Subtractive Structural Modification of Morpho Butterfly Wings

Different from studies of butterfly wings through additive modification, this work for the first time studies the property change of butterfly wings through subtractive modification using oxygen plasma etching. The controlled modification of butterfly wings through such subtractive process results in gradual change of the optical properties, and helps the further understanding of structural optimization through natural evolution. The brilliant color of Morpho butterfly wings is originated from the hierarchical nanostructure on the wing scales. Such nanoarchitecture has attracted a lot of research effort, including the study of its optical properties, its potential use in sensing and infrared imaging, and also the use of such structure as template for the fabrication of high‐performance photocatalytic materials. The controlled subtractive processes provide a new path to modify such nanoarchitecture and its optical property. Distinct from previous studies on the optical property of the Morpho wing structure, this study provides additional experimental evidence for the origination of the optical property of the natural butterfly wing scales. The study also offers a facile approach to generate new 3D nanostructures using butterfly wings as the templates and may lead to simpler structure models for large‐scale man‐made structures than those offered by original butterfly wings.     

Replication of polypyrrole with photonic structures from butterfly wings as biosensor

Polypyrrole (PPy) with photonic crystal structures were synthesized from Morpho butterfly wings using a two-step templating process. In the first step photonic crystal SiO₂ butterfly wings were synthesized from Morpho butterfly wings and in the second step the SiO₂ butterfly wings were used as templates for the replication of PPy butterfly wings using an in situ polymerization method. The SiO₂ templates were then removed from the PPy butterfly wings using a HF solution. The hierarchical structures down to the nanometer level, especially the photonic crystal structures, were retained in the final PPy replicas, as evidenced directly by field-emission scanning electron microscope (FE-SEM) and transmission electron microscopy (TEM). The optical properties of the resultant PPy replicas were investigated using reflectance spectroscopy and the PPy replicas exhibit brilliant color due to Bragg diffraction through its ordered periodic structures. The preliminary biosensing application was investigated and it was found that the PPy replicas showed a much higher biological activity compared with PPy powders through their response to dopamine (DA), probably due to the hierarchical structures as well as controlled porosity inherited from Morpho butterfly wings. It is expected that our strategy will open up new avenues for the synthesis of functional polymers with photonic crystal structures, which may form applications as biosensors.     

Synthetic Butterfly Scale Surfaces with Compliance‐Tailored Anisotropic Drop Adhesion

Many natural surfaces such as butterfly wings, beetles' backs, and rice leaves exhibit anisotropic liquid adhesion; this is of fundamental interest and is important to applications including self‐cleaning surfaces, microfluidics, and phase change energy conversion. Researchers have sought to mimic the anisotropic adhesion of butterfly wings using rigid surface textures, though natural butterfly scales are sufficiently compliant to be deflected by capillary forces exerted by drops. Here, inspired by the flexible scales of the Morpho aega butterfly wing, synthetic surfaces coated with flexible carbon nanotube (CNT) microscales with anisotropic drop adhesion properties are fabricated. The curved CNT scales are fabricated by a strain‐engineered chemical vapor deposition technique, giving ≈5000 scales of ≈10 µm thickness in a 1 cm² area. Using various designed CNT scale arrays, it is demonstrated that the anisotropy of drop roll‐off angle is influenced by the geometry, compliance, and hydrophobicity of the scales; and a maximum roll‐off anisotropy of 6.2° is achieved. These findings are supported by a model that relates the adhesion anisotropy to the scale geometry, compliance, and wettability. The electrical conductivity and mechanical robustness of the CNTs, and the ability to fabricate complex multidirectional patterns, suggest further opportunities to create engineered synthetic scale surfaces.     

