No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
White hairy layer on the Boehmeria nivea leaf - inspiration for reflective coatings
Abstract
Whiteness is an intriguing property in some creature surfaces and usually originates from broadband multi-scattering by the refined structures. In this article, we report that Boehmeria nivea, a widely distributed tropical and subtropical plant, has a highly reflective layer on the lower surface of the leaf. Morphological characterization demonstrates that the layer consists of numerous wrinkled micro-filaments, forming a disordered porous network to efficiently scatter visible light. Moreover, the white layer is shown to exhibit a protection function by reflecting incident light when exposed to high radiation. The reflective layer can slightly improve the absorption by the leaves when light is incident on the upper surface of the leaves. In addition, the porous layer shows hydrophobicity. To mimic the white layer, a well-established electrospinning process is used to fabricate porous polymeric membranes, consisting of nano-wrinkled filaments with micro-sized diameter. Finally, the artificial membranes are demonstrated to have a light-shielding function in a photo-chromic experiment and a light-management ability for quantum dot film.
... Another study focused on the differences in wettability between the leaf surface, wing cells, and foot cells to develop a one-way valve for water conduction and absorption [133]. Other biomimetic examples are a microrobot system, suitable for manipulation in agriculture, based on the hooked trichomes of Galium aparine leaves [134] or reflective coatings that were claimed as being inspired by dense trichome covers [135]. ...
... Various protective functions (e.g., evaporation, herbivores, pathogens, irradiation, heat), water capture, water absorption, climbing, channeling of light, mechanical stabilization, unknown * Capillary water conduction, drag reduction, antifouling, insect control, fog harvesting, valve concepts, adhesion and climbing for robotics, light reflectance, light capture[123][124][125][126][127][128][129][130][131][132][133][134][135][136] ...
As organs of photosynthesis, leaves are of vital importance for plants and a source of inspiration for biomimetic developments. Leaves are composed of interconnected functional elements that evolved in concert under high selective pressure, directed toward strategies for improving productivity with limited resources. In this paper, selected basic components of the leaf are described together with biomimetic examples derived from them. The epidermis (the “skin” of leaves) protects the leaf from uncontrolled desiccation and carries functional surface structures such as wax crystals and hairs. The epidermis is pierced by micropore apparatuses, stomata, which allow for regulated gas exchange. Photosynthesis takes place in the internal leaf tissue, while the venation system supplies the leaf with water and nutrients and exports the products of photosynthesis. Identifying the selective forces as well as functional limitations of the single components requires understanding the leaf as an integrated system that was shaped by evolution to maximize carbon gain from limited resource availability. These economic aspects of leaf function manifest themselves as trade-off solutions. Biomimetics is expected to benefit from a more holistic perspective on adaptive strategies and functional contexts of leaf structures.
... Barthlott et al demonstrated that a floating water fern Salvinia molesta is capable of holding an air film underwater for several weeks with the assistance of its superhydrophobic leaf surface covered by densely distributed eggbeater-shaped micro hairs with hydrophilic pins [8,9]. Recently, our group has reported that the ramie leaf shows moderate hydrophobicity on the lower surface of the leaf covered by a dense white hairy layer [10]. Indeed, many terrestrial plants contain diverse microstructures such as wax crystalloids, cuticular folds, and trichomes on their leafs or petal surfaces to provide exceptional water repellency [10,11]. ...
... Recently, our group has reported that the ramie leaf shows moderate hydrophobicity on the lower surface of the leaf covered by a dense white hairy layer [10]. Indeed, many terrestrial plants contain diverse microstructures such as wax crystalloids, cuticular folds, and trichomes on their leafs or petal surfaces to provide exceptional water repellency [10,11]. Considering the abundance of plants, there still exist many unknown superhydrophobic organism structures to be explored in nature. ...
The Kapok petal is reported for the first time that it shows a superhydrophobic characteristic with a static water contact angle higher than 150°. Intriguingly, there exist single-scale micro-trichomes and no more nanocrystals on a kapok petal in contrast to most natural superhydrophobic surfaces with hierarchical morphologies, such as lotus leaf and rose petal. Experiment results show that kapok petal has an excellent self-cleaning ability either in air or oil. Further scanning electron microscope characterization demonstrates that the superhydrophobic state is induced by densely-distributed microscale trichomes with an average diameter of 10.2 μm and a high aspect ratio of 17.5. A mechanical model is built to illustrate that the trichomes re-entrant curvature should be a key factor to induce the superhydrophobic state of the kapok petal. To support the proposed mechanism, gold-wire trichomes with a re-entrant curvature are fabricated and the results show that a superhydrophobic state can be induced by microstructures with a re-entrant curvature surface. Taking the scalability and cost-efficiency of microstructure fabrication into account, we believe the biomimetic structures inspired by the superhydrophobic kapok petal can find numerous applications that require a superhydrophobic state.
... Note that the outstanding whiteness of Cyphochilus scale promoted numerous bioinspired fabrications. As a versatile method, electrospinning was used to fabricate highly reflective films consisting of nano-or micro-filaments [7]. Caixeiro et al. achieved a cellulose nanocrystal membrane with a TMFP value of 3.5 µm by a sacrificial template method [8]. ...
... Since a short TMFP indicates a stronger scattering ability, the porous films have higher backscattering strength in the blue region, which matches the transmittance and reflectance measurement results. The results indicate that the scattering ability of the made porous films show comparable performance with several typical artificial high-scattering media [7][8][9][10][11][12], which proves the feasibility of the PIPS method for making high-scattering films. ...
Inspired by the porous scale of the bright white beetle Cyphochilus, a polymerization-induced phase separation method is proposed to fabricate bioinspired high-scattering polymer films with porous structures. With an optimized formulation, the porous films with a mean pore size of ∼200nm feature a broadband reflectance of ∼71% at a thickness of 16 µm and are measured to have a transport mean free path of ∼3µm. The porous films with high reflectivity enable the application on light-emitting diodes and have great potential in other similar optoelectronic fields.
... From an industrial perspective this conclusion is fortuitous, as optimal whiteness can be achieved without closely imitating a particular type of disordered structure. The universality of optimal whiteness also explains why there are many different disordered structures in biological systems 14,[63][64][65][66][67][68][69][70][71] , as there is not a strong evolutionary pressure to converge on a single structural class. However, we expect morphology to play a greater role in structures with higher refractive index contrast, where higher order interactions from multiple scattering events are more significant. ...
Maximizing the scattering of visible light within disordered nano-structured materials is essential for commercial applications such as brighteners, while also testing our fundamental understanding of light-matter interactions. The progress in the research field has been hindered by the lack of understanding how different structural features contribute to the scattering properties. Here we undertake a systematic investigation of light scattering in correlated disordered structures. We demonstrate that the scattering efficiency of disordered systems is mainly determined by topologically invariant features, such as the filling fraction and correlation length, and residual variations are largely accounted by the surface-averaged mean curvature of the systems. Optimal scattering efficiency can thus be obtained from a broad range of disordered structures, especially when structural anisotropy is included as a parameter. These results suggest that any disordered system can be optimised for whiteness and give comparable performance, which has far-reaching consequences for the industrial use of low-index materials for optical scattering.
