![Peacock feathers closeup](https://upload.wikimedia.org/wikipedia/commons/9/93/Peacock_feathers_closeup.jpg)
Structural coloration in animals, and a few plants, is the production of colour by microscopically structured surfaces fine enough to interfere with
visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
instead of
pigment
A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compo ...
s, although some structural coloration occurs in combination with pigments. For example,
peacock
Peafowl is a common name for three bird species in the genera ''Pavo (genus), Pavo'' and ''Afropavo'' within the tribe Pavonini of the family Phasianidae, the pheasants and their allies. Male peafowl are referred to as peacocks, and female pea ...
tail
feather
Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and a premier ...
s are pigmented brown, but their microscopic structure makes them also reflect blue, turquoise, and green light, and they are often
iridescent
Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...
.
Structural coloration was first observed by English scientists
Robert Hooke
Robert Hooke FRS (; 18 July 16353 March 1703) was an English polymath active as a scientist, natural philosopher and architect, who is credited to be one of two scientists to discover microorganisms in 1665 using a compound microscope that ...
and
Isaac Newton
Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a "natural philosopher"), widely recognised as one of the grea ...
, and its principle –
wave interference
In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...
– 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
A thin film is a layer of material ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ...
, 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
Butterflies are insects in the macrolepidopteran clade Rhopalocera from the Order (biology), order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The ...
, interference is created by a range of
photonic
Photonics is a branch of optics that involves the application of generation, detection, and manipulation of light in form of photons through emission, transmission, modulation, signal processing, switching, amplification, and sensing. Though ...
mechanisms, including
diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structura ...
s, selective mirrors,
photonic crystals
A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomic ...
, crystal fibres, matrices of nanochannels and proteins that can vary their configuration. Some
cuts of meat
A primal cut or cut of meat is a piece of meat initially separated from the carcass of an animal during butchering. Examples of primals include the round, loin, rib, and chuck for beef or the ham, loin, Boston butt, and picnic for pork.
Different ...
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
''Pollia condensata'', sometimes called the marble berry, is a perennial herbaceous plant with stoloniferous stems and hard, dry, shiny, round, metallic blue fruit. It is found in forested regions of Africa. The blue colour of the fruit, created ...
'', where a spiral structure of cellulose fibrils produces
Bragg's law
In physics and chemistry , Bragg's law, Wulff–Bragg's condition or Laue–Bragg interference, a special case of Laue diffraction, gives the angles for coherent scattering of waves from a crystal lattice. It encompasses the superposition of wave ...
scattering of light. The bright gloss of
buttercup
''Ranunculus'' is a large genus of about almost 1700 to more than 1800 species of flowering plants in the family Ranunculaceae. Members of the genus are known as buttercups, spearworts and water crowfoots.
The genus is distributed in Europe, ...
s 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 application, with
biomimetic
Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from grc, βίος (''bios''), life, and μίμησ ...
surfaces that could provide brilliant colours, adaptive
camouflage
Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the ...
, efficient optical switches and low-reflectance glass.
History
![Micrographia title page](https://upload.wikimedia.org/wikipedia/commons/0/08/Micrographia_title_page.gif)
In his 1665 book ''
Micrographia
''Micrographia: or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses. With Observations and Inquiries Thereupon.'' is a historically significant book by Robert Hooke about his observations through various lenses. It ...
'',
Robert Hooke
Robert Hooke FRS (; 18 July 16353 March 1703) was an English polymath active as a scientist, natural philosopher and architect, who is credited to be one of two scientists to discover microorganisms in 1665 using a compound microscope that ...
described the "fantastical" colours of the
peacock
Peafowl is a common name for three bird species in the genera ''Pavo (genus), Pavo'' and ''Afropavo'' within the tribe Pavonini of the family Phasianidae, the pheasants and their allies. Male peafowl are referred to as peacocks, and female pea ...
's feathers:
[Hooke, Robert. Micrographia. Chapter 36 ('Observ. XXXVI. ''Of Peacoks, Ducks, and Other Feathers of Changeable Colours''.')]