Inspiration from butterfly and moth wing scales: Characterization,modeling, and fabrication

Through billions of years of evolution, nature has created biological materials with remarkable properties. Studying these biological materials can guide the design and fabrication of bio-inspired materials. Many of the complex natural architectures, such as shells, bones, and honeycombs, have been studied to imitate the design and fabrication of materials with improved hardness and stiffness. Recently, an increasing number of researchers have investigated the wings of lepidopterans (butterflies and moths) because these structures may exhibit dazzling colors. Based on previous studies, these iridescent colors are attributable to periodic structures on the scales that constitute the wing surfaces. These complex and diverse structures have recently become a focus of multidisciplinary research due to their promising applications in the display of structural colors, advanced sensors, and solar cells. This review provides a broad overview of the research into these wings, particularly the microstructures in the wing scales. This review investigates the following three fields: structural characterization and optical property analysis of lepidopteran wings, modeling and simulation of the optical properties and microstructure, and the fabrication of artificial structures inspired by these wings.     

A dynamic mesh strategy applied to the simulation of flapping wings

A robust and efficient dynamic grid strategy based on an overset grid coupled with mesh deformation technique is proposed for simulating unsteady flow of flapping wings undergoing large geometrical displacement. The dynamic grid method was implemented using a hierarchical unstructured overset grid locally coupled with a fast radial basis function (RBF)‐based mapping approach. The hierarchically organized overset grid allows transferring the grid resolution for multiple blocks and overlapping/embedding the meshes. The RBF‐based mapping approach is particularly highlighted in this paper in view of its considerable computational efficiency compared with conventional RBF evaluation. The performance of the proposed dynamic mesh strategy is demonstrated by three typical unsteady cases, including a rotating rectangular block in a fixed domain, a relative movement between self‐propelled fishes and the X‐wing type flapping‐wing micro air vehicle DelFly, which displays the clap‐and‐fling wing‐interaction phenomenon on both sides of the fuselage. Results show that the proposed method can be applied to the simulation of flapping wings with satisfactory efficiency and robustness. Copyright © 2016 John Wiley & Sons, Ltd.     

Study of the surface structure of butterfly wings using the scanning electron microscopic moiré method

Scanning electron microscopic (SEM) moiré method was used to study the surface structure of three kinds of butterfly wings: Papilio maackii Menetries, Euploea midamus (Linnaeus), and Stichophthalma howqua (Westwood). Gratings composed of curves with different orientations were found on scales. The planar characteristics of gratings and some other planar features of the surface structure of these wings were revealed, respectively, in terms of virtual strain. Experimental results demonstrate that SEM moiré method is a simple, nonlocal, economical, effective technique for determining which grating exists on one whole scale, measuring the dimension and the whole planar structural character of the grating on each scale, as well as characterizing the relationship between gratings on different scales of each butterfly wing. Thus, the SEM moiré method is a useful tool to assist with characterizing the structure of butterfly wings and explaining their excellent properties.     

Biomimetic Omnidirectional Antireflective Glass via Direct Ultrafast Laser Nanostructuring

Here, a single‐step, biomimetic approach for the realization of omnidirectional transparent antireflective glass is reported. In particular, it is shown that circularly polarized ultrashort laser pulses produce self‐organized nanopillar structures on fused silica (SiO₂). The laser‐induced nanostructures are selectively textured on the glass surface in order to mimic the spatial randomness, pillar‐like morphology, as well as the remarkable antireflection properties found on the wings of the glasswing butterfly, Greta oto, and various Cicada species. The artificial structures exhibit impressive antireflective properties, both in the visible and infrared frequency ranges, which are remarkably stable over time. Accordingly, the laser‐processed glass surfaces show reflectivity smaller than 1% for various angles of incidence in the visible spectrum for s–p linearly polarized configurations. However, in the near‐infrared spectrum, the laser‐textured glass shows higher transmittance compared to the pristine. It is envisaged that the current results will revolutionize the technology of antireflective transparent surfaces and impact numerous applications from glass displays to optoelectronic devices.     