... By mimicking the disordered porous network from numerous wrinkled microfilaments within the layer of the Boehmeria nivea, Yu et al. [144] adopted electrospinning process to fabricate porous polymeric artificial reflective coatings for light-shielding and optoelectronic applications by visible light scattering effect. Fan's team [145] discovered that the cicada Cryptotympana atrata could protect themselves from overheating under hot environment with radiative-cooling properties, attributed to the brilliant golden micro-spikes with nanophotonic porous heart-shaped cross section (Fig. 12a). ...
The structural evolutions of the organisms during the development of billions of years endow them with remarkable thermal-regulation properties, which have significance to their survival against the outer versatile environment. Inspired by the nature, there have been extensive researches to develop thermoregulating materials by mimicking and utilizing the advantages from the natural organisms. In this review, the latest advances in thermal regulation of bioinspired microstructures are summarized, classifying the researches from dimension. The representative materials are described with emphasis on the relationship between the structural features and the corresponding thermal-regulation functions. For one-dimensional materials, wild silkworm cocoon fibers have been involved, and the reasons for unique optical phenomena have been discussed. Pyramid cone structure, grating and multilayer film structure are chosen as typical examples of two-dimensional bionics. The excellent thermal performance of the three-dimensional network frame structures is the focus. Finally, a summary and outlook are given.
... The results also suggest that one has to be careful when inferring the about underlying formation mechanism of biological disordered systems using optical response and S 2 (r) alone, due to non-uniqueness of these characteristics. Conversely this would also explain why there are many different disorder system in nature that exhibit whiteness [33][34][35] , as the results suggest that any disordered system generated by a tuneable mechanism can be optimised for brilliant whiteness. ...
Increasing the light scattering efficiency of nanostructured materials is becoming an active field of research both in fundamental science and commercial applications. In this context, the challenge is to use inexpensive organic materials that come with a lower refractive index than currently used mineral nanoparticles, which are under increased scrutiny for their toxicity. Although several recent investigations have reported different disordered systems to optimise light scattering by morphological design, no systematic studies comparing and explaining how different topological features contribute to optical properties have been reported yet. Using in silico synthesis and numerical simulations, we demonstrate that the reflectance is primarily determined by second order statistics. While remaining differences are explained by surface area and integrated mean curvature, an equal reflectance can be obtained by further tuning the structural anisotropy. Our results suggest a topological invariance for light scattering, demonstrating that any disordered system can be optimised for whiteness.
Conventional inorganic‐nanoparticles‐based scattering systems have dominated many practical applications for years. In contrast, the rise of porous polymers is perceived as a game‐changer due to their low cost, facile preparation, and great abundance. One challenging issue to be tackled is the design and fabrication of porous polymers with light‐scattering properties comparable to those of inorganic nanoparticles. Taking inspiration from nature (e.g., from white beetles Cyphochilus ), scientists have achieved remarkable progress in the field of light‐scattering porous polymers and their related applications in recent years. Therefore, here, an up‐to‐date review about this emerging field is provided. This overview covers materials for making porous polymer structures, detailed fabrication methods, and applications benefitting from their tailorable light‐scattering properties. It is envisioned that more bioinspired light‐scattering porous polymers will be made to be potential alternatives of conventional nanoparticles‐based scatterers.
Inspired by the Cyphochilus scale with extreme whiteness, fluoropolymer is used as the matrix to prepare highly scattering porous polymer films through polymerization-induced phase separation (PIPS) method. The mixture used for the PIPS technique contains a fluoride monomer, a UV photoinitiator, and porogens (cyclohexanol and perfluorooctanol). By carefully tuning the mixture parameters (cyclohexanol–perfluorooctanol and monomer–porogen weight ratios), the porous morphology can be well tailored, which induces different scattering performances. With an optimized formulation (50 wt% monomer, 25 wt% cyclohexanol, and 25 wt% perfluorooctanol), the porous film with a thickness of 55 µm can achieve an average total reflectance of ≈90%, featuring a measured transport mean free path of 1.3–1.7 µm (comparable to the 1.5 µm of the benchmark Cyphochilus scale). Further, the porous films are applied to the light-emitting diode (LED) device and it is demonstrated that they can effectively improve the light extraction efficiency of the LED and thus enhance the luminous performance. Due to the simplicity, cost-effectiveness, and scalability of the PIPS method, the as-developed porous fluoropolymer films with such excellent scattering ability can find many potential applications not only in the field of optoelectronics, but also in various complex scenarios, such as daytime passive radiative cooling.
Due to their high transparency, electrodes fabricated from conductive polymers are often implemented in semitransparent organic solar cells. Opaque solar cells usually employ metal back electrodes with high reflectivity for best photon confinement in the light‐harvesting layer. Herein, a bilayer back electrode comprising conductive polymers and nanofoamed poly(methyl methacrylate) (PMMA) is investigated, the latter of which creates diffuse reflection of the incoming light. By tuning the thickness of the nanofoamed PMMA layer, absorption and transmission of the solar cells can be tailored from opaque to vastly transparent. Due to its diffusive character, this versatile electrode enhances the light absorption in the wavelength regimes with lower absorption coefficient. The solar cells are particularly suited for deployment in frosted window applications. A bilayer polymer electrode comprising conductive polymers and supercritical nanofoamed poly(methyl methacrylate) (PMMA) is applied to organic solar cells. Gas inclusions inside the PMMA act as light‐scattering centers. By adjusting the thickness of the nanofoamed layer, the performance and translucency of the solar cell can be tailored. The logo is reproduced with permission from Karlsruhe Institute of Technology.
We adopt polymerization-induced phase separation to fabricate highly scattering porous films, which serve as a diffusing reflective layer for enhancing the optical efficiency of light emitting diode lamp.
Precise optical and thermal regulatory systems are found in nature, specifically in the microstructures on organisms’ surfaces. In fact, the interaction between light and matter through these microstructures is of great significance to the evolution and survival of organisms. Furthermore, the optical regulation by these biological microstructures is engineered owing to natural selection. Herein, the role that microstructures play in enhancing optical performance or creating new optical properties in nature is summarized, with a focus on the regulation mechanisms of the solar and infrared spectra emanating from the microstructures and their role in the field of thermal radiation. The causes of the unique optical phenomena are discussed, focusing on prevailing characteristics such as high absorption, high transmission, adjustable reflection, adjustable absorption, and dynamic infrared radiative design. On this basis, the comprehensive control performance of light and heat integrated by this bioinspired microstructure is introduced in detail and a solution strategy for the development of low‐energy, environmentally friendly, intelligent thermal control instruments is discussed. In order to develop such an instrument, a microstructural design foundation is provided.
