Structural coloration
Structural coloration in animals, and a few plants, is the production of colour by microscopically structured surfaces fine enough to interfere with
Structural coloration was first described by English scientists
Structural coloration has potential for industrial, commercial and military applications, with
History
In his 1665 book
The parts of the Feathers of this glorious Bird appear, through the Microscope, no less gaudy then do the whole Feathers; for, as to the naked eye 'tis evident that the stem or quill of each Feather in the tail sends out multitudes of Lateral branches, … so each of those threads in the Microscope appears a large long body, consisting of a multitude of bright reflecting parts.
… their upper sides seem to me to consist of a multitude of thin plated bodies, which are exceeding thin, and lie very close together, and thereby, like mother of Pearl shells, do not onely reflect a very brisk light, but tinge that light in a most curious manner; and by means of various positions, in respect of the light, they reflect back now one colour, and then another, and those most vividly. Now, that these colours are onely fantastical ones, that is, such as arise immediately from the refractions of the light, I found by this, that water wetting these colour'd parts, destroy'd their colours, which seem'd to proceed from the alteration of the reflection and refraction.[1]
In his 1704 book Opticks, Isaac Newton described the mechanism of the colours other than the brown pigment of peacock tail feathers.[2] Newton noted that[3]
The finely colour'd Feathers of some Birds, and particularly those of Peacocks Tails, do, in the very same part of the Feather, appear of several Colours in several Positions of the Eye, after the very same manner that thin Plates were found to do in the 7th and 19th Observations, and therefore their Colours arise from the thinness of the transparent parts of the Feathers; that is, from the slenderness of the very fine Hairs, or Capillamenta, which grow out of the sides of the grosser lateral Branches or Fibres of those Feathers.[3]
In his 1892 book
The colours of animals are due either solely to the presence of definite pigments in the skin, or … beneath the skin; or they are partly caused by optical effects due to the scattering, diffraction or unequal refraction of the light rays. Colours of the latter kind are often spoken of as structural colours; they are caused by the structure of the coloured surfaces. The metallic lustre of the feathers of many birds, such as the humming birds, is due to the presence of excessively fine striae upon the surface of the feathers.[6]: 1
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;"[6]: 2 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",[6]: 2 though he does later admit that the Cape golden mole has "structural peculiarities" in its hair that "give rise to brilliant colours".[6]: 32
Principles
Structure not pigment
Structural coloration is caused by interference effects rather than by pigments.
Structural coloration is responsible for the blues and greens of the feathers of many birds (the
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.[2]
Mechanisms
Fixed structures
A number of fixed structures can create structural colours, by mechanisms including diffraction gratings, selective mirrors, photonic crystals, crystal fibres and deformed matrices.[8] 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.[10] Each mechanism offers a specific solution to the problem of creating a bright colour or combination of colours visible from different directions.
A
Selective mirrors to 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
Crystal fibres, formed of hexagonal arrays of hollow nanofibres, create the bright iridescent colours of the
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.[10][23]
Spiral coils, formed of
Thin film with diffuse reflector, based on the top two layers of a buttercup's petals. The brilliant yellow gloss derives from a combination, rare among plants, of yellow pigment and structural coloration. The very smooth upper epidermis acts as a reflective and iridescent thin film; for example, in Ranunculus acris, the layer is 2.7 micrometres thick. The unusual starch cells form a diffuse but strong reflector, enhancing the flower's brilliance. The curved petals form a paraboloidal dish which directs the sun's heat to the reproductive parts at the centre of the flower, keeping it some degrees Celsius above the ambient temperature.[11]
Surface gratings, consisting of ordered surface features due to exposure of ordered muscle cells on
Interference from multiple total internal reflections can occur in microscale structures, such as sessile water droplets and biphasic oil-in-water droplets[27] as well as polymer microstructured surfaces.[28] In this structural coloration mechanism, light rays that travel by different paths of total internal reflection along an interface interfere to generate iridescent colour.
Variable structures
Some animals including
Blue-ringed octopuses spend much of their time hiding in crevices whilst displaying effective camouflage patterns with their dermal chromatophore cells. If they are provoked, they quickly change colour, becoming bright yellow with each of the 50-60 rings flashing bright iridescent blue within a third of a second. In the greater blue-ringed octopus (Hapalochlaena lunulata), the rings contain multi-layer iridophores. These are arranged to reflect blue–green light in a wide viewing direction. The fast flashes of the blue rings are achieved using muscles under neural control. Under normal circumstances, each ring is hidden by contraction of muscles above the iridophores. When these relax and muscles outside the ring contract, the bright blue rings are exposed.[29]
Examples
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European bee-eaters owe their brilliant colours partly to diffraction grating microstructures in their feathers
-
In Morpho butterflies such as Morpho helena the brilliant colours are produced by intricate firtree-shaped microstructures too small for optical microscopes.