蝶翅精细分级结构纳米Ag-Au/蝶翅复合材料的可控制备

通过调控蝶翅的分步浸渍,在蝶翅模板上原位还原生成不同形状的纳米Ag-Au颗粒,并嵌入蝶翅精细分级结构得到纳米Ag-Au/蝶翅复合材料。在Ag-Au/蝶翅复合材料形成过程中,蝶翅既提供了构筑精细分级结构纳米复合材料的基体模板,又通过活性基团（如：—CONH—、—OH）参与控制Ag-Au颗粒的还原。因此,通过调控浸渍过程的温度和浸渍方式等工艺参数,得到30~50 nm的实心球状、50~80 nm空心球、不规则螺母形等不同形状的纳米Ag-Au粒子,这些纳米粒子原位沉积并均匀镶嵌在蝶翅基体上,不仅实现了对蝶翅的精细分级结构的复制,而且调控了所生成纳米Ag-Au粒子的形状。这种基于自然生物模板进行液相浸渍的制备方法为有效制备具有精细分级结构和多组分功能纳米结构的复合材料提供了重要借鉴方法。     

Air-gap-enhanced pearlescent effect in periodic stratified bilayers of Perna viridis shell

Pearlescent effect is a unique optical phenomenon originated by an interference of visible light with biomaterial having periodic structure such as pearls, mother-of-pearls, mollusk shells, beetle wings, and butterfly wings. The pearlescent effect exhibits series of vivid colors at a particular spot as an angle of observation is changed. In this paper, the pearlescent effect in Asian green mussel (Perna viridis) shell was investigated. The nacreous layer of the shells is an assembly of stratified bilayers of alternate aragonite calcium carbonate and organic matrix. A consecutive chemical/thermal treatment was employed to remove the organic layer. The enhanced pearlescent effect, indicated by more vivid and brighter colors, was due to a greater refractive index difference within the treated structure as the organic layers were replaced by air gaps of the same thickness. The modified transfer matrix method developed for the stratified bilayers system was employed to verify enhanced pearlescent effect. The results from theoretical simulation corroborated the visual observation as a 4-time full-width-at-half-maximum increment of the reflectance peak, and a broader spectral coverage was achieved after the replacement of organic layers by air gaps.     

A 3D numerical and experimental investigation of microstructural alterations around non-metallic inclusions in bearing steel

Non-metallic inclusions such as sulfides and oxides are byproducts of steel manufacturing process. When a component is subjected to repetitive loading, fatigue cracks can emanate from these inclusions due to stress concentrations that happen because of mismatch in elastic–plastic properties of inclusions and matrix. In certain applications such as gears and bearings, crack initiation from inclusions is accompanied with microstructural alteration. This paper employs a numerical as well an experimental approach to investigate these microstructural changes which are so-called “butterfly wings”. A 3D finite element model was developed to obtain the stress distribution in a domain subjected to Hertzian loading with an embedded non-metallic inclusion. A formerly introduced 2D model based on continuum damage mechanics (CDM) was developed to simulate the butterfly wing formation in 3D. Wingspan-to-inclusion ratios were observed at different cross sections following an analytical serial sectioning procedure. A closed form solution was suggested for the wingspan-to-observed-inclusion-diameter ratio and the results were corroborated with the data available in the open literature. On the experimental front, nonmetallic inclusions inside a sample made of bearing steel was detected using ultrasonic inspection method. Rolling contact fatigue (RCF) tests were run on the specimen and post-failure serial sectioning was conducted to understand the 3D shape of butterflies formed around an inclusion detected by ultrasound. Comparison of experimental and numerical serial sectioning of the wings showed a close correlation in the butterfly wings geometry.     

Beyond robins: aerodynamic analyses of animal flight

Recent progress in studies of animal flight mechanics is reviewed. A range of birds, and now bats, has been studied in wind tunnel facilities, revealing an array of wake patterns caused by the beating wings and also by the drag on the body. Nevertheless, the quantitative analysis of these complex wake structures shows a degree of similarity among all the different wake patterns and a close agreement with standard quasi-steady aerodynamic models and predictions. At the same time, new data on the flow over a bat wing in mid-downstroke show that, at least in this case, such simplifications cannot be useful in describing in detail either the wing properties or control prospects. The reasons for these apparently divergent results are discussed and prospects for future advances are considered.     

