Iridescent Colors Research Articles

Iridescent Specular Structural Colors of Two-Dimensional Periodic Diffraction Gratings

Specular structural colors generated by two-dimensional periodic binary gratings are investigated theoretically. An approximate mathematical model of the grating specular structural colors is described, based on scalar nonparaxial diffraction theory, and the functional relationships of specular structural color and structural parameters of gratings are analyzed. Through this, the optimal condition for maximizing the color-representation range in the standard CIE 1931 chromaticity diagram is derived.

Tuning the iridescence of chiral nematic cellulose nanocrystal films with a vacuum-assisted self-assembly technique.

Iridescent films composed of the chiral nematic liquid crystal phase of cellulose nanocrystals (CNC) have attracted significant interest due to their fascinating optical properties. However, the current fabrication method, i.e., solution casting with a subsequent evaporation process, has significant limitations and therefore hinders the application of CNC iridescent films. In the present study, we demonstrate, for the first time, that vacuum-assisted self-assembly (VASA) can be used to fabricate highly oriented, large area, smooth, and structurally homogeneous CNC iridescent films. It was found that a long ultrasonic pretreatment is necessary for obtaining CNC iridescent films via VASA. Furthermore, it was also found that the iridescent color of the CNC films can be tuned by the sonication time, suspension volume, and degree of vacuum. By combining CD spectroscopy, SEM, and WAXD techniques, the internal structure of CNC iridescent films prepared by VASA has been investigated in detail. Moreover, the origin of the ultrasonic pretreatment effect on the self-assembly behavior of CNCs is also discussed.

The expression of pre- and postcopulatory sexually selected traits reflects levels of dietary stress in guppies.

Environmental and ecological conditions can shape the evolution of life history traits in many animals. Among such factors, food or nutrition availability can play an important evolutionary role in moderating an animal's life history traits, particularly sexually selected traits. Here, we test whether diet quantity and/or composition in the form of omega-3 long chain polyunsaturated fatty acids (here termed ‘n3LC’) influence the expression of pre- and postcopulatory traits in the guppy (Poecilia reticulata), a livebearing poeciliid fish. We assigned males haphazardly to one of two experimental diets supplemented with n3LC, and each of these diet treatments was further divided into two diet ‘quantity’ treatments. Our experimental design therefore explored the main and interacting effects of two factors (n3LC content and diet quantity) on the expression of precopulatory (sexual behaviour and sexual ornamentation, including the size, number and spectral properties of colour spots) and postcopulatory (the velocity, viability, number and length of sperm) sexually selected traits. Our study revealed that diet quantity had significant effects on most of the pre- and postcopulatory traits, while n3LC manipulation had a significant effect on sperm traits and in particular on sperm viability. Our analyses also revealed interacting effects of diet quantity and n3LC levels on courtship displays, and the area of orange and iridescent colour spots in the males’ colour patterns. We also confirmed that our dietary manipulations of n3LC resulted in the differential uptake of n3LC in body and testes tissues in the different n3LC groups. This study reveals the effects of diet quantity and n3LC on behavioural, ornamental and ejaculate traits in P. reticulata and underscores the likely role that diet plays in maintaining the high variability in these condition-dependent sexual traits.

Now you see me, now you don't: iridescence increases the efficacy of lizard chromatic signals.

The selective forces imposed by primary receivers and unintended eavesdroppers of animal signals often act in opposite directions, constraining the development of conspicuous coloration. Because iridescent colours change their chromatic properties with viewer angle, iridescence offers a potential mechanism to relax this trade-off when the relevant observers involved in the evolution of signal design adopt different viewer geometries. We used reflectance spectrophotometry and visual modelling to test if the striking blue head coloration of males of the lizard Lacerta schreibeiri (1) is iridescent and (2) is more conspicuous when viewed from the perspective of conspecifics than from that of the main predators of adult L. schreibeiri (raptors). We demonstrate that the blue heads of L. schreiberi show angle-dependent changes in their chromatic properties. This variation allows the blue heads to be relatively conspicuous to conspecific viewers located in the same horizontal plane as the sender, while simultaneously being relatively cryptic to birds that see it from above. This study is the first to suggest the use of angle-dependent chromatic signals in lizards, and provides the first evidence of the adaptive function of iridescent coloration based on its detectability to different observers.

Digital color in cellulose nanocrystal films.

Cellulose nanocrystals (CNCs) form chiral nematic phases in aqueous suspensions that can be preserved upon evaporation of water. The resulting films show an intense directional coloration determined by their microstructure. Here, microreflection experiments correlated with analysis of the helicoidal nanostructure of the films reveal that the iridescent colors and the ordering of the individual nematic layers are strongly dependent on the polydispersity of the size distribution of the CNCs. We show how this affects the self-assembly process, and hence multidomain color formation in such bioinspired structural films.