Disordered porous polymer structures have gained tremendous attention due to their wide applications in various fields. As a simple yet versatile technique, supercritical CO2 microcellular foaming has been proposed to fabricate highly scattering porous polymer films, which have been used to enhance the efficiency of quantum dots (QDs) films. In the foaming process, numerous enclosed pores are generated, which induce significant scattering, underpinning the efficiency enhancement in optoelectronic devices. However, the scattering property of foamed porous structures has still not been well investigated, and effective guidelines for engineering the porous structures are still not available. In this work, we use Mie scattering theory and ray-tracing simulation to analyze the optical property of a single pore, pore assembly, and porous film. Furthermore, it is demonstrated that the scattering scheme in the porous QD films leads to a large enhancement of excitation light absorption and QD emission extraction. It is envisioned that our work will contribute to the engineering guidelines of porous structures and boost the application of porous structures in similar fields.
The formation of microscopic cavities and microfibrils at stress hotspots in polymers is typically undesirable and is a contributor to material failure. This type of stress crazing is accelerated by solvents that are typically weak enough not to dissolve the polymer substantially, but which permeate and plasticize the polymer to facilitate the cavity and microfibril formation process1–3. Here we show that microfibril and cavity formation in polymer films can be controlled and harnessed using standing-wave optics to design a periodic stress field within the film⁴. We can then develop the periodic stress field with a weak solvent to create alternating layers of cavity and microfibril-filled polymers, in a process that we call organized stress microfibrillation. These multi-layered porous structures show structural colour across the full visible spectrum, and the colour can be tuned by varying the temperature and solvent conditions under which the films are developed. By further use of standard lithographic and masking tools, the organized stress microfibrillation process becomes an inkless, large-scale colour printing process generating images at resolutions of up to 14,000 dots per inch on a number of flexible and transparent formats5,6.
The inorganic halide perovskite quantum dots (QDs) have been considered as a promising substitute for white light-emitting diodes (WLEDs). In this paper, the green CsPbBr
<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>
QDs and red K
<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>
SiF
<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub>
:Mn
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup>
(KSF) phosphor were used to fabricate the conversion layers. Because the location of the layers is fundamental to the absorption priority of blue light, the location of KSF and QDs were controlled and the QDs-up type and QDs-down type WLEDs were made. The optical power, luminous efficiency, CCE, and luminous intensity in the middle of QDs-up type are 13.83 mW, 13.54 lm/W, 11.94%, and 2.65 cd, meaning 24.26%, 25.72%, 2.63%, and 23.83% higher than those of QDs-down type, respectively. In addition, the QDs-up type has a lower correlated color temperature (CCT) shift of 734 K and a decreased highest temperature of 56.8° (51.5% and 14.9% lower). These key property differences indicate that the QDs-up type is more suitable for the application in display and backlight. In order to explore the reasons for these differences, the emission spectra, CCE, reflection rate, absorption rate, and temperature curves of QDs or KSF films were also analyzed, which provided a better understanding of designing package structures.
Micro-scale patterned arrays and nano-scale rough morphology are promising for improving the light-extraction performance of GaN-based thin film light-emitting diodes (TFLEDs), while the light-extraction mechanisms of the multiscale architectures combining these two structures have not been investigated yet. In this report, we have adopted a pattern transfer and wet etching combined method to fabricate multiscale patterned arrays with rough morphology (msPARM) on n-GaN layers for TFLEDs and investigated their light-extraction mechanisms by the finite-difference time domain and ray-tracing combined method. Results show that the TFLEDs achieve the maximum radiant efficacy using the msPARM with an etching time of 8 min, which is increased by 16.3% and 1.7% compared with that achieved using only the patterned arrays or only the rough morphology, respectively. Most importantly, optical simulation reveals that the msPARM can provide a high transmittance for light with large emission angles from the active region using the inclined surface of micro-scale concave cones, while effectively suppressing the reflection loss for light with small emission angles using the scattering effect of nano-scale rough morphology, resulting in enhancing the light-extraction of TFLEDs. Consequently, this study can provide a better understanding to design the multiscale structures for achieving high efficiency LEDs.
The scales of the white Cyphochilus beetles are endowed with unusual whiteness arising from the exceptional scattering efficiency of their disordered ultrastructure optimized through millions of years of evolution. Here, a simple, one‐step method based on water vapor–induced phase separation is developed to prepare thin polystyrene films with similar microstructure and comparable optical performance. A typical biomimetic 3.5 µm PS film exhibits a diffuse reflectance of 61% at 500 nm wavelength, which translates into a transport mean free path below 1 µm. A complete optical characterization through Monte Carlo simulations reveals how such a scattering performance arises from the scattering coefficient and scattering anisotropy, whose interplay provides insight into the morphological properties of the material. The potential of bright‐white coatings as smart sensors or wearable devices is highlighted using a treated 3.5 µm film as a real‐time sensor for human exhalation.
Materials with extreme photonic properties such as maximum diffuse reflectance, high albedo or tunable bandgaps are essential in many current and future photonic devices and coatings. While photonic crystals (PCs), periodic anisotropic structures, are well established, their disordered counterparts , photonic glasses (PGs), are less understood despite their most interesting isotropy of pho-tonic properties. Here, we introduce the first controlled high index model PG system. It is made of monodisperse spherical TiO2 colloids to exploit strongly resonant Mie scattering for optimal tur-bidity. We report spectrally resolved combined measurements of turbidity and light energy velocity from large monolithic crack-free samples. This new material class reveals pronounced resonances enabled by the possibility to tune both the refractive index of the extremely low polydisperse constituents and their radius. All our results are rationalized by a model based on the energy coherent potential approximation (ECPA) which is free of any fitting parameter. Surprisingly good quantitative agreement is found even at high index and elevated packing fraction. This may be the key to optimized tunable novel photonic materials, but also central to understand fundamental questions such as isotropic structural colors, random lasing or strong light localization in 3D.
Polyacrylonitrile electrospinning nanofiber film was introduced into a light emitting diode (LED) lamp to exploit the strong reflective and scattering effects. The light extraction mechanism was studied systematically for three different electrospinning types in three different types of LED lamps. For the all-electrospinning types, the luminous efficacy increased for the white LED, outwards remote phosphor layer, and inwards remote phosphor layer lamps by 10.98, 16.97, and 18.35%, respectively, compared with the reference lamp. Lamps with stronger backscatter had larger luminous efficacy enhancements. The reflector-electrospinning type helped redirect lights with large emission angles. The substrate-electrospinning type was beneficial for recycling the total interior reflection lights and increasing the yellow to blue ratio. Additionally, the all-electrospinning white LED lamps remains 97.89% luminous flux after a 96-hour aging process. Electrospinning fiber films are favorable luminous efficacy enhancers for the future generation of LED lamps.
