Iridescent Colors Research Articles

Synthesis of novel colored substrate-free pearlescent pigments of vanadium phosphates with large-size layered platelet morphology

Novel colored substrate-free pearlescent pigments based on vanadium phosphates, VOPO4·2H2O (VOP) and H0.6(VO)3(PO4)3(H2O)3·4H2O (HVP), with layered structures, were synthesized using solution process and hydrothermal synthesis. XRD and SAED analyses confirmed the single crystallinity of VOP and HVP. Hydrogen peroxide (H2O2) was crucial for producing larger plate-like crystal grains of VOP and contributing to the phase formation of HVP. FE-SEM analysis revealed that VOP and HVP consist of layered plate-like particles (platelets) with smooth surfaces, achieving small thicknesses and high aspect ratios (diameter to thickness). Notably, this study is the first to report particle diameters of 221 µm for VOP and 220 µm for HVP and to propose VOP and HVP as potential substrate-free pearlescent pigments. The optical properties are characterized by optical reflectance spectra and CIE L∗a∗b∗ coordinates, while the pearlescent effect was also evaluated. Under an optical microscope, all samples exhibited striking iridescent colors, visible even in different regions within the same particle. Furthermore, 1 wt% of the green HVP pearlescent pigments were successfully dispersed in the poly(methyl methacrylate) (PMMA) matrix, indicating a good suitability for PMMA colorization.

Surface Nanopatterning and Structural Coloration of Liquid Metal Gallium Through Hypergravity Nanoimprinting

AbstractNanoscale structuring of gallium‐based liquid metals has emerged as a promising approach for generating unique properties and functionalities in advanced materials and devices. However, their exceptionally high surface tension presents significant challenges for achieving surface nanostructuring using conventional fabrication techniques. Here, a hypergravity nanoimprinting method is introduced, which harnesses horizontal centrifugation to generate hypergravity fields that drive liquid gallium into nanoscale cavities on an elastic polymer stamp surface at subzero temperatures, where it solidifies and preserves imprinted features down to 100 nm lateral resolution. This surpasses previous limits of gallium patterning, enabling phenomena such as iridescent structural colors with a wide range of hues and high saturation. Numerical simulations reveal the intricate fluid dynamic behaviors and interfacial interactions during the imprinting process, providing valuable insights for process optimization and control. By synergistically combining advanced soft lithography techniques with the reversible solid‐liquid phase transition properties of liquid metals, hypergravity nanoimprinting offers a practical nanofabrication approach, facilitating the development of next‐generation nanoscale gallium devices with finely structured nanofeatures across diverse application domains, such as nanoelectronics and photonic metamaterials.

Active Photonic Glass for Hydrogen Generation.

Chirality is vital in many living species since it is responsible for structural iridescent coloration and plays a key role in light harvesting during natural photosynthesis. Developing photoactive materials with such chiral structures is a challenging but promising strategy for energy applications. Here, we describe a straightforward method to establish an active photonic glass obtained through the co-condensation oftetramethyl orthosilicate (TMOS) and titanium diisopropoxide bis(acetylacetonate) (TAA) dissolved in a liquid crystal formed from cellulose nanocrystalline (CNC). The inorganic glass maintains a long range of chiral nematic ordering, displaying iridescent colors characterized by a Bragg peak reflection. The reflected wavelengths are tuned all over the UV-visible range, demonstrating that the replica of the chiral nematic structure generates photonic properties. Incorporation of gold nanoparticles (Au NPs) into the films is further performed by impregnation/chemical reduction. We show that the charge carrier density and photocatalytic H2 generation were amplified when the photonic band gap edges matched the absorbance of the TiO2 and localized surface plasmon resonance (LSPR) of AuNPs. This photocatalytic glass with chiral nematic ordering and a tunable photonic bandgap paves the way for the development of metamaterials with new applications, such as asymmetric photocatalysis.

Structurally Colored Photonic Crystal Biomimetic Microstructures for Daytime Radiative Cooling.