Nanostructured free surfaces in plant-based plywoods driven by chiral capillarity

Periodic nano-structured surfaces in Nature are responsible for functionalities such as iridescent colors observed in some flower petal surfaces that act as diffraction gratings. We present a theoretical scaling and computational analysis of the formation and structural characteristics of surface nano-wrinkling using the well-known cellulose chiral liquid crystals (LC) material model system that displays the twisted plywood architecture. Using a generalized LC shape equation here adapted to cholesteric free surfaces, we derive a model in terms of three capillary pressures whose solution is the ubiquitous periodic relief of plywood surfaces. The period and depth of the surface structure are shown to be dependent on the plywood's helical pitch. The ratio of amplitude to period of the surface undulation scales linearly with the anisotropic surface tension. These new findings can be used to characterize plant-based plywoods as well as to inspire the design of optical devices based on chirality-driven nano-wrinkling.

Feather bacterial load affects plumage condition, iridescent color, and investment in preening in pigeons

Feathers are inhabited by numerous bacteria, some of them being able to degrade feathers, and thus potentially alter thermoregulation and visual communication. To limit the negative effects of feather bacteria on fitness, birds have therefore evolved antimicrobial defense mechanisms, including preening feathers with secretions of the preen gland. However, whether feather bacteria can alter feather condition and color signaling in vivo, and thus whether birds adjust their investment in preening according to feather bacterial load, has barely been investigated. Here, we experimentally decreased and increased feather bacterial load on captive feral pigeons Columba livia and investigated the effects on plumage characteristics and investment in preening. We found that birds of both sexes had a plumage in higher condition and invested less in preen secretion quantity and preening behavior when feather bacterial load was lower. It suggests that preen secretions may be used by pigeons to limit feather degradation by bacteria, but as they are probably costly to produce, their quantity is adjusted depending on feather bacteria load. Birds with lower bacteria load on feathers had brighter iridescent neck feathers, suggesting that feather bacteria may play an important role in the evolution of the signaling function of iridescent color in pigeons. Altogether, our study provides the first experimental evidence for in vivo effects of feather bacteria on plumage degradation and coloration and suggests that preening is an inducible antibacterial defense.

Dynamics of F-actin prefigure the structure of butterfly wing scales

The wings of butterflies and moths consist of dorsal and ventral epidermal surfaces that give rise to overlapping layers of scales and hairs (Lepidoptera, “scale wing”). Wing scales (average length ~200µm) are homologous to insect bristles (macrochaetes), and their colors create the patterns that characterize lepidopteran wings. The topology and surface sculpture of wing scales vary widely, and this architectural complexity arises from variations in the developmental program of the individual scale cells of the wing epithelium. One of the more striking features of lepidopteran wing scales are the longitudinal ridges that run the length of the mature (dead) cell, gathering the cuticularized scale cell surface into pleats on the sides of each scale. While also present around the periphery of other insect bristles and hairs, longitudinal ridges in lepidopteran wing scales gain new significance for their creation of iridescent color through microribs and lamellae. Here we show the dynamics of the highly organized F-actin filaments during scale cell development, and present experimental manipulations of actin polymerization that reveal the essential role of this cytoskeletal component in wing scale elongation and the positioning of longitudinal ribs.

Multipeak aniostropic microfacet model for iridescent surfaces

In this paper, we propose an efficient iridescent rendering method. The iridescent colors on a surface is caused by the different reflection paths of rays with different wavelengths. In microfacet-based rendering model, the dominant reflection path of an incident light ray is statistically described by the microfacet-distribution function (MDF) of the surface. Therefore, the iridescent colors on the surface can be produced by applying different MDFs to different light wave channels. However, it is unnatural for a surface to have significantly different reflection properties in accordance with the light waves. Taking those into account, the proposed method employs identical and anisotropic microfacet distribution function(MDF) for each light wave channel, and rotates the identical anisotropic MDF of each channel with its own angle to produce iridescent reflection. The method also employs multi-peak MDF for the simulation of diffraction effects, and can be successfully applied to iridecent surface such as CD-ROM and mother-of-pearl (nacre) furniture. The experimental results demonstrate that the method enables interactive applications such as games or virtual reality softwares to plausibly express various cases of iridescent surfaces.