从自然到仿生的超疏水表面的微观结构

利用体视显微镜、扫描电子显微镜(SEM)、接触角测量仪对蝴蝶翅膀和蝉翼表面的微细结构及疏水性能进行研究发现:蝴蝶翅膀表面的微米级孔穴和蝉翼表面的纳米阶层柱状结构,是其表面具有超疏水特性的根本原因.受此启发,用激光直写法和软刻蚀法制备出微米级周期排列的方柱、方孔微结构,测量其表面静态接触角分别为151.4°和121.7°.实验结果表明,周期排列的方柱和方孔微结构增强了固体表面的疏水性,且微结构的形态对润湿性能有很大的影响.用经典润湿理论对实验结果进行理论分析发现,Cassie理论与Wenzel理论分别适用于不同程度润湿性能的疏水微结构表面,且微结构的参数影响其表面的润湿性能.     

Nano/Micro‐Manufacturing of Bioinspired Materials: a Review of Methods to Mimic Natural Structures

Through billions of years of evolution and natural selection, biological systems have developed strategies to achieve advantageous unification between structure and bulk properties. The discovery of these fascinating properties and phenomena has triggered increasing interest in identifying characteristics of biological materials, through modern characterization and modeling techniques. In an effort to produce better engineered materials, scientists and engineers have developed new methods and approaches to construct artificial advanced materials that resemble natural architecture and function. A brief review of typical naturally occurring materials is presented here, with a focus on chemical composition, nano‐structure, and architecture. The critical mechanisms underlying their properties are summarized, with a particular emphasis on the role of material architecture. A review of recent progress on the nano/micro‐manufacturing of bio‐inspired hybrid materials is then presented in detail. In this case, the focus is on nacre and bone‐inspired structural materials, petals and gecko foot‐inspired adhesive films, lotus and mosquito eye inspired superhydrophobic materials, brittlestar and Morpho butterfly‐inspired photonic structured coatings. Finally, some applications, current challenges and future directions with regard to manufacturing bio‐inspired hybrid materials are provided.     

Confined blue iridescence by a diffracting microstructure: an optical investigation of the Cynandra opis butterfly

When illuminated and viewed along certain well-defined directions, segments on the wings of the butterfly Cynandra opis shows a striking violet-blue to blue-green. We quantify the spectral and the directional properties of these areas of the wings of the insect. Electron microscopy shows that wing scales from these iridescent regions of the wings contain two gratinglike microstructures crossed at right angles. Application of the diffraction theory, as formulated by the Stratton-Silver-Chu integral, to the microstructure can explain all the important features observed experimentally.     

Grazing-incidence iridescence from a butterfly wing

The Troides magellanus butterfly exhibits a specialized iridescence that is visible only when its hind wings are both illuminated and viewed at near-grazing incidence. The effect is due to the presence of a constrained bigrating structure in its wing scales that has been previously observed in only one other species of butterfly (Ancyluris meliboeus). However, whereas the Ancyluris presents wide-angle flickering iridescence, the Troides butterfly uses pigmentary coloration at all but a narrow tailored range of angles, producing a characteristic effect.     

Bio-inspired wing morphing for unmanned aerial vehicles using intelligent materials

Biologically inspired engineering or Biomimicry is the practice of developing designs and technologies inspired by nature. This conscious use of examples from nature is a form of applied case-based reasoning thus treating nature itself as a database of solutions that have survived for millions of years-survival of the fittest. Inspired by nature, we have developed aircraft wings that imitate the amazing flight of birds. This bio-inspired effort, which is the result of a collaborative research program with Defence Science Organisation National Laboratories of Singapore, is concerned with the design and development of a novel wing prototype for unmanned aerial vehicles (UAVs) that morphs seamlessly without the use of complex hydraulics and/or servo motors. The novel design, selected from a number of designs, is characterised by a high degree of flight adaptability, enhanced manoeuvrability and improved performance with a limited added weight. These characteristics were attained through the use of shape memory alloys as actuators in an antagonistic fashion. Unlike compliant actuators that require continued input of thermal energy, antagonistic setup does not suffer from this difficulty. This is because they require the thermal energy to deform the wing but not to maintain its morphed shape. Structural analysis based upon safety factors specified by FAR23 standards and aerodynamic analysis using FLUENT were conducted on the novel designs to validate their suitability as viable wings for UAVs. In addition, conditioning of the shape memory actuators was conducted using a specially designed circuitry that imposes the appropriate heating and cooling cycles at set periodic times. The outcome of this study is manifest in the new designs that satisfy the missions of different UAVs.     

