Abstract

Bright colours often originate from tiny mechanisms in feathers or wings or skin in the natural world that modify the way light reacts as it is reflected. The bright hues of birds, insects and fruits  are responsible for this structural colour. In recent years, structural coloration has drawn considerable attention from scientists and engineers, due to the obsession with different brilliant examples seen in nature as well as the exciting applications of bio-inspired colored functional structures and materials. In order to expose and mimic the physical structures that underlie the structural colours observed in nature, much study has been undertaken.

Background

In order to make a textile material colorful or print different motifs on it dyes and pigments are used. Dyes commonly have synthetic roots and have chemical forms that are aromatic. The dye effluents are heavily coloured and can be particularly harmful for their disposal into the atmosphere. Their involvement is aesthetically undesirable in watercourses and can be noticeable at concentrations as low as 1 ppm.[3] In aquatic environments, they also control photosynthetic behaviour by reducing light penetration. The low biodegradability of colourants leads to another challenge. The methods of treatment for these effluents are very costly and often have operating difficulties. Some dyes have also been forbidden because they have been classified as allergic and carcinogenic. [1]As environmental standards are becoming more strict, the need for alternative textile structural coloring techniques has increased. Structural colour has excellent durability and does not fade, whereas pigment shades do not have these properties owing to the iridescent effect, polarisation effect, and UV effect.

 It can profoundly enrich the biological use of colour to fulfil the specifications of the different types of biological functions. Through the research of the living essence of systemic colour optics, we can investigate the use of the structure to accomplish a lot of complex functions.[2]

 

Introduction

Due to the selective sensitivities of the light receptors in the retinas of our eyes, humans experience the environment in a colourful manner. Several colour production processes in materials, including preferential absorption , emission, diffraction, and photochromism, are known. For a non-luminous object, whether the object reflects only a certain spectrum of visible wavelengths, we see a certain hue. In two cases, this can occur.[1] On the one side, due to collisions between the illuminating photons and electrons in the substance, which is the ultimate coloration process of pigments, dyes, and metals, the sample absorbs part of the light spectrum. For starters, much of the red and blue light from the sun is absorbed by the chlorophyll pigment present in many bacteria , algae, and plants and reflects green light, allowing most vegetation to appear green. On the other side, owing to the interference of light with the surface of an object, light may be intensely reflected and/or deflected from touching the eye, as is the case with so-called structural shades. The ambient rainbow, caused by the absorption, refraction, and dispersion of light in water droplets, is a familiar example of structural colour, resulting in a variety of colours occurring in the sky.[1] One of the emerging new methods of coloration of textile materials is through the interference phenomenon that is the basis for coloration of butterfly wings. The butterfly wing usually consists of two or three layers of small scales living on a membrane , causing diffraction to take place. Material scientists were also influenced by the structural colours of living objects to create new photonic crystals and other optical materials. In order to understand and mimic the hierarchical structures that create biological structural colours, especially in insects, several approaches have been used.

 

Physical basis and mechanism of structural color

In general, the division of structural colours are done  into the types of thin film and multi-film reflections (interference), diffraction gratings, scattering and photonic crystals has been found to be beneficial by biologists.[2] Structural color can also be categorized as either iridescent or non-iridescent. According to the research done on structural colors, iridescent structures come from work on insects. Broadly defined, surfaces of which the color changes with the viewing angle are called iridescent colors,  while non-iridescent colors remain similar in appearance regardless of the angle of observation. Some other mechanism due to which structural color is induced has been discussed below. [1]

Film interference

In nature, film interference entails interference with thin-film and multi-film. The iridescent soap bubble is a well-known example of colour caused by thin film interference. In nature, thin-film interaction is observed, when light is absorbed and interferes with the upper and lower borders. When light waves bouncing off the top and bottom surfaces of a thin film interact with each other, thin film interference occurs.[2]