Near-Infrared Selective and Angle-Independent Backscattering from Magnetite Nanoparticle-Decorated Diatom Frustules

Periodic heterogeneous structures exhibit color with brilliance through constructive interference of electromagnetic waves in accordance with Bragg’s law. However, the wavelength of diffracted light is strictly angle-dependent, and such periodic structures generate only iridescent color. Here we report that periodically porous, microellipsoidal shells of diatom Pinnularia sp. wavelength-selectively backscatter light at arbitrary incidence. The biosilica frustules could be approximated as many polygonal faces of two-dimensional photonic crystal slabs. Furthermore, surface decoration of the frustule with magnetite nanoparticles produced photonic band gaps in the near-infrared. Magnetite nanoparticle-decorated frustules behave as angle-independent near-infrared reflectors and near-infrared contrast agents in optical coherence tomography.

The lustre of pearls

A bout 15 years ago, I learned from a gemologist friend that the value of pearls is appraised by evaluating the following factors: Size; shape; colour and lustre (a term used for the quality of shinyness). The first three are easily quantified, but what about lustre? I decided to look into the problem as an intriguing piece of applied optics. Pearlescence? Iridescence? What optical phenomenon is responsible for the unique appearance of nacre (i.e. motherof-pearl)? The appearance of abalone shell, especially New Zealand Paua shell, gives a clue: Interference colours. So it must be something to do with the structure, maybe it is due to something like thin film interference? This is confirmed by scanning electron microscopysee Fig.1 – and explained by marine biologists as follows: Nacre consists of thin, tabular crystals of aragonite, which is Calcium Carbonate that the mollusk concentrates from seawater. But that is not all: aragonite normally forms needle-shaped crystals. However in seashells, it is constrained into tabular, platelet-shaped crystals by means of a protein secreted by the animal. This protein has dangling bonds with a spacing that is comparable with that of calcium carbonate molecules arranged in flat layers. So the biology of the situation constrains a change in crystal ‘architecture’ – from acicular, i.e. needle-shaped, to tabular. This, incidentally, gives the resulting composite structure quite remarkable mechanical strength, apart from the interesting optical properties. That seems to explain it, I thought: In abalone shells, the thickness of the “bricks” must be very uniform so that thin-film interference gives rise to iridescent colours. But what about ordinary nacre (mother-of-pearl) and indeed pearls themselves? My first thoughts were that maybe the layers are too thick or maybe of non-uniform thickness in ordinary nacre. I wasted a lot of time trying to calculate the reflection properties of thin films of slightly variable (i.e. poly-disperse) thickness. This turned out to be an interesting exercise, rather more difficult than I thought. I even enlisted the expert help of Professor John Lekner from Victoria University, New Zealand, who spent a brief Sabbatical with us in Melbourne and is a noted specialist in reflection phenomena, random matrices and so forth. It turned out to be a wild goose chase and eventually the real answer dawned on me and turned out to be much simpler than I had thought. Before coming to the real explanation, I must remind you why sugar, salt, snow, etc. are white. Rather surprisingly, many students and even more accomplished physicists are unaware of the answer. Starting with an analogy, the random walk of a drunk near a cliff-top, even if initially stepping away from the cliff face, inevitably ends up at the bottom of the cliff! In a completely analogous way, a photon (or a ray, if you prefer) entering a substance consisting of small transparent crystals gets randomly refracted (scattered) and ends up coming back to the surface, whence it emerges in some random direction, never to return to the bulk. This is a form of diffuse reflection, which – if the crystallites are totally non-absorbing – corresponds to 100% reflection. Even better than the best metallic mirrors! So, a white surface is a better reflector than a metallic surface. Even with a certain amount of absorption caused by impurities, this gives rise to strong diffuse reflection, hence the utility of white paint. The mean depth to which the photon (or ray) penetrates depends on the scattering power of the crystallites. This, in cartoon by melbourne artist Leunig

Iridescent thin films of porous anodic aluminum oxide with embedded silver nanowires

Anodic aluminum oxide (AAO) embedded Ag with iridescent colors were fabricated by using an AC electrochemical deposition technique. It was found that by changing the oxidation time of the aluminum in a phosphoric acid electrolyte, anodic aluminum oxide composite films can be made to have different colors. Compared to the color saturation of alumina films, the color saturation of AAO/Ag composite films was dramatically increased. The mechanisms for films displaying brilliant colors were discussed, and provide experimental results for iridescent patterns that were prepared by an organics-assisted method. AAO/Ag films with structural color can be used in many areas, including display, decoration, and anti-counterfeiting technology.