Daytime radiative cooling offers a novel solution to the energy crisis, enabling green and efficient thermal management in space. High reflectance in the solar spectrum is essential for passive radiative cooling, rendering dye coloring and similar methods unsuitable for colored coolers. This paper presents a structurally colored photonic crystal biomimetic microstructured radiative cooler. Inspired by natural biological systems, this cooler features a dual-layered microtruncated-cone array structure on the surface and bottom membrane layers. The silver reflector and 3D micrograting surface structure produce continuous iridescent colors through multiple interference effects. Optimized lithographic process enable the fabrication of the ordered dual-layer surface microstructure arrays with precise angular combinations. The dual-layered microtruncated-cone introduces a gradient refractive index, reducing impedance mismatch at the interface. As a result, the radiative cooler achieves high solar spectral reflectance (0.95) and high mid-infrared emissivity (0.95). Notably, the net theoretical cooling power and the subambient temperature drop are 106.9 W m-2 and 7.4 °C, respectively, at an ambient temperature of 40 °C, with a measured average temperature reduction of 6.1 °C under direct sunlight. This performance matches that of advanced radiative coolers, striking a balance between aesthetics and radiative cooling capability.

Direct Ink Writing of Cephalopod Skin‐Like Core‐Shell Fibers From Cholesteric Liquid Crystal Elastomers and Dyed Solutions

AbstractDirect ink writing (DIW) of core‐shell structures allows for patterning hollow or composite structures for shape morphing and color displays. Cholesteric liquid crystal elastomers (CLCEs) with liquid crystal mesogens assembled in a helix superstructure are attractive for generating tunable iridescent structural colors. Here, by fine‐tuning the rheology of the core and shell materials, respectively, this study creates droplets or a continuous filament in the core from the precursors of polydimethylsiloxane (PDMS) or poly(vinyl alcohol), whereas CLCE forms the outer shell. By introducing a dye in the droplets, the skin structures of cephalopods, consisting of chromatophores and iridocytes, are mimicked for enhanced color saturation, lightness, and camouflage. After removal of the core material, a CLCE hollow fiber is obtained, which can switch colors upon mechanical stretching and pneumatic actuation, much like papilla along with iridocytes. Further, liquid crystal mesogens assembled in the bulk of the fiber are in polydomain. Thus, the skin appears opalescent at room temperature, much like how leucophores enhance reflectins. Upon heating above the nematic to isotropic transition temperature, the skin becomes transparent. Lastly, a cephalopod model is constructed, where different parts of the model can change colors independently based on different mechanisms.

Sexual dimorphism in the structural colours of the wings of the black soldier fly (BSF) Hermetia illucens (Diptera: Stratiomyidae)

The black soldier fly (BSF) Hermetia illucens (Diptera: Stratiomyidae) plays a significant role at the larval stage in the circular economy due to its ability to convert organic waste into valuable products for energy, food, feed, and agricultural applications. Many data are available on larval development and biomass generation, but basic research on this species is lacking and little is known about adult biology, in particular about the cues involved in sexual recognition. In the present study, using various instruments (stereomicroscope, scanning and transmission electron microscope, hyperspectral camera and spectrophotometer), wing ultrastructure of both sexes was analysed, reflectance and transmission spectra of the wings were measured and behavioural bioassays were carried out to measure male response to specific visual stimuli. The collected data showed the existence of sexual dimorphism in the wings of H. illucens due to iridescent structural colouration generated by a multilayer of melanin located in the dorsal lamina of the central part of the wing. Wing sexual dimorphism is particularly evident regarding the strong emission of blue light of female wings. Blue colour induces in males a strong motivation to mate. The obtained results can help to improve and optimize the breeding techniques of BSF.

Investigation of the origin of structural colors in calliphoridae flies for bioinspiration

The article presents a study of the origin of iridescent structural coloration in the thorax of the Calliphoridae fly, intending to inspire in this fly optical and structural properties of interest for use in industry. We carried out SEM microscopic analyses and modeled optical properties using the transfer matrix. The results indicate that this coloration is due to the presence of a one-dimensional photonic crystal composed of two alternating layers of specific thickness. Microscopic analysis using a scanning electron microscope led to this conclusion. Based on these results, a model was proposed describing the structure as consisting of chitin and air. By modulating the optical properties of this structure at different angles of incidence, it was observed that the iridescent colors, notably green, blue, and violet, matched the predictions made by this modulation. These colors are the result of constructive interference. In addition, we observed the presence of a photonic band gap when exploring the influence of the periodicity of chitin and air multilayer in the fly on reflection intensity. Thus, a comparative study of the fly and that emerged in water with a different refractive index revealed consistency in our model. Finally, the results obtained improve our understanding of the mechanisms responsible for this coloration and pave the way for the development of new materials inspired by nature, with potential applications in the fields of biomimetic engineering and optics.