Diffraction gratings 

In 1818, in a physics laboratory-diffraction grating, another form of physical structure with reflective properties was developed, and it was not understood to exist in nature until 1995. It is a surface that is regularly corrugated around the surface in a certain direction.A diffraction grating is an optical feature of optics with a periodic arrangement that separates and diffracts light through several beams that pass to various directions.[1] A type of structural coloration is the emerging coloration. In the science and industrial worlds of optics, diffraction gratings have become big players and have become refined and varied to create an array of optical results.They are responsible for the metallic-like colored holograms. [3]

Scattering Light (Coherent and incoherent)

In the development of blue coloration, scattering is widely used in nature. In general, the term dispersion implies the interference of light in a positive or disruptive way with various wavelengths transmitted from dispersing objects.[2] 

It is possible to identify structural colour processing processes as modes of either incoherent or coherent scattering. When independent light-scattering objects scatter visible wavelengths differentially, incoherent scattering occurs. Incoherent scattering processes take spatially independent light-scattering structures (i.e. uniformly scattered with regard to visible wavelengths) such that the phase relationships of the spread waves are arbitrary.[9] Consequently, incoherent scattering models neglect the phase associations between the scattered waves and characterise the development of colour as a function of the individual scatterers’ differential scattering of wavelengths themselves. Coherent scattering, on the other hand, happens as the spatial distribution of scatterers is non-random with respect to visible light wavelengths, so that dispersed wave phases are non-random [13] Color is a function of the properties of individual scatterers in incoherent scattering, while the spatial distribution of light-scattering interfaces is defined in coherent scattering colour. Incoherent scattering models include Rayleigh scattering (also incorrectly known as Tyndall scattering) and Mie scattering, which is a mathematically precise definition of light scattering by a single particle. Coherent scattering encompasses various optical phenomena that can also be described as diffraction, reinforcement and interference.[13] Well-known examples include the structural colours produced by brilliant iridescent butterfly wing scales and avian feather barbules such as the peacock’s tail. Coherent scattering often produces the phenomenon of iridescence – a prominent change in hue with angle of observation or illumination.[9]

Structural color in nature

Photonic structures are present in naturally formed structures such as opal stones and undoubtedly in many biological materials, considering their complexity. Indeed, their occurrence is remarkable in a wide variety of living organisms, since these mammals, insects and plants manage to use the simple physical processes mentioned above to produce varied colours for their biological needs.[11]

 

Morpho Butterflies 

A pretty magnificent example is the bright blue on the wings of the Morpho butterflies, among the structural colours shown in nature. Morpho wings, which produces a deep blue metallic hue. It can be found under a low-power optical microscope that the rear sides of the main and hind wings are coated with tilted scales.[2] Closer optical analysis shows that a single scale consists of apparently parallel rows, while a cross-section of a scale transmission electron microscopy (TEM) shows that the rows themselves have complicated nanostructures of ridges and lamellae. [11] Three types of photonic crystal-like structures constitute the clusters of ridges and lamellae. Second, thin films with air gaps and lamellae form a multilayer structure that, according to the interference state, gives rise to the blue colour. Secondly, on the right and left sides, a ridge is composed of a collection of staggered lamella (‘shelves’) which create two phase-shifted photonic crystals.[6] The blue portion disperses back to the source as incident white light interacts with the two photonic crystals. Third, from a grating-like structure, the ridges of random height, but each ridge ‘s width, height, and lamella shape retains some randomness that changes the reflected wave phases and cancels out the diffraction grating effect.[2]