Quantum dots (QDs) show a great potential for light-emitting diodes (LEDs) packaging, which still face great challenges compared with the matured phosphor downconversion materials. These are probably caused by the unique scattering and absorption properties of QDs, which are extremely different to the traditional phosphor due to their several nanometers size, while their effect on QDs-converted LEDs (QCLEDs) is barely studied. In this paper, we have experimentally and theoretically investigated the effect of scattering and absorption of CdSe/ZnS QDs on the optical performance for QCLEDs by comparing with the traditional yttrium aluminum garnet phosphor. Results indicate that the strong absorption (reabsorption) of QDs causes low radiant efficacy and stability for QCLEDs; their weak scattering also leads to a low color uniformity. It demonstrates that their unique scattering and absorption properties are key factors leading to low optical performance of QCLEDs compared with the traditional phosphor-converted LEDs. For purpose of gaining the white LED with high efficiency and stability, it is highly suggested to use a low QD concentration to reduce the reabsorption loss and the total internal reflection loss. We believe that this paper can provide a better understanding of improving the optical performance for QCLEDs from the prospective of scattering and absorption. In the future, it is important to use low QD concentration to gain high-performance white LEDs with high downconversion efficiency by optimizing packaging structures.
The Nicobar pigeon (Caloenas Nicobarica) belongs to the extinct dodo-bird family and has been declared as an endangered species. Here, microscopic and spectroscopic measurements are carried out on the bird's feathers to study the structural coloration originating from the barbule nanostructures. A range of color shades is recorded with changing viewing and illumination angles at different locations of the feathers. A spectacular variation in colors is generated by photonic structures; red, green, and blue and their blends are observed. Hydrophobicity of the optical material is also investigated. A contact angle of ≈156° is observed demonstrating it to be superhydrophobic. Experimental observations of the optical properties are analyzed on these feathers for sensing made possible due to the material and structural properties at the interface between barbule's surface and solution. An optical response is observed with a redshift in optical spectra with increasing refractive index of the solution, which is correlated with concentration values. The structural coloration in Nicobar pigeon can be adopted for many practical applications such as color selective filters, nonreflecting coatings, and refractive index-based sensing.
A common strategy to optimize whiteness in living organisms consists in using 3D random networks with dense and polydisperse scattering elements constituted by relatively low refractive index materials. Inspired by these natural architectures, a fast and scalable method to produce highly scattering porous polymer films via phase separation is developed. By varying the molecular weight of the polymer, the morphology of the porous films is modified, and therefore their scattering properties are tuned. The achieved transport mean free paths are in the micrometer range, improving the scattering strength of analogous low refractive index systems, e.g., standard white paper, by an order of magnitude. The produced porous films show a broadband reflectivity of ≈75% while only 4 µm thick. In addition, the films are flexible and can be readily index-matched with water (i.e., they become transparent when wet), allowing for various applications such as coatings with tunable transmittance and responsive paints.
Based on electrospinning technology, in this study, we fabricated poly(lactic-coglycolic acid) (PLGA) nanofiber films with high reflectivity and scattering properties. Various films with different thicknesses and fiber diameters were fabricated by changing the electrospinning time and solution concentration, respectively. Detailed optical measurements demonstrate that the film reflectance and scattering ability increase with the thickness, whereas fiber diameter contributes little to both properties. With optimized film thickness and fiber diameter, nanofiber films feature whiteness with a reflectance of 98.8% compared to the BaSO4 white plate. Furthermore, when deposited on the reflector surface of a remote phosphor-converted light-emitting diode lamp, nanofiber films witness a correlated color temperature deviation decrease from 8880 K to 1407 K and a luminous efficiency improvement of 11.66% at 350 mA. Therefore, the nanofiber films can be applied in lighting systems as a highly reflective coating to improve their light efficacy and quality.
Most studies of structural color in nature concern periodic arrays, which through the interference of light create color. The “color” white however relies on the multiple scattering of light within a randomly structured medium, which randomizes the direction and phase of incident light. Opaque white materials therefore must be much thicker than periodic structures. It is known that flying insects create “white” in extremely thin layers. This raises the question, whether evolution has optimized the wing scale morphology for white reflection at a minimum material use. This hypothesis is difficult to prove, since this requires the detailed knowledge of the scattering morphology combined with a suitable theoretical model. Here, a cryoptychographic X-ray tomography method is employed to obtain a full 3D structural dataset of the network morphology within a white beetle wing scale. By digitally manipulating this 3D representation, this study demonstrates that this morphology indeed provides the highest white retroreflection at the minimum use of material, and hence weight for the organism. Changing any of the network parameters (within the parameter space accessible by biological materials) either increases the weight, increases the thickness, or reduces reflectivity, providing clear evidence for the evolutionary optimization of this morphology.
Inspired by the white beetle of the genus Cyphochilus, we fabricate ultra-thin, porous PMMA films by foaming with CO_2 saturation. Optimising pore diameter and fraction in terms of broad-band reflectance results in very thin films with exceptional whiteness. Already films with 60 µm-thick scattering layer feature a whiteness with a reflectance of 90%. Even 9 µm thin scattering layers appear white with a reflectance above 57%. The transport mean free path in the artificial films is between 3.5 µm and 4 µm being close to the evolutionary optimised natural prototype. The bio-inspired white films do not lose their whiteness during further shaping, allowing for various applications.
Cellulose is the most abundant biopolymer on Earth. Cellulose fibers, such as the one extracted form cotton or woodpulp, have been used by humankind for hundreds of years to make textiles and paper. Here we show how, by engineering light-matter interaction, we can optimize light scattering using exclusively cellulose nanocrystals. The produced material is sustainable, biocompatible, and when compared to ordinary microfiber-based paper, it shows enhanced scattering strength (×4), yielding a transport mean free path as low as 3.5 μm in the visible light range. The experimental results are in a good agreement with the theoretical predictions obtained with a diffusive model for light propagation.
Cellulose is the most abundant bio-polymer on earth. Cellulose fibres, such as the one extracted form cotton or woodpulp, have been used by humankind for hundreds of years to make textiles and paper. Here we show how, by engineering light matter-interaction, we can optimise light scattering using exclusively cellulose nanocrystals. The produced material is sustainable, biocompatible and, when compared to ordinary microfibre-based paper, it shows enhanced scattering strength (x4) yielding a transport mean free path as low as 3.5 um in the visible light range. The experimental results are in a good agreement with the theoretical predictions obtained with a diffusive model for light propagation.