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The maleParotia lawesiibird of paradise signals to the female with his breast feathers that switch from blue to yellow.
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Brilliant green of emerald swallowtail, Papilio palinurus, is created by arrays of microscopic bowls that reflect yellow directly and blue from the sides.
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Emerald-patched cattleheart butterfly, Parides sesostris, creates its brilliant green using photonic crystals.
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Iridescent scales of Lamprocyphus augustus weevil contain diamond-based crystal lattices oriented in all directions to give almost uniform green.
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Iridescent scales on Entimus imperialis weevil
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Electron micrograph of the three-dimensional photonic crystals within the scales on Entimus imperialis weevil
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Hollow nanofibre bristles of Aphrodita aculeata (a species of sea mouse) reflect light in yellows, reds and greens to warn off predators.
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Longfin inshore squid,Doryteuthis pealeii, has been studied for its ability to change colour.
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Thin-film interference in a soap bubble. Colour varies with film thickness.
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Wasps of thesculpturingof their otherwise black chitin.
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Two photographs of the same Eupholus weevil exhibit the unique expression of structural color.
In technology
Gabriel Lippmann won the Nobel Prize in Physics in 1908 for his work on a structural coloration method of colour photography, the Lippmann plate. This used a photosensitive emulsion fine enough for the interference caused by light waves reflecting off the back of the glass plate to be recorded in the thickness of the emulsion layer, in a monochrome (black and white) photographic process. Shining white light through the plate effectively reconstructs the colours of the photographed scene.[30][31]
In 2010, the dressmaker Donna Sgro made a dress from Teijin Fibers' Morphotex, an undyed fabric woven from structurally coloured fibres, mimicking the microstructure of Morpho butterfly wing scales.[32][33][34] The fibres are composed of 61 flat alternating layers, between 70 and 100 nanometres thick, of two plastics with different refractive indices, nylon and polyester, in a transparent nylon sheath with an oval cross-section. The materials are arranged so that the colour does not vary with angle.[35] The fibres have been produced in red, green, blue, and violet.[36]
Several countries and regions, including the U.S., European Union, and Brazil, use
Structural coloration could be further exploited industrially and commercially, and research that could lead to such applications is under way. A direct parallel would be to create active or adaptive military camouflage fabrics that vary their colours and patterns to match their environments, just as chameleons and cephalopods do. The ability to vary reflectivity to different wavelengths of light could also lead to efficient optical switches that could function like transistors, enabling engineers to make fast optical computers and routers.[10]
The surface of the compound eye of the housefly is densely packed with microscopic projections that have the effect of reducing reflection and hence increasing transmission of incident light.[37] Similarly, the eyes of some moths have antireflective surfaces, again using arrays of pillars smaller than the wavelength of light. "Moth-eye" nanostructures could be used to create low-reflectance glass for windows, solar cells, display devices, and military stealth technologies.[38] Antireflective biomimetic surfaces using the "moth-eye" principle can be manufactured by first creating a mask by lithography with gold nanoparticles, and then performing reactive-ion etching.[39]
See also
References
- ^ a b c Hooke, Robert. Micrographia. Chapter 36 ('Observ. XXXVI. Of Peacoks, Ducks, and Other Feathers of Changeable Colours.')
- ^ a b "Iridescence in Lepidoptera". Natural Photonics (originally in Physics Review Magazine). University of Exeter. September 1998. Archived from the original on April 7, 2014. Retrieved April 27, 2012.
- ^ a b Newton, Isaac (1730) [1704]. Opticks (4th ed.). William Innys at the West-End of St. Paul's, London. pp. Prop. V., page 251. Retrieved April 27, 2012.
- S2CID 110408369.
- ^ Shamos, Morris (1959). Great Experiments in Physics. New York: Holt Rinehart and Winston. pp. 96–101.
- ^ ISBN 978-0-543-91406-4.
- ^ Structural colour under the microscope! Feathers, beetles and butterflie!!
- ^ ISBN 978-163-081-797-8
- doi:10.1016/j.optlastec.2005.06.037.)