Beetles 

There are several kinds of beetles reflecting vivid iridescent colours that tend to change with the vision and/or lighting angle(s). Beetles come in various different colors with a metallic shine, such as gold (Chrysina resplendens, Aspidomorpha tecta), silver (Chrysina batesi, Chrysina strasseni), gray (Chrysina limbata), blue (Eupholus schoenherri petiti), red (Plusiotis chrysargirea), and purple (Smaragdesthes Africana oertzeni, Chrysina purulhensis), which justifies the expression ‘‘living jewelry’’ sometimes used to refer to them.[4] Beetles develop their peculiar tints, sometimes referred to as ‘‘structural colors’’, through a microstructure with dimensions comparable or shorter than the wavelength of light. Three categories can be defined as the structures responsible for these colours: multilayer reflectors, 3D photonic crystals, and diffraction gratings. In the sizes of beetles, different kinds of 3D photonic crystal structures have been discovered.[7] The photonic crystals found in the Pachyrrhynchus and Metapocyrtus scales, for example, have a close-packed hexagonal structure similar to opal, while the Lamprocyphus photonic crystal has a diamond-based layer. In beetles, a third type of structural colour is caused by grating structures made of parallel ridges or slits, such as the beetle Serica sericea, which diffract white light through its constituent wavelengths in order to provide a rainbow-like transmittance. [4,7]

Fruits

An fascinating example of this adaptation is the fruit of the South American tropical vine, Margaritaria nobilis, widely called “bastard hogberry.” A more fleshy and edible rival mimics the ultra-bright blue fruit, which is poor in edible content. Deceived birds consume the fruit and gradually release their seeds over a wide geographical region. [15] An incredibly useful and fascinating technical concept has been influenced by the light-manipulation architecture the surface layer provides, which has evolved to fulfil a particular biological purpose.Vukusic and his Harvard colleagues researched the systemic roots of the vivid colour of the crop. They discovered that a curved, repeated pattern is found in the upper cells in the skin of the seed, which produces colour by the intrusion of light waves. (The vivid colours of soap bubbles are responsible for a similar mechanism.)[11] The team’s study found that several layers of cells in the seed coat are each composed of a cylindrical structure of high nanoscale regularity. In order to build lightweight, stretchable and colour-changing photonic fibres using a revolutionary roll-up mechanism mastered in the Harvard laboratories, the team replicated the core structural elements of the fruit.[15]

Peacock 

Another outstanding example of biological structural colours is shown by the peacock feather. One of the most stunning birds is the male peacock: it has bright, iridescent, diversified colours, and detailed, vivid eye patterns. On either foot, the peacock tail feather has a central stem with an assortment of barbs.[2] Peacock feathers, alternatively coloured, shine like metallic vibrancy. Interestingly, as we take a closer look at the peacocks feather from one hand to the other, their shades are steadily changing, we will see their colour moving from blue-green to yellow-green.[10]This is what makes physical form colouring more distinctive and charmingly mystical in a way than conventional chemical dyeing.Infrared spectrum analysis has shown that there is no major variation between the chemical components of peacock feathers in the yellow and blue regions. This indicates that the colours of the peacock feather are not produced by colouring but by its own composition based on the principle of coloration. [10] 

We can see certain layers organised within small barbs examining a feather crack on a small barb with a scanning electron microscope at magnification of 1,500, 8,000, 15,000 and 25,000 times. The diameter of this very thin protein fibre can exceed 150-160 nanometers.[2] A two-dimensional photonic crystal fibre structure of similar thickness and transparent structure is created by their arrangement and goes along the axial path of small barbs.This microscopic physical structure at the nanometer level, when illuminated by a beam of light, can form a good interference and superposition of photons, which is equivalent to white light moving through a triangular prism to form seven-color light and then reflect a single colour to form a colour in one direction. Many birds’ colourful feathers are connected to the nano micro-structure within their feathers. [10]

Other Birds 

Many other birds have vivid colours caused by nanostructures in their feathers, such as hummingbirds and kingfishers. In China, the kingfisher ‘s blue feathers have historically been used to decorate jewellery, even after hundreds of years, their colour has not disappeared.[14] is also possible to categorise the colours of bird feathers as iridescent colours, that is, varying in appearance with the angle(s) of view  and/or lighting, and non-iridescent colours that remain more or less visually unchanged regardless of the angle of view. [9] In avian animals, iridescent structural colours are usually attributed to barbules consisting of well-ordered granules of melanin. In projecting structural colours, the granules, arranged in a normal arrangement, play an important part, while absorption of the unwanted complementary colours further improves the display’s vibrancy. The hummingbird ‘s head and breast have iridescent green – blue shades, both.[14]

