs hydrogel sensor in response to temperature and organic solvents. We proved that the structural color analysis based on the hue value (H) could achieve an analysis accuracy close to the spectrum analysis. In addition, we have obtained stimulus-responsive PhCs structure color images from references and quantitatively analyzed them through the HSB color space. The results show that the H of the structural color can establish a high correlation with the specified target. In some cases, its best fitness exceeds traditional spectroscopy methods. This analysis method will provide a general and quantitative analysis technology for the structural color of PhCs by consumer-grade computers and smartphones.

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As an eco-friendly sustainable iridescent coating, cholesteric cellulose nanocrystal (CNC) is susceptible to substrate effects or shearing effects. In this work, interface interaction and liquid crystal phase transition were evaluated for fabricating iridescent cast or shear coatings of CNCs onto substrates of polystyrene, glass, ceramic, wood, stainless steel, metal, or metal alloy. Three types of substrate effects and four categories of shearing effects on the structure color mechanism of CNC coatings were proposed. Practically, several efficient approaches, such as increasing colloidal concentration, enhancing water-retention of substrates, raising processing temperature, slowing down shearing speed, or doping functional additives were involved. Hence, a feasible strategy was provided for preparing sustainable, iridescent, stable, and industrially scalable coatings of CNCs.

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Colloidal crystals are of great interest to researchers because of their excellent optical properties and broad applications in barcodes, sensors, displays, drug delivery, and other fields. Therefore, the preparation of high quality colloidal crystals in large quantities with high speed is worth investigating. After decades of development, microfluidics have been developed that provide new choices for many fields, especially for the generation of functional materials in microscale. Through the design of microfluidic chips, colloidal crystals can be prepared controllably with the advantages of fast speed and low cost. In this Review, research progress on colloidal crystals from microfluidics is discussed. After summarizing the classifications, the generation of colloidal crystals from microfluidics is discussed, including basic colloidal particles preparation, and their assembly inside or outside of microfluidic devices. Then, applications of the achieved colloidal crystals from microfluidics are illustrated. Finally, the future development and prospects of microfluidic-based colloidal crystals are summarized.

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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.

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A brand-new approach to realizing visible-infrared compatible camouflage is proposed based on a metal-based graphical hetero-structure (MGHS) SiO2/Ag/ZnS/Ag. For different thicknesses (20, 40, and 60 nm) of color-controlling sub-layer, high-contract and large-span structure colors (yellow, navy, and cyan) were observed due to reintroducing constructive interference with a matching intensity of reflected waves. Ultra-low infrared emissivity values of 0.04, 0.05, and 0.04 (with high average reflectance values of 95.46%, 95.31%, and 95.09%) were obtained at 3⁻14 µm. In addition, the well-performing trisecting-circle structure further indicates that it is feasible to design on-demand compatible camouflage patterns using the easily-prepared MGHS.

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A flexible, magnetic field induced structurally colored films with brilliant colors and high physical rigidity were reported in this article. Using an external magnetic field, the photocurable colloidal suspensions that containing superparamagnetic Fe3O4@C colloidal nanocrystal clusters (CNCs) could polymerize under UV light. After polymerization, the films with different colors (red, green, blue) were obtained. Through combination of suspensions which contains Fe3O4@C CNCs with different sizes, a series of multi-colored films were obtained. Moreover, these structural colors can be patterned easily by photolithography and various structural colored patterns were shown in the article. The structural colored patterns could conceal or display its color according to the changing of background which makes them hold significant potential applications for security materials.

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With the rapid development of display-related markets, transparent conductive films (TCFs) with wide viewing angles, high transmittance and low sheet resistance are in high demand. However, as a promising TCF material, metallic membranes with a submicrometer-sized periodicity pattern fabricated by currently available techniques always reveal the angle-dependent structure color which can be a major issue in the development of wide-angle viewing display-related applications. In this work, we demonstrate an Au nanomesh with disordered dual-size apertures as a novel TCF with wide viewing angles which is made via a modified nanosphere lithography technique. The as-prepared Au nanomesh film shows good optoelectronic properties (Rs = 160 Ω sq(-1), T = 80%; Rs = 8 Ω sq(-1), T = 57%) that are similar to the Au nanomesh with single size apertures, while the former exhibits excellent wide-angle viewing performance. There is no obvious change in the film when the viewing angle, the light incidence angle or the orientation of substrate vary in the range of 0-90°. In contrast, a rainbow color is observed with the film with ordered single-size apertures. Electrochromic devices based on the novel metallic film show more uniform color distribution than the devices based on metallic film with ordered single-size apertures under indoor natural light irradiation. These findings demonstrate the applicability of the Au nanomesh film with dual-size apertures in enhancing display quality of high-performance optoelectronic devices.

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The coloration of butterflies that exhibit human visible iridescence from violet to green has been elucidated. Highly tilted multilayers of cuticle on the ridges, which were found in the scales of male S. charonda and E. mulciber butterflies, produce a limited-view, selective wavelength iridescence (ultraviolet (UV)~green) as a result of multiple interference between the cuticle-air layers. The iridescence from C. ataxus originates from multilayers in the groove plates between the ridges and ribs. The interference takes place between the top and bottom surfaces of each layer and incoherently between different layers. Consequently, the male with the layers that are ~270 nm thick reflects light of UV~560 nm (green) and the female with the layers that are ~191 nm thick reflects light of UV~400 nm (violet). T. aeacus does not produce the iridescent sheen which T. magellanus does. No iridescent sheen is ascribed to microrib layers, which are perpendicular to the scale plane, so that they cannot reflect any backscattering. The structures of these butterflies would provide us helpful hints to manipulate light in photoelectric devices, such as blue or UV LEDs.