Interplay between iridescent and non-iridescent coloration in bio-inspired electrically-tunable nanostructures

This work is inspired by biological nanostructured surfaces possessing structural color generation and wettability control. Nanostructured films consisting of a tantalum pentoxide (Ta2O5) nanopost array (nanograss) formed on top of a continuous Ta2O5 layer on a reflective Ta thin-film have been fabricated and investigated. A non-iridescent coloration typical for short-range order was produced by the nanograss while iridescent coloration was produced by the underlying continuous Ta2O5 layer. When the nanograss surface is wetted with organic liquids, a different non-iridescent blue color appears. Moreover, this blue color can be dynamically and reversibly switched on and off by applying electrical current to an indium-tin-oxide electrode.

Scared by shiny? The value of iridescence in aposematic signalling of the hibiscus harlequin bug

Studies on aposematism have generally focused on the benefits of red or yellow coloration, occasionally in contrast with green or brown, but rarely blue or orange. Furthermore, almost no studies have explicitly studied the utility of iridescent coloration in aposematism. To evaluate the survival benefit of iridescent coloration, we tested the ability of the natural colour extremes of Tectocoris diophthalmus jewelbugs to induce initial avoidance, learned avoidance, discrimination from palatable alternatives and broad generalization against avian predators: naive hand-reared and experienced wild-caught great tits, Parus major. Artificial baits were created by hollowing out bugs and inserting pieces of mealworm. Preference tests presented iridescent and orange baits simultaneously, then birds were divided into training groups and sequentially exposed to palatable black baits alternated with iridescent or orange baits made unpalatable by soaking mealworms in quinine solution. This was followed by simultaneous black/coloured discrimination tests, then a generalization test with both previously experienced and novel baits (all palatable). All groups showed a preference for orange baits over ones with iridescent patches. For wild-caught birds, attack latencies of iridescent and orange training groups were statistically indistinguishable, although only orange caused increased attack latency over the sequence. Hand-reared birds showed no change in attack rate/latency towards iridescent bugs over the sequence. In postlearning discrimination tests, all groups showed equally strong preference for palatable black baits and their unpalatable training baits. In generalization tests, hand-reared birds were most averse towards trained baits, but increased avoidance of iridescent-and-black baits suggests iridescence alone can contribute to aposematism. Wild-caught birds showed strong aversion to iridescent and novel orange-and-black baits regardless of training, suggesting birds may be broadly generalizing experience from local red-and-black aposematic bugs. Results suggest iridescent coloration and patterning can be an effective aposematic signal, especially in the presence of alternative palatable prey and/or other aposematic species.

The rheology and processing of “edge sheared” colloidal polymer opals

This paper is concerned with the rheology and processing of solvent-free core shell “polymer opals” that consist of a soft outer shell grafted to hard colloidal polymer core particles. Strong iridescent colors can be produced by shearing the material in a certain way that causes the initially disordered spheres to rearrange into ordered crystalline structures and produce colors by diffraction and interference of multiple light scattering, similar to gemstone opals. The basic linear viscoelastic rheology of a polymer opal sample was determined as a function of temperature, and the material was found to be highly viscoelastic at all tested temperatures. A Cambridge multipass rheometer was specifically modified in order to make controlled mechanical measurements of initially disordered polymer opal tapes that were sandwiched between protective polyethylene terephthalate sheets. Axial extension, simple shear, and a novel “edge shearing” geometry were all evaluated, and multiple successive experiments of the edge shearing test were carried out at different temperatures. The optical development of colloidal ordering, measured as optical opalescence, was quantified by spectroscopy using visible backscattered light. The development of opalescence was found to be sensitive to the geometry of deformation and a number of process variables suggesting a complex interaction of parameters that caused the opalescence. In order to identify aspects of the deformation mechanism of the edge shearing experiment, a separate series of in situ optical experiments were carried out and this helped indicate the extent of simple shear generated with each edge shear deformation. The results show that strong ordering can be induced by successive edge shearing deformation. The results are relevant to polymer opal rheology, processing, and mechanisms relating to ordering within complex viscoelastic fluids.

Facile Assembly of 3D Binary Colloidal Crystals from Soft Microgel Spheres

It still remains a big challenge to fabricate binary colloidal crystals (binary CCs) from hard colloidal spheres, although a lot of efforts have been made. Here, for the first time, binary CCs are assembled from soft hydrogel spheres, PNIPAM microgels, instead of hard spheres. Different from hard spheres, microgel binary CCs can be facilely fabricated by simply heating binary microgel dispersions to 37 °C and then allowing them to cool back to room temperature. The formation of highly ordered structure is indicated by the appearance of an iridescent color and a sharp Bragg diffraction peak. Compared with hard sphere binary CCs, the assembly of PNIPAM microgel binary CCs is much simpler, faster and with a higher "atom" economy. The easy formation of PNIPAM microgel binary CC is attributed to the thermosensitivity and soft nature of the PNIPAM microgel spheres. In addition, PNIPAM microgel binary CCs can respond to temperature change, and their stop band can be tuned by changing the concentration of the dispersion.