In his 1704 book ''
Opticks
''Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light'' is a book by English natural philosopher Isaac Newton that was published in English in 1704 (a scholarly Latin translation appeared in 1706). (''Opti ...
'',
Isaac Newton
Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a "natural philosopher"), widely recognised as one of the grea ...
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
In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.
They vary greatly in size or quantity, from s ...
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
Interference is the act of interfering, invading, or poaching. Interference may also refer to:
Communications
* Interference (communication), anything which alters, modifies, or disrupts a message
* Adjacent-channel interference, caused by extr ...
patterns.
In his 1892 book ''
Animal Coloration'',
Frank Evers Beddard
Frank Evers Beddard FRS FRSE (19 June 1858 – 14 July 1925) was an English zoologist. He became a leading authority on annelids, including earthworms. He won the Linnean Medal in 1916 for his book on oligochaetes.
Life
Beddard was born in ...
(1858–1925) acknowledged the existence of structural colours:
![NHM Chrysospalax trevelyani (cropped)](https://upload.wikimedia.org/wikipedia/commons/1/18/NHM_Chrysospalax_trevelyani_%28cropped%29.JPG)
But Beddard then largely dismissed structural coloration, firstly as subservient to pigments: "in every case the
tructuralcolour 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
The Cape golden mole (''Chrysochloris asiatica'') is a small, insectivorous mammal of the family Chrysochloridae, the golden mole
Golden moles are small insectivorous burrowing mammals endemic to Sub-Saharan Africa. They comprise the family ...
has "structural peculiarities" in its hair that "give rise to brilliant colours".
Principles
Structure not pigment
![Thin film interference](https://upload.wikimedia.org/wikipedia/commons/5/51/Thin_film_interference.svg)
Structural coloration is caused by interference effects rather than by pigments.
Colour
Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...
s 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
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
.
Structural coloration is responsible for the blues and greens of the feathers of many birds (the
bee-eater
The bee-eaters are a group of non-passerine birds in the family Meropidae, containing three genera and thirty species. Most species are found in Africa and Asia, with a few in southern Europe, Australia, and New Guinea. They are characterised by ...
,
kingfisher
Kingfishers are a family, the Alcedinidae, of small to medium-sized, brightly colored birds in the order Coraciiformes. They have a cosmopolitan distribution, with most species found in the tropical regions of Africa, Asia, and Oceania, ...
and
roller, for example), as well as many
butterfly
Butterflies are insects in the macrolepidopteran clade Rhopalocera from the Order (biology), order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The ...
wings,
beetle
Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...
wing-cases (
elytra
An elytron (; ; , ) is a modified, hardened forewing of beetles (Coleoptera), though a few of the true bugs (Hemiptera) such as the family Schizopteridae are extremely similar; in true bugs, the forewings are called hemelytra (sometimes alterna ...
) and (while rare among
flowers
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechani ...
) the gloss of
buttercup
''Ranunculus'' is a large genus of about almost 1700 to more than 1800 species of flowering plants in the family Ranunculaceae. Members of the genus are known as buttercups, spearworts and water crowfoots.
The genus is distributed in Europe, ...
petals.
These are often
iridescent
Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...
, as in
peacock
Peafowl is a common name for three bird species in the genera ''Pavo (genus), Pavo'' and ''Afropavo'' within the tribe Pavonini of the family Phasianidae, the pheasants and their allies. Male peafowl are referred to as peacocks, and female pea ...
feathers and
nacreous shells such as of
pearl oyster
''Pinctada'' is a genus of saltwater oysters, marine bivalve mollusks in the family Pteriidae. These pearl oysters have a strong inner shell layer composed of nacre, also known as "mother of pearl".
Pearl oysters are not closely related to ei ...
s (
Pteriidae
Pteriidae, also called the feather oysters, is a family of medium-sized to large saltwater clams. They are pearl oysters, marine bivalve molluscs in the order Pteriida.
Some of the species in this family are important economically as the sou ...
) and ''
Nautilus
The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina.
It comprises six living species in ...
''. 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
Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
,
while buttercup petals have both
carotenoid pigments for yellowness and thin films for reflectiveness.