An overview of plant surface structures and their evolution is presented. It combines surface chemistry and architecture with their functions and refers to possible biomimetic applications. Within some 3.5 billion years biological species evolved highly complex multifunctional surfaces for interacting with their environments: some 10 million living prototypes (i.e., estimated number of existing plants and animals) for engineers. The complexity of the hierarchical structures and their functionality in biological organisms surpasses all abiotic natural surfaces: even superhydrophobicity is restricted in nature to living organisms and was probably a key evolutionary step with the invasion of terrestrial habitats some 350–450 million years ago in plants and insects. Special attention should be paid to the fact that global environmental change implies a dramatic loss of species and with it the biological role models. Plants, the dominating group of organisms on our planet, are sessile organisms with large multifunctional surfaces and thus exhibit particular intriguing features. Superhydrophilicity and superhydrophobicity are focal points in this work. We estimate that superhydrophobic plant leaves (e.g., grasses) comprise in total an area of around 250 million km², which is about 50% of the total surface of our planet. A survey of structures and functions based on own examinations of almost 20,000 species is provided, for further references we refer to Barthlott et al. (Philos. Trans. R. Soc. A 374: 20160191, 1). A basic difference exists between aquatic non-vascular and land-living vascular plants; the latter exhibit a particular intriguing surface chemistry and architecture. The diversity of features is described in detail according to their hierarchical structural order. The first underlying and essential feature is the polymer cuticle superimposed by epicuticular wax and the curvature of single cells up to complex multicellular structures. A descriptive terminology for this diversity is provided. Simplified, the functions of plant surface characteristics may be grouped into six categories: (1) mechanical properties, (2) influence on reflection and absorption of spectral radiation, (3) reduction of water loss or increase of water uptake, moisture harvesting, (4) adhesion and non-adhesion (lotus effect, insect trapping), (5) drag and turbulence increase, or (6) air retention under water for drag reduction or gas exchange (Salvinia effect). This list is far from complete. A short overview of the history of bionics and the impressive spectrum of existing and anticipated biomimetic applications are provided. The major challenge for engineers and materials scientists, the durability of the fragile nanocoatings, is also discussed.
The extremely brilliant whiteness shown by the Cyphochilus beetle is generated by multiple scattering of light inside the ultrathin scales that cover its body, whose interior is characterized by an anisotropic nanostructured network of chitin filaments. It is demonstrated that the structural anisotropy of the network is crucial in order to achieve high broadband reflectance from such a thin, low-refractive-index system.
Whiteness arises from diffuse and broadband reflection of light typically achieved through optical scattering in randomly structured media. In contrast to structural colour due to coherent scattering, white appearance generally requires a relatively thick system comprising randomly positioned high refractive-index scattering centres. Here, we show that the exceptionally bright white appearance of Cyphochilus and Lepidiota stigma beetles arises from a remarkably optimised anisotropy of intra-scale chitin networks, which act as a dense scattering media. Using time-resolved measurements, we show that light propagating in the scales of the beetles undergoes pronounced multiple scattering that is associated with the lowest transport mean free path reported to date for low-refractive-index systems. Our light transport investigation unveil high level of optimisation that achieves high-brightness white in a thin low-mass-per-unit-area anisotropic disordered nanostructure.
Nature’s color has three main sources: pigments, structural colors, and bioluminescence. Structural color is a special one, which is the color produced by micro- or nano- structures, and is bright and dazzling. The most common mechanisms of structural colors are film interference, diffraction grating, scattering, and photonic crystal. The biological colors are mainly derived from film interference which includes thin-film and multi-film interference. The diffraction grating mechanism is found in, for example, seed shrimp, mollusk Haliotis Glabra, and flower of Hibiscus trionum). Scattering includes coherent and incoherent scattering. Well-known examples of coherent scattering include colors produced by brilliant iridescent butterfly wing scales and avian feather barbules such as the peacock’s tail. Examples of colors produced by photonic crystal structures include opal color in the beetle and iridescent spines in the sea mouse. Coloration changes occur through the structural changes for camouflage, predation, signal communication, and sex choice. This paper presents an overview of lessons from nature and various relevant mechanisms. Examples of bioinspired fabrication methods and applications are also presented in the paper.
Biological communication by means of structural color has existed for at least 500 million years. Structural color is commonly observed in the animal kingdom, but has been little studied in plants. We present a striking example of multilayer-based strong iridescent coloration in plants, in the fruit of Pollia condensata. The color is caused by Bragg reflection of helicoidally stacked cellulose microfibrils that form multilayers in the cell walls of the epicarp. We demonstrate that animals and plants have convergently evolved multilayer-based photonic structures to generate colors using entirely distinct materials. The bright blue coloration of this fruit is more intense than that of any previously described biological material. Uniquely in nature, the reflected color differs from cell to cell, as the layer thicknesses in the multilayer stack vary, giving the fruit a striking pixelated or pointillist appearance. Because the multilayers form with both helicoidicities, optical characterization reveals that the reflected light from every epidermal cell is polarized circularly either to the left or to the right, a feature that has never previously been observed in a single tissue.
Inspired by the self-cleaning silver ragwort leaf, we have recently fabricated a biomimetic super-hydrophobic fibrous mat surface comprising micro/nanoporous polystyrene (PS) microfibres via electrospinning. The rough surface of the silver ragwort leaf fibres, with nanometre-sized grooves along the fibre axis, was imitated by forming micro-and nanostructured pores on the electrospun fibre surface. The solvent composition ratios of tetrahydrofuran (THF) to N,N-dimethylformamide (DMF) in PS solutions were proved to be the key parameter to affect the fibre surface structures due to the various phase separation speeds of the solvents from PS fibres during electrospinning. The combination of the hierarchical surface roughness inherent in electrospun microfibrous PS mats and the low surface free energy of PS yielded a stable super-hydrophobicity with water contact angles as high as 159.5 • for a 12 mg water droplet, exceeding that (147 •) of the silver ragwort leaf. Moreover, the hydrophobicity of the porous PS mat surface was found to increase on increasing the surface roughness of the microfibres. (Some figures in this article are in colour only in the electronic version)
Pubescence layers with their native structure and orientation were isolated from the leaves of Olea europaea L. and Olea chrysophylla L. They were almost transparent in the visible, but considerable absorptance was evident in the ultraviolet-B region (UV-B), with maximum at 310 nm. Methanolic extracts of hairs from Olea and a variety of other pubescent species consistently showed the existence of UV-screening pigments. Absorptance of trichomes varied, but a trend towards more effective UV-B radiation attenuation in the sub-alpine Verbascum species may be claimed. In all cases, pigments were located within hair cells and in Olea they were characterized as phenolics with considerable flavonoid contribution. It is suggested that leaf hairs, besides other functions, may constitute a shield against UV-B radiation.