Bioinspired Three-Component System to Prepare Full-Color Functional Biomimetic Pigments.

Nature provides a great source of inspiration for the development of sustainable materials with excellent properties, among which melanin with optical, electronic, and radiation protection properties are considered to be promising coloring materials. However, compared to chemical pigments, the single color, complex oxidation process, and poor solubility of natural melanin strongly limit their further applications. Here, we introduce a series of melanin-like polymeric pigments with amino acid-encoded physicochemical properties by a simple three-component reaction system. Our protocol enables artificial control of the chromophore structures through the rational design of the substrates and dopants, thereby combining the safety and functionality of biopigments with the color richness of chemical dyes. Similar to the photoprotective effect of natural melanin, the polymeric pigments showed excellent antioxidant activity in reducing free radicals and have the advantages of iridescent color, strong tinting strength, stability, and affordability. Furthermore, due to their ability to dye substrates, these biomimetic are expected to become new low-cost bioactive chromophores and find various biochemical applications such as in clothing and hair dyeing, food addition, and anticounterfeiting detection.

Rapid preparation of full color spectrum structural color coatings with iridescence effects via spray methods

Differing from conventional spray methods that only produce amorphous photonic crystals, this study innovatively achieves the ordered assembly of poly(styrene-methyl methacrylate-acrylic acid) (P(St-MMA-AA)) microspheres into photonic crystals on the substrate surface by adjusting the spray distance, a simple parameter. The resulting coating exhibits a uniform and iridescent structural color. Simultaneously, employing this technique, P(St-MMA-AA) microspheres with three different diameters (234 nm, 178 nm, and 152 nm) were mixed pairwise and co-organized in an ordered manner on the substrate surface. By adjusting the mass ratio of the two types of microspheres, structural color coatings covering the full visible spectrum were facilely and efficiently prepared. The coatings exhibit iridescence, and the structural colors are true colors with high color quality. Additionally, violet, unattainable in traditional color mixing systems, was achieved. The structural color coatings obtained in this study demonstrate promising applications in areas such as environmentally friendly finishes, optical anti-counterfeiting, and true-color displays.

Cellulose nanocrystals-based optical organohydrogel fiber with customizable iridescent colors for strain and humidity response

Stimuli-responsive optical hydrogels are widely used in various fields including environmental sensing, optical encryption, and intelligent display manufacturing. However, these hydrogels are susceptible to water losses when exposed to air, leading to structural damage, significantly shortened service lives, and compromised durability. This study presents mechanically robust, environmentally stable, and multi-stimuli responsive optical organohydrogel fibers with customizable iridescent colors. These fibers are fabricated by incorporating tunicate cellulose nanocrystals, alginate, and acrylamide in a glycerol-water binary system. The synthesized fibers exhibit high strength (1.38 MPa), moisture retention capabilities, and elastic properties. Furthermore, a sensor based on these fibers demonstrates high- and low-temperature resistance along with stimuli-responsive characteristics, effectively detecting changes in environmental humidity and strains. Moreover, the fiber sensor demonstrates continuous, repeatable, and quantitatively predictable moisture discoloration responses across a humidity range of 11 % and 98 %. During strain sensing, the optical-organohydrogel-based sensor demonstrates a large working strain (50 %) and excellent cycling stability, underscoring its potential for effectively monitoring a wide range of intricate human motions. Overall, the synthesized fibers and their simple fabrication method can elicit new avenues for numerous related applications including the large-scale implementation of advanced wearable technology.