Principle of iridescence
![Reversal of handedness of circularly polarized light reflected by mirror 2s](https://upload.wikimedia.org/wikipedia/commons/5/5f/Reversal_of_handedness_of_circularly_polarized_light_reflected_by_mirror_2s.gif)
Iridescence, as explained by
Thomas Young in 1803, is created when extremely
thin film
A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ap ...
s 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
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
of the film, and the angle at which the light fell – the two sets of waves are
out of phase
In physics and mathematics, the phase of a periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted \phi(t) and expressed in such a scale that it ...
. 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
![Butterfly magnification series collage](https://upload.wikimedia.org/wikipedia/commons/b/bd/Butterfly_magnification_series_collage.jpg)
A number of 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.
![Morpho butterfly scale christmas tree multilayer structures drawing from electron micrograph](https://upload.wikimedia.org/wikipedia/commons/b/b4/Morpho_butterfly_scale_christmas_tree_multilayer_structures_drawing_from_electron_micrograph.jpg)
A
diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structura ...
constructed of layers of
chitin
Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...
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 microscope
The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsco ...
s. 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
The morpho butterflies comprise many species of Neotropical butterfly under the genus ''Morpho''. This genus includes more than 29 accepted species and 147 accepted subspecies, found mostly in South America, Mexico, and Central America. ''Morph ...
'' butterflies (see drawing). Yet another variant exists in ''Parotia lawesii'',
Lawes's parotia
Lawes's parotia (''Parotia lawesii''), is a medium-sized (up to 27 cm long) passerine of the bird-of-paradise family, Paradisaeidae. It is distributed and endemic to mountain forests of southeast and eastern Papua New Guinea. Occasionally, th ...
, 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. 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
Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
.
Photonic crystal
A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of Crystal structure, natural crystals gives rise to X-ray crystallograp ...
s can be formed in different ways. In ''
Parides sesostris
''Parides sesostris'', the emerald-patched cattleheart or southern cattleheart, is a species of butterfly in the family Papilionidae.
Subspecies
* ''Parides sesostris sesostris'' (Cramer, 1779) (Suriname to eastern Ecuador and south to northern ...
'', 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
nanometre
330px, Different lengths as in respect to the molecular scale.
The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...
s 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
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
, 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
In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device.
In most digital display devices, pixels are the sm ...
about a micrometre wide. Each such pixel is a single crystal and reflects light in a direction different from its neighbours.
![Emerald Swallowtail Papilio palinurus scale structure diagrams](https://upload.wikimedia.org/wikipedia/commons/6/61/Emerald_Swallowtail_Papilio_palinurus_scale_structure_diagrams.svg)
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
emerald swallowtail
''Papilio palinurus'', the emerald swallowtail, emerald peacock, or green-banded peacock, is a butterfly of the genus ''Papilio'' of the family Papilionidae. It is native to Southeast Asia, but is regularly kept in butterfly houses around the wor ...
butterfly. These act as highly selective
mirror
A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...
s 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, formed of hexagonal arrays of hollow nanofibres, create the bright iridescent colours of the
bristles
A bristle is a stiff hair or feather (natural or artificial), either on an animal, such as a pig, a plant, or on a tool such as a brush or broom.
Synthetic types
Synthetic materials such as nylon are also used to make bristles in items such as br ...
of ''Aphrodita'', the
sea mouse
''Aphrodita'' is a genus of marine polychaete worms found in the Mediterranean sea and the eastern and western Atlantic Ocean.
Several members of this genus are known as "sea mice".
Etymology
The name of the genus is taken from Aphrodite ...
, a non-wormlike genus of marine annelids.
The colours are
aposematic
Aposematism is the advertising by an animal to potential predators that it is not worth attacking or eating. This unprofitability may consist of any defences which make the prey difficult to kill and eat, such as toxicity, venom, foul taste o ...
, 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.
![Ara ararauna Luc Viatour](https://upload.wikimedia.org/wikipedia/commons/e/ec/Ara_ararauna_Luc_Viatour.jpg)
Deformed matrices, consisting of randomly oriented nanochannels in a spongelike
keratin
Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
matrix, create the diffuse non-iridescent blue colour of ''Ara ararauna'', the
blue-and-yellow macaw
The blue-and-yellow macaw (''Ara ararauna''), also known as the blue-and-gold macaw, is a large South American parrot with mostly blue top parts and light orange underparts, with gradient hues of green on top of its head. It is a member of the la ...
