Structural coloration in animals, and a few plants, is the production of colour by microscopically structured surfaces fine enough to interfere with visible light instead of pigments, although some structural coloration occurs in combination with pigments. For example, peacock tail feathers are pigmented brown, but their microscopic structure makes them also reflect blue, turquoise, and green light, and they are often iridescent.
Structural coloration was first described by English scientists Robert Hooke and Isaac Newton, and its principle—wave interference—explained by Thomas Young a century later. Young described iridescence as the result of interference between reflections from two or more surfaces of thin films, combined with refraction as light enters and leaves such films. The geometry then determines that at certain angles, the light reflected from both surfaces interferes constructively, while at other angles, the light interferes destructively. Different colours therefore appear at different angles.
In animals such as on the feathers of birds and the scales of butterflies, interference is created by a range of photonic mechanisms, including diffraction gratings, selective mirrors, photonic crystals, crystal fibres, matrices of nanochannels and proteins that can vary their configuration. Some cuts of meat also show structural coloration due to the exposure of the periodic arrangement of the muscular fibres. Many of these photonic mechanisms correspond to elaborate structures visible by electron microscopy. In the few plants that exploit structural coloration, brilliant colours are produced by structures within cells. The most brilliant blue coloration known in any living tissue is found in the marble berries of Pollia condensata, where a spiral structure of cellulose fibrils produces Bragg's law scattering of light. The bright gloss of buttercups is produced by thin-film reflection by the epidermis supplemented by yellow pigmentation, and strong diffuse scattering by a layer of starch cells immediately beneath.
Structural coloration has potential for industrial, commercial and military applications, with biomimetic surfaces that could provide brilliant colours, adaptive camouflage, efficient optical switches and low-reflectance glass.
History
In his 1665 book Micrographia, Robert Hooke described the "fantastical" colours of the peacock's feathers:Hooke, Robert. Micrographia. Chapter 36 ('Observ. XXXVI. Of Peacoks, Ducks, and Other Feathers of Changeable Colours.')
In his 1704 book Opticks, Isaac Newton described the mechanism of the colours other than the brown pigment of peacock tail feathers. Newton noted that
Thomas Young (1773–1829) extended Newton's particle theory of light by showing that light could also behave as a wave. He showed in 1803 that light could diffract from sharp edges or slits, creating interference patterns.
In his 1892 book Animal Coloration, Frank Evers Beddard (1858–1925) acknowledged the existence of structural colours:
But Beddard then largely dismissed structural coloration, firstly as subservient to pigments: "in every case the [structural] colour needs for its display a background of dark pigment;" and then by asserting its rarity: "By far the commonest source of colour in invertebrate animals is the presence in the skin of definite pigments", though he does later admit that the Cape golden mole has "structural peculiarities" in its hair that "give rise to brilliant colours".
Principles
Structure not pigment
Structural coloration is caused by interference effects rather than by pigments. Structural colour under the microscope! Feathers, beetles and butterflie!! Colours are produced when a material is scored with fine parallel lines, or formed of one or more parallel thin layers, or otherwise composed of microstructures on the scale of the colour's wavelength.
Structural coloration is responsible for the blues and greens of the feathers of many birds (the bee-eater, kingfisher and roller, for example), as well as many butterfly wings, beetle wing-cases (elytra) and (while rare among flowers) the gloss of buttercup petals. These are often iridescent, as in peacock feathers and nacreous shells such as of pearl oysters (Pteriidae) and Nautilus. This is because the reflected colour depends on the viewing angle, which in turn governs the apparent spacing of the structures responsible. Structural colours can be combined with pigment colours: peacock feathers are pigmented brown with melanin, while buttercup petals have both carotenoid pigments for yellowness and thin films for reflectiveness.
Principle of iridescence
Iridescence, as explained by Thomas Young in 1803, is created when extremely thin films reflect part of the light falling on them from their top surfaces. The rest of the light goes through the films, and a further part of it is reflected from their bottom surfaces. The two sets of reflected waves travel back upwards in the same direction. But since the bottom-reflected waves travelled a little farther – controlled by the thickness and refractive index of the film, and the angle at which the light fell – the two sets of waves are out of phase. When the waves are one or more whole wavelengths apart – in other words, at certain specific angles, they add (interfere constructively), giving a strong reflection. At other angles and phase differences, they can subtract, giving weak reflections. The thin film therefore selectively reflects just one wavelength – a pure colour – at any given angle, but other wavelengths – different colours – at different angles. So, as a thin-film structure such as a butterfly's wing or bird's feather moves, it seems to change colour.
