Butterfly Scales Research Articles

Pretreated Butterfly Wings for Tuning the Selective Vapor Sensing.

Photonic nanoarchitectures occurring in the scales of Blue butterflies are responsible for their vivid blue wing coloration. These nanoarchitectures are quasi-ordered nanocomposites which are constituted from a chitin matrix with embedded air holes. Therefore, they can act as chemically selective sensors due to their color changes when mixing volatile vapors in the surrounding atmosphere which condensate into the nanoarchitecture through capillary condensation. Using a home-built vapor-mixing setup, the spectral changes caused by the different air + vapor mixtures were efficiently characterized. It was found that the spectral shift is vapor-specific and proportional with the vapor concentration. We showed that the conformal modification of the scale surface by atomic layer deposition and by ethanol pretreatment can significantly alter the optical response and chemical selectivity, which points the way to the efficient production of sensor arrays based on the knowledge obtained through the investigation of modified butterfly wings.

Vapour sensitivity of an ALD hierarchical photonic structure inspired by Morpho

The unique architecture of iridescent Morpho butterfly scales is known to exhibit different optical responses to various vapours. However, the mechanism behind this phenomenon is not fully quantitatively understood. This work reports on process developments in the micro-fabrication of a Morpho-inspired photonic structure in atomic layer deposited (ALD) materials in order to investigate the vapour optical sensitivity of such artificial nanostructures. By developing recipes for dry and wet etching of ALD oxides, we micro-fabricated two structures: one combining Al2O3 and TiO2, and the other combining Al2O3 and HfO2. For the first time, we report the optical response of such ALD Morpho-like structures measured under a controlled flow of either ethanol or isopropyl alcohol (IPA) vapour. In spite of the small magnitude of the effect, the results show a selective vapour response (depending on the materials used).

Brilliant iridescence of Morpho butterfly wing scales is due to both a thin film lower lamina and a multilayered upper lamina

Butterflies belonging to the nymphalid subfamily, Morphinae, are famous for their brilliant blue wing coloration and iridescence. These striking optical phenomena are commonly explained as to originate from multilayer reflections by the ridges of the wing scales. Because the lower lamina of the scales of related nymphalid butterflies, the Nymphalinae, plays a dominant role in the wing coloration, by acting as a thin film reflector, we investigated single blue scales of three characteristic Morpho species: M. epistrophus, M. helenor and M. cypris. The experimental data obtained by spectrophotometry, scatterometry and scanning electron microscopy demonstrated that also in the Morpho genus the lower lamina of both the cover and ground scales acts as an optical thin film reflector, contributing importantly to the blue structural coloration of the wings. Melanin pigment has a contrast-enhancing function in a sub-class of ground scales.Electronic supplementary materialThe online version of this article (doi:10.1007/s00359-016-1084-1) contains supplementary material, which is available to authorized users.

Research on surface lossy electromagnetic scattering of the scales of Morpho rhetenor butterflies

In view to the attenuation of the electromagnetic (EM) wave propagating energy in media, the EM scattering of the simple model of the iridescent tropical butterfly Morpho rhetenor in visible light irradiation is investigated by using finite-difference time-domain (FDTD) method, combined with extensive surface technique. And it is compared with that of the lossless media, and prospective scattering characteristics are verified. As the scattering performance of the structure is characteristic with not only the structural stealth but also the material stealth characteristic, taking all kinds of factors that influence the properties of the media into account, it will help to accurately select and develop late-model high performance stealth material to make a further investigation.

Polymerisation of Aniline on the Butterfly Scale: Bio-Interface Polymerisation

Synthesis of polyaniline on butterfly scale was carried out. Polyaniline emeraldine salt was deposited on the surface of the buttefly scales to form bio/synthetic polymer composite. This research demonstrates bio-interface polymerisation on the butterfly scales.

Polymerisation of Aniline on the Butterfly Scale: Bio-Interface Polymerisation

Synthesis of polyaniline on butterfly scale was carried out. Polyaniline emeraldine salt was deposited on the surface of the buttefly scales to form bio/synthetic polymer composite. This research demonstrates bio-interface polymerisation on the butterfly scales.

Bio-inspired Fabrication of Complex Hierarchical Structure in Silicon.