Influences of Heat Treatment, Particle Arrangement, and Binder Addition on the Mechanical Robustness of Structurally Colored Coatings Formed with Monodisperse SiO2 Spheres

This article highlights the critical factors influencing the mechanical robustness of structurally colored coatings formed as arrays of monodisperse SiO2 particles. Various types of coatings can be fabricated using electrophoretic deposition (EPD) techniques. Anodic EPD generates coatings with colloidal crystalline structures, iridescent structural color, and noteworthy mechanical robustness owing to interparticle necking during heat treatment, even in the absence of binder additives. Conversely, coatings prepared via cathodic EPD exhibit noniridescent structural color and colloidal amorphous structure, with only marginally improved mechanical robustness after high‐temperature treatment. However, when Mg(OH)2 is codeposited during EPD in the coating, the resultant film exhibits strong adhesion between particles and the substrate—as evidenced by minimal peeling from the substrate even after ultrasonication for 6 h—owing to the binding effect of MgO, which is converted from Mg(OH)2 via heat treatment. The robustness of the film is not dictated by the size of the SiO2 particles, suggesting that this approach can be applied to structurally colored coatings of various colors. Consequently, the results suggest that EPD with the addition of a binder and heat treatment can be used to fabricate particle‐array‐type coatings with extremely high mechanical robustness.

Design and Fabrication of Biosensor for a Specific Microbe by Silicon-Based Interference Color System.

In this paper, one of the great challenges faced by silicon-based biosensors is resolved using a biomaterial multilayer. Tiny biomolecules are deposited on silicon substrates, producing devices that have the ability to act as iridescent color sensors. The color is formed by a coating of uniform microstructures through the interference of light. The system exploits a flat, RNA-aptamer-coated silicon-based surface to which captured microbes are covalently attached. Silicon surfaces are encompassed with the layer-by-layer deposition of biomolecules, as characterized by atomic force microscopy and X-ray photoelectron spectroscopy. Furthermore, the results demonstrate an application of an RNA aptamer chip for sensing a specific bacterium. Interestingly, the detection limit for the microbe was observed to be 2 × 106 CFUmL-1 by visually observed color changes, which were confirmed further using UV-Vis reflectance spectrophotometry. In this report, a flexible method has been developed for the detection of the pathogen Sphingobium yanoikuyae, which is found in non-beverage alcohols. The optimized system is capable of detecting the specific target microbe. The simple concept of these iridescent color changes is mainly derived from the increase in thickness of the nano-ordered layers.

Switchable and conspicuous retroreflective sensors inspired by the wing scale of an emerald swallowtail

Butterfly wings possess distinct micro/nanostructures that contribute to their vibrant coloration, light-trapping capabilities, and sensitivity to various stimuli. These complex features have inspired the creation of diverse devices and systems, such as sensors, photovoltaics, photocatalysis, and robotics. Specifically, the wing scales of the Emerald Swallowtail (Papilio palinurus) display iridescent, polarization-sensitive, and retroreflective colors due to their hierarchical structures. However, current technologies fail to mimic these natural designs fully, limiting their practical application in everyday life. In this study, we introduce a groundbreaking method for fabricating artificial wing scales that emulate the biological structure's functionality with a much simpler geometry. By integrating self-graded lossy media into metallic micro-concavity arrays, we achieve pronounced iridescent effects in both coaxial and non-coaxial arrangements, while preserving retroreflective properties. In particular, the simplified design allows for switchable color patterns based on the viewing angle. Demonstrating the concept, we successfully employ these conspicuous retroreflectors in hydrogen gas detection and the bi-directional/switchable recognition of patterned signals.

Slow Photonic Effect Inducing Improved H2 Generation in Photonic Films with Chiral Nematic Structure

AbstractIntegrating photonic crystals (PCs) into the design of a photocatalyst can significantly enhance its light‐harvesting capability. PCs can manipulate the propagation of light uniquely within a material and reduce its group velocity, thereby enhancing the absorption factor for photocatalysts. However, the slow photon effect in photoactive films with chiral nematic structures has not been reported yet, especially at the blue edge of the photonic bandgap. This work proposes a straightforward one‐pot method to fabricate various photonic films with chiral nematic, namely g‐C3N4/SiO2, TiO2/SiO2, and g‐C3N4/TiO2/SiO2. The sol‐gel biotemplating formulation using cellulose nanocrystals successfully leads to the elaboration of films exhibiting variable iridescent colors with photonic bandgap from UV to visible range. The tunable wavelength of the Bragg peak reflection offers the opportunity to access a region with a slow photonic effect, which directly impacts the light‐harvesting properties of the photoactive material. It is demonstrated that the H2 generation is significantly enhanced when the blue edge of the photonic bandgap position overlapped with the absorbance band of the photocatalyst. These results offer the opportunity to design photonic materials with chiral nematic structure and optimize the photocatalytic performance for energy application.