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- ^ Wallin, Margareta (2002). "Nature's Palette: How animals, including humans, produce colours" (PDF). Bioscience Explained. 1 (2): 1–12. Retrieved November 17, 2011.
- ^ Smyth, S.; et al. (2007). "What Makes the Peacock Feather Colorful?" (PDF). NNIN REU Journal.
- ^ Smyth, S. (2009). "What Makes the Peacock Feather Bright and Colorful". University of Alaska, Fairbanks (Honors Thesis). Archived from the original on 2016-03-04. Retrieved 2015-09-21.
- PMID 21159676.[permanent dead link]
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- .
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: CS1 maint: multiple names: authors list (link - ^ The Photonic Beetle: Nature Builds Diamond-like Crystals for Future Optical Computers Archived 2012-11-02 at the Wayback Machine. Biomimicry News, 21 May 2008.
- ^ "Sea mouse promises bright future". BBC News. BBC. January 3, 2001. Retrieved April 26, 2012.
- ^ McPhedran, Ross; McKenzie, David; Nicorovici, Nicolae (3 April 2002). "A Natural Photonic Crystal" (PDF). University of Sydney School of Physics. Archived from the original (PDF) on 25 August 2012. Retrieved 18 May 2012.
- S2CID 4413969.)
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- ^ "Visual Ecology" by Cronin, T.W., Johnson, S., Marshall, N.J. and Warrant, E.J. (2014) Princeton University Press
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- ^ Biedermann, Klaus (15 May 2005). "Lippmann's and Gabor's Revolutionary Approach to Imaging". Nobel Prize.
- ^ Cherny-Scanlon, Xenya (29 July 2014). "Seven fabrics inspired by nature: from the lotus leaf to butterflies and sharks". The Guardian. Retrieved 23 November 2018.
- ^ Sgro, Donna. "About". Donna Sgro. Retrieved 23 November 2018.
- ^ Sgro, Donna (9 August 2012). "Biomimicry + Fashion Practice". Fashionably Early Forum, National Gallery Canberra. pp. 61–70. Retrieved 23 November 2018.
- ^ "Teijin Limited | Annual Report 2006 | R&D Efforts" (PDF). Teijin Japan. July 2006. Archived from the original (PDF) on 17 November 2016. Retrieved 23 November 2018.
MORPHOTEX, the world's first structurally colored fiber, features a stack structure with several tens of nano-order layers of polyester and nylon fibers with different refractive indexes, facilitating control of color using optical coherence tomography. Structural control means that a single fiber will always show the same colors regardless of its location.
- ^ "Fabric | Morphotex". Transmaterial. 12 October 2010. Retrieved 23 November 2018.
- S2CID 7184882.)
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: CS1 maint: multiple names: authors list (link - ^ Boden, S.A., Bagnall, D.M. "Antireflection". University of Southampton. Retrieved May 19, 2012.
{{cite web}}
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{{cite journal}}
: CS1 maint: multiple names: authors list (link
Bibliography
Pioneering books
- Animal Coloration, An Account of the Principal Facts and Theories Relating to the Colours and Markings of Animals. Swan Sonnenschein, London.
- --- 2nd Edition, 1895.
- Hooke, Robert (1665). Micrographia, John Martyn and James Allestry, London.
- Newton, Isaac (1704). Opticks, William Innys, London.
Research
- Fox, D.L. (1992). Animal Biochromes and Animal Structural Colours. University of California Press.
- Johnsen, S. (2011). The Optics of Life: A Biologist's Guide to Light in Nature. Princeton University Press.
- Kolle, M. (2011). Photonic Structures Inspired by Nature . Springer.
General books
- Brebbia, C.A. (2011). Colour in Art, Design and Nature. WIT Press.
- Lee, D.W. (2008). Nature's Palette: The Science of Plant Color. University of Chicago Press.
- Kinoshita, S. (2008). "Structural Color in the Realm of Nature". World Scientific Publishing
- Mouchet, S. R., Deparis, O. (2021). "Natural Photonics and Bioinspiration". Artech House
External links
- National Geographic News: Peacock Plumage Secrets Uncovered
- Doucet, S. M.; Shawkey, M. D.; Hill, G. E.; Montgomerie, R. (2006). "Iridescent plumage in satin bowerbirds: Structure, mechanisms and nanostructural predictors of individual variation in colour". Journal of Experimental Biology. 209 (2): 380–390. S2CID 14595674.
- Causes of Color: Peacock feathers
- Butterflies and Gyroids – Numberphile