Structural whiteness in nature is one of the useful optical phenomena contributed by refined microstructures and serves important biological functions. However, whiteness residing in wings, antennae, and bodies of butterflies often draws less attention, with undetected physical mechanisms and applications. Here, it is discovered that bright silver scales on the ventral side of butterfly Curetis acuta Moore have enhanced broadband reflection with particular angle dependency, which is dominated by the irregularity of scales. The broadband reflection is the result of color mixing effect caused by laminated scales with irregular layer space and thickness. The special angle dependency suggests that the reflection intensity changes with azimuth angles but remains unvaried at different viewing angles along given direction. This is the consequence of diffraction caused by ridges and irregular holes on the scales, which generates the shining effect and extends the viewing angle of broadband reflection. The characteristic reflection property is speculated to be used for intraspecific communication. Furthermore, it is found that the broadband reflection property of the silver scales is helpful to lower the body temperature under direct sunshine. These findings for light and thermo controlling would offer a potential strategy to design bioinspired advanced materials for low‐energy, reflectance‐based applications. The bright silver brilliancy on ventral wings of Curetis acuta Moore originates from the irregularity of scales. First, the irregular layer space and thickness of scales cause enhanced broadband reflection. Second, silver brilliancy has special angular dependency with wide viewing angles induced by anisotropic ridges and irregular holes. The reflection properties of wings can function as intraspecific communication and thermoregulation.
Quantum dot (QD) white light-emitting diodes (LEDs) are promising devices in illumination and display applications, and the low thermal stability of QDs is one of the biggest problems limiting their practical use. In this paper, we proposed a metal-based inverted packaging (MIP) structure to enhance the thermal performance and stability of QD white LEDs. The results indicate that the thermal power of LED chips can greatly increase the surface temperature of QD white LEDs, especially that of the color-converted layer and the base. Therefore, the LED chip was separated from the QD layer and isolated with low thermal conductivity layers of air and glass using the MIP structure. This successfully reduces the base temperature and constrains the thermal power of the LED chip on the upper phosphor layer, thus enhancing the thermal performance for the QD layer. Consequently, MIP-QD white LEDs have a small deviation of 57.3 K in their correlated color temperature (CCT) and a deviation of 0.29 in their color rendering index (CRI) at injection power values in a wide range of 0.08-1.3 W, with mean values of 4382 K and 90.3, respectively. Moreover, these devices have negligible reduction in their electroluminescence intensity, and CCT and CRI values after aging for 10 h under severe conditions at an injection power of 0.9 W. Consequently, this paper can provide a general guide to enhance the thermal performance and stability of QD white LEDs by separating the thermal power of the LED chip from the QD layer.
A stronger, cooler wood
One good way to reduce the amount of cooling a building needs is to make sure it reflects away infrared radiation. Passive radiative cooling materials are engineered to do this extremely well. Li et al. engineered a wood through delignification and re-pressing to create a mechanically strong material that also cools passively. They modeled the cooling savings of their wood for 16 different U.S. cities, which suggested savings between 20 and 50%. Cooling wood would be of particular value in hot and dry climates.
Science , this issue p. 760
The color of materials usually originates from a combination of wavelength‐dependent absorption and scattering. Controlling the color without the use of absorbing dyes is of practical interest, not only because of undesired bleaching properties of dyes but also regarding minimization of environmental and health issues. Color control without dyes can be achieved by tuning the material's scattering properties in controlling size and spatial arrangement of scatterers. Herein, calibrated photonic glasses (PGs), which are isotropic materials made by random aggregation of nonabsorbing, monodisperse colloidal polystyrene spheres, are used to generate a wide spectral range of purely structural, angular‐independent colors. Experimental reflectance spectra for different sized spheres compare well with a recent theoretical model, which establishes the latter as a tool for color mapping in PGs. It allows to determine the range of visible colors accessible in PGs as function of size, packing fraction, and refractive index of scatterers. It also predicts color saturation on top of the white reflectance as function of the sample's optical thickness. Blue, green, and red are obtained even with low index, while saturated green, cyan, yellow, and magenta can be reached in higher index PGs over several orders of magnitude of sample thickness.
Structural whiteness, stemming from the biological evolutionarily refined structures, provides inspirations for promising, reflectance-based materials design. White beetles Goliathus goliatus, which can survive in high-temperature-equatorial forests, may hinter undiscovered new physical mechanisms for thermoregulation. The scales’ whiteness is created by the exquisite shell/hollow cylinders structure with two thermoregulatory effects, contributing to lower equilibrium temperature of elytra under direct sunlight. In the visible regime, they enhance the broadband omnidirectional reflection significantly by synergy structural effects originated from the thin-film interference, Mie resonance and total reflection. In the mid-infrared (MIR) regime, white scales act as antireflective layers to increase the emissivity in the MIR, enabling the elytra to reradiate heat to the environment, and help the beetles reduce the temperature by as much as ~7.8℃ in air. These biologic strategies for thermoregulation could provide new approaches for bioinspired coatings towards passive radiative cooling.
A common strategy to optimize whiteness in living organisms consists in using three-dimensional random networks with dense and polydisperse scattering elements constituted by relatively low-refractive index materials. Inspired by these natural architectures, we developed a fast and scalable method to produce highly scattering porous polymer films via phase separation. By varying the molecular weight of the polymer, we modified the morphology of the porous films and therefore tuned their scattering properties. The achieved transport mean free paths are in the micrometer range, improving the scattering strength of analogous low-refractive index systems, e.g. standard white paper, by an order of magnitude. The produced porous films show a broadband reflectivity of approximately 75 % whilst only 4 m thick. In addition, the films are flexible and can be readily index-matched with water (i.e. they become transparent when wet), allowing for various applications such as coatings with tunable transmittance and responsive paints.
Undesired growth of biofilms represents a fundamental problem for all surfaces in long-term contact with aqueous media. Mature biofilms resist most biocide treatments and often are a pathogenic threat. One way to prevent biofilm growth on surfaces is by using slippery liquid-infused porous surfaces (SLIPS). SLIPS consist of a porous substrate which is infused with a lubricant immiscible with the aqueous medium in which the bacteria are suspended. Because of the lubricant, bacteria cannot attach to the substrate surface and thus formation of the biofilm is prevented. For this purpose, we manufactured substrates with different porosity and surface roughness values via UV-initiated free-radical polymerization in Fluoropor. Fluoropor is a class of highly fluorinated bulk-porous polymers with tunable porosity, which we recently introduced. We investigated the growth of the biofilm on the substrates, showing that a reduced surface roughness is beneficial for the reduction of biofilm growth. Samples of low roughness effectively reduced Pseudomonas aeruginosa biofilm growth for 7 days in a flow chamber experiment. The low-roughness samples also become transparent when infused with the lubricant, making such surfaces ideal for real-time observation of biofilm growth by optical examination.
Painting on the cool
Passive radiative cooling materials emit heat. They can reduce the need for air conditioning by providing daytime cooling but are often challenging to apply to rooftops and other building surfaces. Mandal et al. fabricated porous poly(vinylidene fluoride-co-hexafluoropropene) to create an excellent radiative cooling material. Better yet, the polymer is easy to paint or spray onto a wide range of surfaces, has good durability, and can even be dyed. This makes it a promising candidate for widespread use as a high-performance passive radiative cooling material.