 

Application in Textiles 

Inspired by the scale structure of Morpho butterflies, the fibre manufacturer Kuraray Corp. produced a polyester material with low reflectivity but vivid colour. This substance was made of rectangular cross section fibres, called Diphorl. Two kinds of polyester with different thermal properties are spun from the fibres, which are heat treated after weaving and create several twists in the yarn (about 80-120 twists / inch).[8][13] This arrangement is believed to produce alternate longitudinal and transverse configurations, resulting in numerous scattering and absorption of incoming light, creating vibrant colours.[6] Another Japanese firm, Teijin Fibres Ltd, has created a fibre called Morphotex, which is often believed to resemble the microstructure of the Morpho butterfly and its structural colours.The thickness of the layers, their number, and the various refractive indices of the polymers used (nylon and polyester) eventually lead to structural colours.[11] Inspired by the study of the arrangement of peacock feathers, China’s Beijing Institute of Clothing Technology has investigated the use of physical and optical technologies to produce nanophotonic crystals with unique colours.[12] The congregational status of the two-dimensional photonic crystal structure of keratin in feather barbs produces a periodic two-dimensional structure that has a clear reflection of light along the surface in a certain wave band to form different colour arrays.

Inspired by the change in humidity resulting in color-changing activity in longhorn beetles, an artificial colloidal element sensitive to humidity was created by penetration into the interstice of the opal prototype and subsequent photopolymerization of a hydrophilic polyacrylamide (PAAm) solution.[12] The colour of the PA hydrogel was humidity sensitive; under different humidity conditions, it could reversibly vary from clear to violet, blue, cyan, green and red, covering the entire visible spectrum.In addition to helping to gain insight into the biological functionality of structural coloration, the technique of structural colour alteration may also motivate the creation of innovative artificial optical systems.[12] A biomimetic thin film-type humidity sensor with nanoporous structures (three-dimensional photonic crystals) has been designed, inspired by the humidity-dependent colour change observed in the Hercules beetle. As the ambient humidity rose, the visible colour of the fabricated humidity sensor shifted from blue-green to red. [13]

Conclusion

In insects , birds and plants, structural colour does not fade and has iridescent and UV properties that are not present in the features of pigment colour. The biological use of colour satisfies the demands of all kinds of biological functions. Centered on coherent and incoherent light scattering including film interference, diffraction grating, and photonic crystals, Rayleigh & Tyndall effect this paper reviews the physical structures and mechanisms of species with the existence of structural colours. In certain species, the basic roles of colour variations act as camouflage, predation, transmission of signals and reproductive activity. In order to manufacture systems for industrial purposes, a detailed understanding of the physical system as well as a better manufacturing approach will help. The more lessons we discover and understand from nature, give rise to more scientific implementations. From the point of mimicking iridescent effects, polarisation effects and colour shifts, bioinspired fabrication and applications are also addressed. Biomimetics have been influenced by the replication of structural colour to create diverse optical coatings, films, textiles and apparel, and anti-counterfeiting products. The strong convergence of biology, physics, chemistry and materials science at the nanoscale will likely affect many of the ways we see and understand the world, and the ways we produce objects.

Reference

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  3. Isha Tyagi & Dr. Alka Goel, ,2012, “Biomimetics : Colouring Textiles without Dyes
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  7.  Hiroshi Yabu, Yuta Saito and Masatsugu Shimomura, (2015) “Unique light reflectors that mimic the structural colors of tiger beetles”
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  13.  Richard O. Prum,  Rodolfo Torres, (2003) “Structural colouration of avian skin: convergent evolution of coherently scattering dermal collagen arrays”
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www.seas.harvard.edu/ “bioinspired-fibers-change-color-when-stretched”