A Tunable Hydrophilic-Hydrophobic, Stimulus Responsive, and Robust Iridescent Structural Color Bionic Film with Chiral Photonic Crystal Nanointerface.

Bio-inspired in nature, using nanomaterials to fabricate the vivid bionic structural color and intelligent stimulus responsive interface as smart skin or optical devices are widely concerned and remain a huge challenge. Here, the bionic flexible film is designed and fabricated with chiral nanointerface and tunable hydrophilic-hydrophobic by the ultrasonic energy perturbation strategy and crosslinking of the cellulose nanocrystals (CNC). An intelligent nanointerface with adjustable hydrophilic and hydrophobic properties is constructed by the supramolecular assembly using a smart ionic liquid molecule. The bionic flexible film possessed the variable hydrophilic-hydrophobic, stimulus responsive, and robust iridescent structural color. The reflective wavelength and the helical pitch of the film can be easily modulated through the ultrasonic energy perturbation strategy. The bionic flexible film by covalent cross-linking has excellent robustness, good elasticity and flexibility. The tunable brilliant structural color of the chiral nanointerface is attributed to the surface charge change of the CNC photonic crystal, which is disturbed by ultrasonic energy perturbation. The bionic flexible film with tunable structure color has intelligent hydrophilic and hydrophobic stimulus response properties. The chiral bionic materials have potential applications in smart skin, optical devices, bionic materials, robots, anti-counterfeiting, colorful displays, and stealth materials.

Tunable Construction of Chiral Nematic Cellulose Nanocrystals/ZnO Films for Ultra-Sensitive, Recyclable Sensing of Humidity and Ethanol.

The investigation of functional materials derived from sustainable and eco-friendly bioresources has generated significant attention. Herein, nanocomposite films based on chiral nematic cellulose crystals (CNCs) were developed by incorporating xylose and biocompatible ZnO nanoparticles (NPs) via evaporation-induced self-assembly (EISA). The nanocomposite films exhibited iridescent color changes that corresponded to the birefringence phenomenon under polarized light, which was attributed to the formation of cholesteric structures. ZnO nanoparticles were proved to successfully adjust the helical pitches of the chiral arrangements of the CNCs, resulting in tunable optical light with shifted wavelength bands. Furthermore, the nanocomposite films showed fast humidity and ethanol stimuli response properties, exhibiting the potential of stimuli sensors of the CNC-based sustainable materials.

Blaze-angle led dark-blue iridescence and superhydrophobicity features of non-morpho Euploea midamus butterfly wing scale

We report on backlit iridescent blue structural coloration as well as superhydrophobicity in a non-morpho butterfly of Euploea midamus (blue-spotted crow) belonging to the Lepidoptera order. Select forewing and hindwing parts were characterized by employing optical microscopy, field emission electron microscopy, UV–vis-NIR spectrophotometry, and an advanced contact angle meter. As substantiated from variable incident angle reflectance spectra and chromaticity plots, the apparent visual effect is most pronounced in the forewing case and at an incident angle of 30–40°, with reflectance peak maxima positioned at ~ 412 nm and 478 nm. Additionally, the forewing scale of this butterfly acts as an anti-reflection filter (< 460 nm) for p-polarized light, showing greater polarization anisotropy in the lower wavelength region. Numerical simulationand microstructure-based analytical calculations with blaze angle grating effects have been considered to elucidate the observed dark-blue iridescence at large. Moreover, both the forewing and hindwing of the butterfly exhibit the ‘lotus effect’, with a contact angle as high as of ~ 150°, low contact angle hysteresis (16° and 13°) as well as low roll-off angles (10° and 7°) to favor self-cleaning action. Theoretical calculations attributing to dual roughnesses would encompass micro-textured and nanoscale asperities within the wing scale interface. The scope of the bifunctional features including optical and dewetting responses in natural systems would provide valuable insights and clues for biomimetics, particularly in nanophotonic and nanocoating applications.