In this paper, we developed a top-down method to fabricate complex three dimensional silicon structure, which was inspired by the hierarchical micro/nanostructure of the Morpho butterfly scales. The fabrication procedure includes photolithography, metal masking, and both dry and wet etching techniques. First, microscale photoresist grating pattern was formed on the silicon (111) wafer. Trenches with controllable rippled structures on the sidewalls were etched by inductively coupled plasma reactive ion etching Bosch process. Then, Cr film was angled deposited on the bottom of the ripples by electron beam evaporation, followed by anisotropic wet etching of the silicon. The simple fabrication method results in large scale hierarchical structure on a silicon wafer. The fabricated Si structure has multiple layers with uniform thickness of hundreds nanometers. We conducted both light reflection and heat transfer experiments on this structure. They exhibited excellent antireflection performance for polarized ultraviolet, visible and near infrared wavelengths. And the heat flux of the structure was significantly enhanced. As such, we believe that these bio-inspired hierarchical silicon structure will have promising applications in photovoltaics, sensor technology and photonic crystal devices.

Exploring the molecular basis of monarch butterfly color pattern variation: a response to A. Hume's 'Myosin--a monarch of pigment transport?'.

Exploring the molecular basis of monarch butterfly color pattern variation: a response to A. Hume's 'Myosin--a monarch of pigment transport?'.

Polarization-sensitive color in butterfly scales: polarization conversion from ridges with reflecting elements.

Polarization-sensitive color originates from polarization-dependent reflection or transmission, exhibiting abundant light information, including intensity, spectral distribution, and polarization. A wide range of butterflies are physiologically sensitive to polarized light, but the origins of polarized signal have not been fully understood. Here we systematically investigate the colorful scales of six species of butterfly to reveal the physical origins of polarization-sensitive color. Microscopic optical images under crossed polarizers exhibit their polarization-sensitive characteristic, and micro-structural characterizations clarify their structural commonality. In the case of the structural scales that have deep ridges, the polarization-sensitive color related with scale azimuth is remarkable. Periodic ridges lead to the anisotropic effective refractive indices in the parallel and perpendicular grating orientations, which achieves form-birefringence, resulting in the phase difference of two different component polarized lights. Simulated results show that ridge structures with reflecting elements reflect and rotate the incident p-polarized light into s-polarized light. The dimensional parameters and shapes of grating greatly affect the polarization conversion process, and the triangular deep grating extends the outstanding polarization conversion effect from the sub-wavelength period to the period comparable to visible light wavelength. The parameters of ridge structures in butterfly scales have been optimized to fulfill the polarization-dependent reflection for secret communication. The structural and physical origin of polarization conversion provides a more comprehensive perspective on the creation of polarization-sensitive color in butterfly wing scales. These findings show great potential in anti-counterfeiting technology and advanced optical material design.

The Effect of ‘Butterfly skin’ with Rough Internal Surface on the Performances of an Oscillating Airfoil

An experimental investigation was conducted of the effect of ‘butterfly skin’ (metallic version of the butterfly scale) with rough internal surface on vibration and aerodynamic performances of a two-dimensional, 100-mm chord, NACA-230 airfoil oscillating through simulation of flare maneuver, in a 250 x 750 mm low-speed straight through a wind tunnel. Attention was initially directed to this problem by observation of the rough surfaces of butterfly wing scale as well as other studies indicating the vibration suppression of flying butterfly and the better lift of these wings by wing appendages. Results indicated that the ‘butterfly skin’ increased the lift force by a factor of 1.05, and reduced both the duration of vibration by a factor of 1.13, and the frequency of an oscillating airfoil by a factor of 1.5. The modification of the vibration effects on the rotor blade model was due both to an increase in the added mass, which influences the ‘the butterfly skin’, and to a decrease of the internal air flow ve...

Gas sensing using hierarchical micro/nanostructures of Morpho butterfly scales

We have demonstrated the application of the hierarchical micro/nanostructures of Morpho didius butterfly scales for gas sensing. A reflective optical system was set up to detect the reflectance spectra of butterfly wing samples in a specific gas environment. Principle component analysis method was employed to discriminate and identify nitrogen, methanol and ethanol vapors according to the distribution of the projection points. Meanwhile, we constructed an appropriate 2D optical model of the butterfly scales with Rsoft software and theoretically simulated the gas sensing performance by rigorous coupled wave analysis technique. In the theoretical study, exposing the structures to vapors was modeled as vapor adsorption, resulting in the formation of nanometre-thick liquid films onto the structures, where the thickness of the film increased with the vapor concentration increasing. The experimental and simulation results match well, confirming that the butterfly scales indeed have the sensitivity and selectivity for vapors sensing. As such, we believe an optimized artificial structure mimicking Morpho butterfly scales can be used as gas sensors for sensitive and selective detection of closely related vapors.