Modifying the anti-wetting property of butterfly wings and water strider legs by atomic layer deposition coating: surface materials versus geometry

Although butterfly wings and water strider legs have an anti-wetting property, their working conditions are quite different. Water striders, for example, live in a wet environment and their legs need to support their weight and bear the high pressure during motion. In this work, we have focused on the importance of the surface geometrical structures in determining their performance. We have applied an atomic layer deposition technique to coat the surfaces of both butterfly wings and water strider legs with a uniform 30?nm thick hydrophilic Al(2)O(3) film. By keeping the surface material the same, we have studied the effect of different surface roughness/structure on their hydrophobic property. After the surface coating, the butterfly wings changed to become hydrophilic, while the water strider legs still remained super-hydrophobic. We suggest that the super-hydrophobic property of the water strider is due to the special shape of the long inclining spindly cone-shaped setae at the surface. The roughness in the surface can enhance the natural tendency to be hydrophobic or hydrophilic, while the roughness in the normal direction of the surface is favorable for forming a composite interface.     

Replication of butterfly wing in TiO2 with ordered mesopores assembled inside for light harvesting

The replication of butterfly wing in TiO2 with ordered mesopores assembled inside in situ was prepared by the method of ultrasonication and then calcination. The resultant replica presents high surface area, excellent light absorbance in visible range of 400-500 nm and a narrowest band-gap at 2.94 eV in comparison with TiO2 replica without ordered mesopores and commercial TiO2 powder, attributing to the combination of the functionality from the inorganic oxide and the fine hierarchical biological structures and well-distributed mesopores. The facile method is expected to be used for mass product of TiO2 replicas from butterfly wings for potential application in Dye-Sensitized Solar Cells.     

Elytra boost lift,but reduce aerodynamic efficiency in flying beetles

Flying insects typically possess two pairs of wings. In beetles, the front pair has evolved into short, hardened structures, the elytra, which protect the second pair of wings and the abdomen. This allows beetles to exploit habitats that would otherwise cause damage to the wings and body. Many beetles fly with the elytra extended, suggesting that they influence aerodynamic performance, but little is known about their role in flight. Using quantitative measurements of the beetle''s wake, we show that the presence of the elytra increases vertical force production by approximately 40 per cent, indicating that they contribute to weight support. The wing-elytra combination creates a complex wake compared with previously studied animal wakes. At mid-downstroke, multiple vortices are visible behind each wing. These include a wingtip and an elytron vortex with the same sense of rotation, a body vortex and an additional vortex of the opposite sense of rotation. This latter vortex reflects a negative interaction between the wing and the elytron, resulting in a single wing span efficiency of approximately 0.77 at mid downstroke. This is lower than that found in birds and bats, suggesting that the extra weight support of the elytra comes at the price of reduced efficiency.     

Controlled replication of butterfly wings for achieving tunable photonic properties

The fine structure of the wing scale of a Morpho Peleides butterfly was examined carefully, and the entire configuration was completely replicated by a uniform Al(2)O(3) coating through a low-temperature ALD process. An inverted structure was achieved by removing the butterfly wing template at high temperature, forming a polycrystalline Al(2)O(3) shell structure with precisely controlled thickness. Other than the copy of the morphology of the structure, the optical property, such as the existence of PBG, was also inherited by the alumina replica. Reflection peaks at the violet/blue range were detected on both original wings and their replica, while a simple alumina coating shifted the reflection peak to longer wavelength because of the change of periodicity and refraction index. The alumina replicas also exhibited similar functional structures as waveguide and beam splitter, which may be used as the building blocks for photonic ICs with high reproducibility and lower fabrication cost compared to traditional lithography techniques.