Science , this issue p. 315
The understanding of the interaction between light and complex, random structures is the key for designing and tailoring the optical appearance and performance of many materials that surround us, ranging from everyday consumer products, such as those for personal care, paints, and paper, to light diffusers used in the LED-lamps and solar cells. Here, it is demonstrated that the light transport in membranes of pure cellulose nanofibrils (CNFs) can be controlled to achieve bright whiteness in structures only a few micrometers thick. This is in contrast to other materials, such as paper, which require hundreds of micrometers to achieve a comparable appearance. The diffusion of light in the CNF membranes is shown to become anomalous by tuning the porosity and morphological features. Considering also their strong mechanical properties and biocompatibility, such white coatings are proposed as a new application for cellulose nanofibrils.
Nature’s evolution provides a multitude of answers to scientific and key technological challenges such as the light harvesting. In this work, we investigate the optical properties of the unique texture of viola petals for the purpose of improved light harvesting in photovoltaics. We find that crystalline silicon solar cells encapsulated with a transparent coating show a 6% improvement in power conversion efficiency if the viola petal texture is replicated onto the front surface. This gain is based on a broadband enhancement in current generation which originates from the exceptional optical properties of the viola surface texture, combining micro- and nano-texture. The micro-cones of this hierarchical texture demonstrate strong and broadband light incoupling effects as well as retro-reflection capabilities, and the nano-wrinkles further decrease the reflection losses. Using rigorous optical simulation, we analyze and explain the working principle ruling the light harvesting properties of this dual-scale texture.
Through the use of the limited materials palette, optimally designed micro- and nanostructures, and tightly regulated processes, nature demonstrates exquisite control of light–matter interactions at various length scales. In fact, control of light–matter interactions is an important element in the evolutionary arms race and has led to highly engineered optical materials and systems. In this review, we present a detailed summary of various optical effects found in nature with a particular emphasis on the materials and optical design aspects responsible for their optical functionality. Using several representative examples, we discuss various optical phenomena, including absorption and transparency, diffraction, interference, reflection and antireflection, scattering, light harvesting, wave guiding and lensing, camouflage, and bioluminescence, that are responsible for the unique optical properties of materials and structures found in nature and biology. Great strides in understanding the design principles adapted by nature have led to a tremendous progress in realizing biomimetic and bioinspired optical materials and photonic devices. We discuss the various micro- and nanofabrication techniques that have been employed for realizing advanced biomimetic optical structures.
Most studies of structural color in nature concern periodic arrays, which through the interference of light create color. The "color" white however relies on the multiple scattering of light within a randomly structured medium, which randomizes the direction and phase of incident light. Opaque white materials therefore must be much thicker than periodic structures. It is known that flying insects create "white" in extremely thin layers. This raises the question, whether evolution has optimized the wing scale morphology for white reflection at a minimum material use. This hypothesis is difficult to prove, since this requires the detailed knowledge of the scattering morphology combined with a suitable theoretical model. Here, a cryoptychographic X-ray tomography method is employed to obtain a full 3D structural dataset of the network morphology within a white beetle wing scale. By digitally manipulating this 3D representation, this study demonstrates that this morphology indeed provides the highest white retroreflection at the minimum use of material, and hence weight for the organism. Changing any of the network parameters (within the parameter space accessible by biological materials) either increases the weight, increases the thickness, or reduces reflectivity, providing clear evidence for the evolutionary optimization of this morphology.
Superwettability is a centuries-old concept that has been rediscovered in past decades, largely owing to new understanding of the mechanisms of special wetting phenomena in nature. Combining multiscale structures and surface chemical compositions is crucial to fabricate interfacial materials with superwettability. In this Review, we detail the historical development and summarize the various combined superwetting states in superwettability systems. Nature-inspired design principles of superwettable materials are also briefly introduced. Superwettability systems can be extended from 2D surfaces to 0D nanoparticles, 1D fibres and channels, and 3D integrated materials. We discuss new phenomena and the advantages that superwettability-based systems have for chemical reactions and materials fabrication, including emerging applications that utilize single extreme wetting states or that combine two extreme wetting states. Finally, we provide our perspective for future research directions.
Significance
Whiteness, although frequently apparent on the wings, legs, antennae, or bodies of many species of moths and butterflies, has often escaped our attention. Here, we investigate the nanostructure and microstructure of white spots on the wings of Carystoides escalantei , a dusk-active and shade-inhabiting Costa Rican rain forest butterfly (Hesperiidae). We identify two types of whiteness: angle dependent and angle independent. We speculate that the biological functions and evolution of Carystoides spot patterns, scale structures, and their varying whiteness are adaptations to the butterfly’s low light habitat and to airflow experienced on the wing base vs. wing tip during flight. Sex and species differences in the location of angle-dependent white spots on the wings may function in both intraspecific and interspecific communication.
Butterflies of the family Pieridae are brightly colored, ranging from white to red, caused by various pterin pigments concentrated in scattering spheroidal beads in the wing scales. Given the sparsity of the beads in the wing scales, the high brightness suggests a scattering strength of the beads that significantly surpasses that of typical cuticular chitin beads with the areal density found in the wing scales. To elucidate this apparent contradiction, the optical signature of the pierids' highly saturated pigmentary colors are analyzed by using Jamin–Lebedeff interference microscopy combined with Kramers–Kronig theory and light scattering modeling. This study shows that extreme pterin pigment concentrations cause a very high refractive index of the beads with values above 2 across the visible wavelength range, thus creating one of the most highly light scattering media thus far discovered in the animal kingdom.
The cleanup of accidental oil spills in water is an enormous challenge; conventional oil sorbents absorb large amounts of water in addition to oil and other cleanup methods can cause secondary pollution. In contrast, fresh leaves of the aquatic ferns Salvinia are superhydrophobic and superoleophilic, and can selectively absorb oil while repelling water. These selective wetting properties are optimal for natural oil absorbent applications and bioinspired oil sorbent materials. In this paper we quantify the oil absorption capacity of four Salvinia species with different surface structures, water lettuce (Pistia stratiotes) and Lotus leaves (Nelumbo nucifera), and compare their absorption capacity to artificial oil sorbents. Interestingly, the oil absorption capacities of Salvinia molesta and Pistia stratiotes leaves are comparable to artificial oil sorbents. Therefore, these pantropical invasive plants, often considered pests, qualify as environmentally friendly materials for oil spill cleanup. Furthermore, we investigated the influence of oil density and viscosity on the oil absorption, and examine how the presence and morphology of trichomes affect the amount of oil absorbed by their surfaces. Specifically, the influence of hair length and shape is analyzed by comparing different hair types ranging from single trichomes of Salvinia cucullata to complex eggbeater-shaped trichomes of Salvinia molesta to establish a basis for improving artificial bioinspired oil absorbents.