Temperature and saturation dependence in the vapor sensing of butterfly wing scales

The sensing of gasses/vapors in the ambient air is the focus of attention due to the need to monitor our everyday environment. Photonic crystals are sensing materials of the future because of their strong light-manipulating properties. Natural photonic structures are well-suited materials for testing detection principles because they are significantly cheaper than artificial photonic structures and are available in larger sizes. Additionally, natural photonic structures may provide new ideas for developing novel artificial photonic nanoarchitectures with improved properties. In the present paper, we discuss the effects arising from the sensor temperature and the vapor concentration in air during measurements with a photonic crystal-type optical gas sensor. Our results shed light on the sources of discrepancy between simulated and experimental sensing behaviors of photonic crystal-type structures. Through capillary condensation, the vapors will condensate to a liquid state inside the nanocavities. Due to the temperature and radius of curvature dependence of capillary condensation, the measured signals are affected by the sensor temperature as well as by the presence of a nanocavity size distribution. The sensing materials used are natural photonic nanoarchitectures present in the wing scales of blue butterflies.

Absence of Circular Polarisation in Reflections of Butterfly Wing Scales with Chiral Gyroid Structure

The single Gyroid, a triply-periodic ordered chiral network of cubic symmetry, appears as a nanostructure in the green-colored wing scales of various butterflies. In lossless and perfectly ordered single Gyroid materials, the structural chirality leads to circularly polarized reflections from crystals oriented in the [100] direction. Here we report a circular polarisation study of the macroscopic reflections of the wing scales of Callophrys rubi and Teinopalpus imperialis that reveals no circular dichroism, that is, we find no significant difference in the reflectance values for left- and right-circularly polarized light. The reasons for the absence of circularly polarized reflections is likely to be a compound effect of various factors, including crystallite orientation, presence of both left- and right-handed single Gyroid enantiomers, and structural disorder. Each of these factors weakens, but does not fully extinguish, the circular polarisation signal. We further find a substantial amount of blue-absorbing pigment in those wing scales of C. rubi that are structured according to the single Gyroid. Numerical simulations demonstrate that absorption, while evidently reducing overall reflectance, does generally not reduce the circular dichroism strength. The experimental findings of this paper, however, clearly demonstrate that circular dichroism is absent from the reflections of the butterfly wing scale. Henceforth, the chiro-optical response of the idealised structure does not fulfil a biological photonic function.

Discovery of the surface polarity gradient on iridescent Morpho butterfly scales reveals a mechanism of their selective vapor response

For almost a century, the iridescence of tropical Morpho butterfly scales has been known to originate from 3D vertical ridge structures of stacked periodic layers of cuticle separated by air gaps. Here we describe a biological pattern of surface functionality that we have found in these photonic structures. This pattern is a gradient of surface polarity of the ridge structures that runs from their polar tops to their less-polar bottoms. This finding shows a biological pattern design that could stimulate numerous technological applications ranging from photonic security tags to self-cleaning surfaces, gas separators, protective clothing, sensors, and many others. As an important first step, this biomaterial property and our knowledge of its basis has allowed us to unveil a general mechanism of selective vapor response observed in the photonic Morpho nanostructures. This mechanism of selective vapor response brings a multivariable perspective for sensing, where selectivity is achieved within a single chemically graded nanostructured sensing unit, rather than from an array of separate sensors.

Theoretical and experimental analysis of the structural pattern responsible for the iridescence of Morpho butterflies

Morpho butterflies are well-known for their iridescence originating from nanostructures in the scales of their wings. These optical active structures integrate three design principles leading to the wide angle reflection: alternating lamellae layers, "Christmas tree" like shape, and offsets between neighboring ridges. We study their individual effects rigorously by 2D FEM simulations of the nanostructures of the Morpho sulkowskyi butterfly and show how the reflection spectrum can be controlled by the design of the nanostructures. The width of the spectrum is broad (≈ 90 nm) for alternating lamellae layers (or "brunches") of the structure while the "Christmas tree" pattern together with a height offset between neighboring ridges reduces the directionality of the reflectance. Furthermore, we fabricated the simulated structures by e-beam lithography. The resulting samples mimicked all important optical features of the original Morpho butterfly scales and feature the intense blue iridescence with a wide angular range of reflection.