Glass-based confined structures fabricated by sol-gel and radio frequency sputtering

Some of the main results obtained in the field of glass-based photonic crystal (PC) systems using complementary techniques, such as radio frequency (RF) sputtering and sol-gel route, are presented. Initially, rare earth-activated one-dimensional PCs fabricated by RF-sputtering technique will be discussed, specifically the cavity is constituted by an Er 3 + -doped SiO 2 active layer inserted between two Bragg reflectors consisting of 10 pairs of SiO 2 / TiO2/ layers. Moreover, from near infrared, transmittance and variable angle reflectance spectra have verified the presence of a stop band from 1500 to 2000 nm with a cavity resonance centered at 1749 nm at 0 deg and quality factor of 890. In the second case, a composite system based on polystyrene colloidal nanoparticles assembled and embedded in an elastomeric matrix will be presented in detail. This system has been designed as a structure that displays an iridescent green color that can be attributed to the PC effect. This feature has been exploited to create a chemical sensor; in fact optical measurements have evidenced that this system presents a different optical response as a function of the solvent applied on the surface, showing: (1) high sensitivity, (2) fast response, and (3) reversibility of the signal change.

Nanostructured Surfaces: Bioinspiration for Transparency, Coloration and Wettability

Natural nanostructures rarely come with a single biological function to fulfil. Moreover, from a bio-inspiration perspective, it sounds attractive to develop multifunctional coatings, devices or sensors. Suppression of light reflection from body parts, such as the wings of insects, is useful for hiding from predators. The transparent parts of the wings of Cacostatia ossa (moth) inherit their antireflective property from non-close-packed nano-scale nipple arrays on both sides of the wings. Through modelling and optical simulations, we show that effective medium approaches, commonly used to characterize antireflection, slightly overestimate the reflectance with respect to detailed Rigorous Coupled Wave Analysis calculation. Coloration due to light interference in nanostructure, on the other hand, sometimes comes with an additional, unexpected and maybe non-biologically significant function: hygrochromism, i.e. change of color with humidity. The male Hoplia coerulea (beetle), for instance, exhibits iridescent blue-violet color which turns to emerald green when the elytron is put in contact with water. Impregnation experiments with various liquids revealed intriguing color change dynamics which could be related to the wetting properties of porous chitinous cuticle. Super-hydrophobicity is another function of biological significance, which helps, for instance, insects to keep dry in humid environments. Through morphological, optical and contact angle measurements, we show the existence of entangled levels of functionality on the wings of Cicada orni, namely a superhydrophobic stage at the upper part of the nipple array corrugated surface and an antireflective stage and the lower part.

Condition-dependent mate assessment and choice by peahens: implications for sexual selection

Condition-dependent mate choice is thought to affect the strength and direction of sexual selection. Although there is ample evidence of this from studies that experimentally manipulate female condition, few studies to date have examined condition dependence of natural mate choice wherein females interact with a pool of available males. We examined mate assessment by free-living Pavo cristatus peafowl, focusing on 3 measures of the choosing female’s condition: mass condition (scaled mass index), ectoparasite load, and white blood cell (WBC) count. Females with greater mass condition were more active on peacock leks, approaching and visiting males more often, and spending more time near the males they visited. Condition did not affect the total number of males visited or the probability that a female would visit a male that she had already approached. Peacock mating success is strongly correlated with the iridescent colors of the eyespots on their train, suggesting that color signals are a major focus of female choice. Here, we show that females in better condition (higher mass condition and lower WBC count) allocate a greater proportion of their visit time to the most iridescent males displaying on leks. Our results provide evidence of strong condition-dependent effects on female mate assessment and choice in the absence of experimentally induced stress. We suggest that condition-dependent mate choice may help maintain variation in sexually selected male color traits.

Toward bioinspired nanostructures for selective vapor sensing: diverse vapor-induced spectral responses within iridescent scales of Morpho butterflies

ABSTRACTThe iridescent colors of Morpho butterflies have captured scientific intrigue for over a century. However, only recently photonic structures of the wing scales of Morpho butterflies have inspired new ideas in the diverse areas of technology including sensing. In this study, we performed theoretical and experimental evaluation of vapor-induced reflectance changes of the Morpho scales. These experiments provided additional details of the origin and the magnitude of vapor response selectivity in these natural photonic nanostructures and facilitated our design and fabrication of highly selective biomimetic photonic nanostructures.