Light-harvesting micro-/nanohierarchical structures replicated from plants' epidermal cells are exploited for photovoltaic applications. Their broadband and omnidirectional antireflection properties, together with their light-trapping capability, are analyzed experimentally. Power conversion efficiency gains are reported after integrating those replicas onto optimized state-of-the-art organic solar cells. The proposed approach can be applied to different plant species and photovoltaic technologies.
The natural world exhibits numerous examples of efficient optical designs with novel hierarchical microstructures and specialized functionality after millions of years of evolution. Materials scientists have long been deriving understanding and inspiration from nature's optical ingenuity, such as vivid structural colors, light antireflection, light focusing, and chirality. Progress in engineering bioinspired optical functional materials has been exciting in the past years. In this review, the focus is on the state-of-the-art achievements of bioinspired optical materials with applications in various areas including efficient light manipulation, optical sensors, light-energy conversion, plasmonic materials with ultrahigh surface plasmon resonance (SPR) efficiency and metamaterials. The major challenges and perspectives for bioinspired designs of optical functional materials in the future are also briefly addressed.
Keeping cool
Silver ants inhabit one of the hottest and driest environments on Earth, the Saharan sands, where most insects shrivel and die moments after contact. Shi et al. show that the triangular shape of the silver hairs that cover their bodies enables this existence. The hairs both increase the reflection of near-infrared rays and dissipate heat from the ants' bodies, even under full sun conditions. Evolution's simple solution to intense heat management in this species could lead to better designs for passive cooling of human-produced objects.
Science , this issue p. 298
This is the second part of a world-wide revision of the genus Boehmeria, the previously-published part having dealt with the New World species. The Old World species are widely distributed in the tropics and subtropics from West Africa to islands in the Pacific Ocean and from Japan and China to Southern Africa, Madagascar and Australia, with the highest species richness in the Himalayas and their extension into China and Indochina. No indigenous species is common to both the Old and New World. The species represent taxonomic units of very different complexity: most species exhibit little infraspecific variation; in several others formal taxonomic infraspecific units can be recognised; however, in two, B. virgata and B. japonica, a highly complex variation is seen, fitting with difficulty into the normal hierarchy of taxonomic classification. With the conclusions reached here, 33 species, including 31 varieties, are recognised and over one hundred previously established names are placed in synonymy. Four new taxa are described: B. pilosiuscula var. suffruticosa, B. virgata subsp. macrophylla var. minuticymosa, B. virgata subsp. virgata var. velutina and B. virgata subsp. virgata var. maxima. The following new combinations are made: B. densiflora var. boninensis, B. heterophylla var. blumei, B. japonica var. silvestrii, B. japonica var. tenera, B. sieboldiana var. fuzhouensis, B. ternifolia var. kamley, B. virgata subsp. macrophylla, B. virgata subsp. macrophylla var. canescens, B. virgata subsp. macrophylla var. densiglomerata, B. virgata subsp. macrophylla var. longissima, B. virgata subsp. macrophylla var. macrostachya, B. virgata subsp. macrophylla var. molliuscula, B. virgata subsp. macrophylla var. rotundifolia, B. virgata subsp. macrophylla var. scabrella, B. virgata subsp. macrophylla var. strigosa, B. virgata subsp. macrophylla var. sumatrana, B. virgata subsp. macrophylla var. tomentosa and B. virgata subsp. virgata var. austroqueenslandica.
What do lotus flowers have in common with human bones, liquid crystals
with colloidal suspensions, and white beetles with the beautiful stones
of the Taj Mahal? The answer is they all feature disordered structures
that strongly scatter light, in which light waves entering the material
are scattered several times before exiting in random directions. These
randomly distributed rays interfere with each other, leading to
interesting, and sometimes unexpected, physical phenomena. This Review
describes the physics behind the optical properties of disordered
structures and how knowledge of multiple light scattering can be used to
develop new applications. The field of disordered photonics has grown
immensely over the past decade, ranging from investigations into
fundamental topics such as Anderson localization and other transport
phenomena, to applications in imaging, random lasing and solar energy.
The hair layer consisting of hollow fibers provides the poplar leaf with an energy efficient “cool roof” to protect it from being burned by strong light. Inspired by the hair structure, we use coaxial electro-spinning technology to achieve a highly reflective and superhydrophobic white coating towards making an eco-friendly and effective “cool roof”.
Tropical Morpho butterflies are famous for their brilliant iridescent colours, which arise from ordered arrays of scales on their wings. Here we show that the iridescent scales of the Morpho sulkowskyi butterfly give a different optical response to different individual vapours, and that this optical response dramatically outperforms that of existing nano-engineered photonic sensors. The reflectance spectra of the scales provide information about the nature and concentration of the vapours, allowing us to identify a range of closely related vapours–water, methanol, ethanol and isomers of dichloroethylene when they are analysed individually. By comparing the reflectance as a function of time for different vapours, we deduce that wing regions with scale structures of differing spatial periodicity give contributions to the overall spectral response at different wavelengths. Our optical model explains the effect of different components of the wing scales on the vapour response, and could steer the design of new man-made optical gas sensors.
Flavonoids of non-glandular leaf hairs from Quercus ilex were analysed. The main compounds were acylated kaempferol glycosides. Acylation shifted the absorption peak into the ultraviolet-B region of the spectrum in which intact trichome layers absorbed strongly. Ultraviolet-B radiation caused a considerable reduction of photosystem II photochemical efficiency only in dehaired leaves. It is suggested that leaf hairs, besides other roles, may function as an effective filter against the harmful ultraviolet-B radiation.
Two types of films that are structurally colored by exploiting the self-assembly of colloidal polymer nanoparticles were created. Monodisperse PS spheres were synthesized using a surfactant-free polymerization technique. Particles were electrostatically stabilized by copolymerization with sodium 4-vinylbenzenesulfonate. After synthesis, particle suspensions were washed by centrifugation and resuspension at least three times with deionized (DI) water. Particle sizes were determined by SEM image analysis with an accelerating voltage of 10 kV, after being coated with a thin layer of gold. To prepare bidisperse suspensions, equal volumes of two monodisperse suspensions were mixed by pipetting approximately 20 times and vortexing for at least 30 s. It was observed that isotropic structures with a characteristic length-scale comparable to the wavelength of visible light can produce structural color when wavelength-independent scattering is suppressed.
Self-assembly techniques are widely used to grow ordered structures such as, for example, opal-based photonic crystals. Here, we report on photonic glasses, new disordered materials obtained via a modified self-assembling technique. These random materials are solid thin films which exhibit rich novel light diffusion properties originating from the optical properties of their building blocks. This novel material inaugurated a wide range of nanophotonic materials with fascinating applications, such as resonant random lasers or Anderson localization.