Characterizing the Microparticles Deposition Structure and its Photonic Nature in Surfactant-Laden Evaporating Colloidal Sessile Droplets.

This work presents a simple method to create photonic microstructures via the natural evaporation of surfactant-laden colloidal sessile droplets on a flat substrate. In the absence of dissolved surfactant, the evaporating colloidal droplet forms a well-known coffee ring deposition. In contrast, the presence of surfactant leads to the formation of multiple ring structures due to the repetitive pinning-depinning behavior of the droplet contact line (CL). It is found that the multiring structure shows vibrant iridescent structural colors while the coffee ring lacks a photonic nature. This difference in the structural color for the presence and absence of the surfactant is found to be dependent on the arrangement of the particles in the deposition structure. The particle arrangement in the multirings is monolayered and well-ordered. The ordering of the particles is strongly influenced by the particle dynamics, contact angle (CA), and CL dynamics of the evaporating colloidal solution droplet. Furthermore, the iridescent nature of the multiring deposition is demonstrated and explained. The dependence of the multiring deposition structure on the concentration of the dissolved surfactant and the suspended particles is also studied. The findings demonstrate that an intermediate surfactant concentration is desirable for the formation of a multiring structure. Further, the pinning-depinning CL dynamics that causes the formation of the multiring deposition structure is discussed. Finally, we demonstrate the applicability of the approach to smaller droplet volumes.

Nanoscale millefeuilles produce iridescent bill ornaments in birds.

Colors are well studied in bird plumage but not in other integumentary structures. In particular, iridescent colors from structures other than plumage are undescribed in birds. Here, we show that a multilayer of keratin and lipids is sufficient to produce the iridescent bill of Spermophaga haematina. Furthermore, that the male bill is presented to the female under different angles during display provides support for the hypothesis that iridescence evolved in response to sexual selection. This is the first report of an iridescent bill, and only the second instance of iridescence in birds in which melanosomes are not involved. Furthermore, an investigation of museum specimens of an additional 98 species, showed that this evolved once, possibly twice. These results are promising, as they suggest that birds utilize a wider array of physical phenomena to produce coloration and should further stimulate research on nonplumage integumentary colors.

A new strategy for structural color fabrics: Transfer of photonic crystal coatings from slides to fabrics

The construction of three-dimensional photonic crystals on textile surfaces has emerged as a highly effective method to create fabrics with structural color, generating significant interest among researchers. However, the strict requirements of the substrate fabrics and the challenges associated with patterning coatings for bright structural colors have hindered the widespread application of photonic crystal materials in the textile industry. In this study, we present a strategy for obtaining patterned structural color coatings on fabrics made of different materials. Pre-crystallized poly(styrene-methyl methacrylate-acrylic acid) (P(St-MMA-AA)) liquid photonic crystals (LPCs) are used as raw material for the rapid preparation of solid photonic crystals (SPCs). These SPCs with structural colors are used as templates and transferred to various fabrics through transfer printing techniques. Importantly, the SPCs maintain an ordered face-centered cubic structure both before and after transfer. Such a feature ensures that the processed structural color fabrics exhibit vibrant and iridescent structural colors. This result is achieved by modification of the substrate fabric with viscous polymer and the intact retention of SPCs. The sandwich structure formed by the adhesive polymer on the base fabric and the encapsulation of the film-forming material on the SPCs imparts good stability to the structural color fabric. It is worth noting that patterned photonic crystal coatings can be easily obtained on the fabrics made of different materials (cotton, linen, wool, and silk fabrics) by manipulating the shape of the viscous polymer. Additionally, a wide range of common materials, such as paper, wood, steel, and others, can be used as target substrates. This novel dyeing strategy not only offers new ideas for preparing patterned structural color fabrics but also further promotes the application of photonic crystal materials in textile printing or display.