Butterfly scales as bionic templates for complex ordered nanophotonic materials: A pathway to biomimetic plasmonics

In this paper we propose a possible use of butterfly scales as templates for ordered 2D or 3D nanophotonic materials, with complexity not easily reproducible by conventional micro/nanofabrication methods. Functionalization through laminar nanocompositing is utilized to impart novel properties to the biological scaffold. An extremely wide variability of butterfly scale forms, shapes, sizes and fine structures is observed in nature, many of them already possessing peculiar optical properties. Their nanophotonic functionalization ensures a large choice of forms and functions, including enhanced light localization, light and plasmon waveguiding and general metamaterial behavior, to mention a few. We show that one is able to achieve a combination of plasmonics and bionics, resulting in functionalities seldom if ever met in nature. As an illustration we have analyzed the photonic properties of the nanostructured scales on the wings of Purple Emperor butterflies Apatura ilia, Apatura iris and Sasakia charonda. Their intricate nanometer-sized structures produce remarkable ultraviolet-blue iridescence, spectrally and directionally narrow. We present our analysis of their plasmonic/nanophotonic functionalization including preliminary calculations and initial experimental results. As a simple example, we used radiofrequent sputtering to produce nanoaperture-based plasmonic structures at a fraction of the cost and necessary engineering efforts compared to the conventional top-down methods. We conclude that the described pathway to biomimetic plasmonics offers potentials for significant expansion of the nanophotonic and nanoplasmonic material toolbox.

A Discussion of the Cover Photograph, “Arranged Butterfly Scales”

The mathematics of how diffraction works to separate white light into component colors is a well-studied, predictable phenomenon. But how some iridescent butterfly scales keep showing brilliant iridescent colors when viewed from different angles has been a long-standing mystery. The answer to that question is revealed in a paper by Saranathan et al. (2010), who found that butterflies, through millions of years of selection, evolved a geometric nanostructure within their scale cells, previously known only to mathematicians. The first three-dimensional picture of the arrangement was computer generated by NASA materials scientist A. H. Schoen in 1970. He named the almost impossible-to-describe geometric shape a gyroid . Probably the best way to introduce students to the complexities of a gyroid is to let them climb through one – at least the educators at San Francisco’s Exploratorium think so. They built a 10-foot-high gyroid in the institute’s outdoor Geometry Playground, viewable at http://www.exploratorium.edu/geometryplayground/gyroid.html. A gyroid is a labyrinth-like structure in which there are no straight lines or plane reflectional surfaces (Figure 1). The mindboggling shape is no doubt the key to the omnidirectional irradiance these gyroids, when nano sized, can emit. Saranathan …

Large-visual-angle microstructure inspired from quantitative design of Morpho butterflies’ lamellae deviation using the FDTD/PSO method

The wide angular range of the treelike structure in Morpho butterfly scales was investigated by finite-difference time-domain (FDTD)/particle-swarm-optimization (PSO) analysis. Using the FDTD method, different parameters in the Morpho butterflies' treelike structure were studied and their contributions to the angular dependence were analyzed. Then a wide angular range was realized by the PSO method from quantitatively designing the lamellae deviation (Δy), which was a crucial parameter with angular range. The field map of the wide-range reflection in a large area was given to confirm the wide angular range. The tristimulus values and corresponding color coordinates for various viewing directions were calculated to confirm the blue color in different observation angles. The wide angular range realized by the FDTD/PSO method will assist us in understanding the scientific principles involved and also in designing artificial optical materials.

Structural integration design for enhanced photoluminescence in butterfly scales

Here we present a structural integration design in butterfly scales that helps to enhance the photoluminescence of the scale pigment. The optical mechanism lying behind blue scales of the butterfly Papilio oribazus and green ones from Papilio nireus is illustrated through respective detailed investigation. Microscopic characterization reveals a nano-scaled optical architecture comprised of a network on top of a two-dimensional (2D) photonic crystal slab (PCS) and a multilayer structure located under the PCS. The top side of the PCS is infused with pigments. Combining the experimental reflection spectra and computational simulation through finite difference time domain (FDTD) and plane wave expansion (PWE) methods, we find that the network acts as an antireflection layer harvesting UV excitation light for the pigment, and the 2D PCS works as a light filter and waveguide, functioning together with the multilayer structure to extract needed light from the pigment emission and guide it out of the scales. This finding provides a working prototype to enrich the horizon for the design of sophisticated optical devices with high functionality.

Reply to Roberts et al.: Reflectivity and pointillist structural color on land and in water

Roberts et al. (1) point out the dominance of water creatures when it comes to multilayer interference in nature, both qualitatively and in terms of quantity. This is perhaps unsurprising when we consider that 70% of the earth is covered by water and that marine life has had a considerably longer time frame than terrestrial plants in which to develop and refine optical innovations. In our comparison of the reflectivity of the Pollia fruit to that of structurally colored animals, we focused only on terrestrial species. Although these are most conspicuous to us, Roberts et al. are correct in pointing out the dominance of fish and squid. An accurate revision of our text (2) would state: “The bright blue coloration of this fruit is more intense than that of many previously described biological materials” and “this is the highest reported reflectivity of any land-based biological organism including beetle exoskeleton, bird feathers, and the famously intense blue of Morpho butterfly scales.”