Mechanisms
Fixed structures
Multiple fixed structures can create structural colours, by mechanisms including diffraction gratings, selective mirrors, photonic crystals, crystal fibres and deformed matrices. Structures can be far more elaborate than a single thin film: films can be stacked up to give strong iridescence, to combine two colours, or to balance out the inevitable change of colour with angle to give a more diffuse, less iridescent effect. Each mechanism offers a specific solution to the problem of creating a bright colour or combination of colours visible from different directions.
Diffraction gratings
A diffraction grating constructed of layers of chitin and air gives rise to the iridescent colours of various butterfly wing scales as well as to the tail feathers of birds such as the peacock. Hooke and Newton were correct in their claim that the peacock's colours are created by interference, but the structures responsible, being close to the wavelength of light in scale (see micrographs), were smaller than the striated structures they could see with their light microscopes. Another way to produce a diffraction grating is with tree-shaped arrays of chitin, as in the wing scales of some of the brilliantly coloured tropical Morpho butterflies (see drawing). Yet another variant exists in Parotia lawesii, Lawes's parotia, a bird of paradise. The barbules of the feathers of its brightly coloured breast patch are V-shaped, creating thin-film microstructures that strongly reflect two different colours, bright blue-green and orange-yellow. In many iridescent bird feathers, ordered arrays of melanosomes within the barbules produce the structural colour, while hollow melanosomes heighten brightness and saturation by adding an air–melanin refractive-index contrast to the nanostructure. When the bird moves the colour switches sharply between these two colours, rather than drifting iridescently. During courtship, the male bird systematically makes small movements to attract females, so the structures must have evolved through sexual selection.
Photonic crystals
Photonic crystals can be formed in different ways. In Parides sesostris, the emerald-patched cattleheart butterfly, photonic crystals are formed of arrays of nano-sized holes in the chitin of the wing scales. The holes have a diameter of about 150 nanometres and are about the same distance apart. The holes are arranged regularly in small patches; neighbouring patches contain arrays with differing orientations. The result is that these emerald-patched cattleheart scales reflect green light evenly at different angles instead of being iridescent. In Lamprocyphus augustus, a weevil from Brazil, the chitin exoskeleton is covered in iridescent green oval scales. These contain diamond-based crystal lattices oriented in all directions to give a brilliant green coloration that hardly varies with angle. The scales are effectively divided into pixels about a micrometre wide. Each such pixel is a single crystal and reflects light in a direction different from its neighbours.The Photonic Beetle: Nature Builds Diamond-like Crystals for Future Optical Computers . Biomimicry News, 21 May 2008. The ctenophore Beroe cucumis has two-dimensional photonic crystals formed by the densely packed cilia of the comb plates. The regular nanoscale arrangement of the cilia causes constructive interference of light across the visible spectrum, producing the characteristic shifting iridescence of the comb rows. The high refractive index of seawater, compared with air, broadens the reflected wavelength range relative to what the same structure would produce in a terrestrial setting, enriching the iridescent effect.
Selective mirrors
Selective mirrors that create interference effects are formed of micron-sized bowl-shaped pits lined with multiple layers of chitin in the wing scales of Papilio palinurus, the emerald swallowtail butterfly. These act as highly selective mirrors for two wavelengths of light. Yellow light is reflected directly from the centres of the pits; blue light is reflected twice by the sides of the pits. The combination appears green, but can be seen as an array of yellow spots surrounded by blue circles under a microscope.
Crystal fibres
Crystal fibres, formed of hexagonal arrays of hollow nanofibres, create the bright iridescent colours of the bristles of Aphrodita, the sea mouse, a non-wormlike genus of marine annelids. The colours are aposematic, warning predators not to attack. The chitin walls of the hollow bristles form a hexagonal honeycomb-shaped photonic crystal; the hexagonal holes are 0.51 μm apart. The structure behaves optically as if it consisted of a stack of 88 diffraction gratings, making Aphrodita one of the most iridescent of marine organisms.
Deformed matrices
Deformed matrices, consisting of randomly oriented nanochannels in a spongelike keratin matrix, create the diffuse non-iridescent blue colour of Ara ararauna, the blue-and-yellow macaw. Since the reflections are not all arranged in the same direction, the colours, while still magnificent, do not vary much with angle, so they are not iridescent.
Spiral coils