. 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.
![Pollia](https://upload.wikimedia.org/wikipedia/commons/f/f3/Pollia.jpg)
Spiral coils, formed of
helicoid
The helicoid, also known as helical surface, after the plane and the catenoid, is the third minimal surface to be known.
Description
It was described by Euler in 1774 and by Jean Baptiste Meusnier in 1776. Its name derives from its similarit ...
ally stacked
cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
microfibrils A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and spermatozoon, sperm tail. Its mos ...
, create
Bragg reflection
In physics and chemistry , Bragg's law, Wulff–Bragg's condition or Laue–Bragg interference, a special case of Laue diffraction, gives the angles for coherent scattering of waves from a crystal lattice. It encompasses the superposition of wave ...
in the "marble berries" of the African herb ''
Pollia condensata
''Pollia condensata'', sometimes called the marble berry, is a perennial herbaceous plant with stoloniferous stems and hard, dry, shiny, round, metallic blue fruit. It is found in forested regions of Africa. The blue colour of the fruit, created ...
'', resulting in the most intense blue coloration known in nature.
The berry's surface has four layers of cells with thick walls, containing spirals of transparent cellulose spaced so as to allow
constructive interference
In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...
with blue light. Below these cells is a layer two or three cells thick containing dark brown
tannin
Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids.
The term ''tannin'' (from Anglo-Norman ''tanner'', ...
s. ''Pollia'' produces a stronger colour than the wings of ''
Morpho
The morpho butterflies comprise many species of Neotropical butterfly under the genus ''Morpho''. This genus includes more than 29 accepted species and 147 accepted subspecies, found mostly in South America, Mexico, and Central America. ''Morph ...
'' butterflies, and is one of the first instances of structural coloration known from any plant. Each cell has its own thickness of stacked fibres, making it reflect a different colour from its neighbours, and producing a
pixellated
In computer graphics, pixelation (or pixellation in British English) is caused by displaying a bitmap or a section of a bitmap at such a large size that individual pixels, small single-colored square display elements that comprise the bitmap, a ...
or
pointillist
Pointillism (, ) is a technique of painting in which small, distinct dots of color are applied in patterns to form an image.
Georges Seurat and Paul Signac developed the technique in 1886, branching from Impressionism. The term "Pointillism" wa ...
effect with different blues speckled with brilliant green, purple, and red dots. The fibres in any one cell are either left-handed or right-handed, so each cell
circularly polarizes the light it reflects in one direction or the other. ''Pollia'' is the first organism known to show such random polarization of light, which, nevertheless does not have a visual function, as the seed-eating birds who visit this plant species are not able to perceive polarised light. Spiral microstructures are also found in
scarab beetles
The family Scarabaeidae, as currently defined, consists of over 30,000 species of beetles worldwide; they are often called scarabs or scarab beetles. The classification of this family has undergone significant change in recent years. Several sub ...
where they produce iridescent colours.
![Buttercup petal structural and pigment coloration](https://upload.wikimedia.org/wikipedia/commons/6/6a/Buttercup_petal_structural_and_pigment_coloration.svg)
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
''Ranunculus acris'' is a species of flowering plant in the family Ranunculaceae, and is one of the more common buttercups across Europe and temperate Eurasia. Common names include meadow buttercup, tall buttercup, common buttercup and giant but ...
'', 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.
Surface gratings, consisting of ordered surface features due to exposure of ordered muscle cells on
cuts of meat
A primal cut or cut of meat is a piece of meat initially separated from the carcass of an animal during butchering. Examples of primals include the round, loin, rib, and chuck for beef or the ham, loin, Boston butt, and picnic for pork.
Different ...
. The structural coloration on meat cuts appears only after the ordered pattern of muscle
fibrils is exposed and light is diffracted by the proteins in the fibrils. The coloration or wavelength of the diffracted light depends on the angle of observation and can be enhanced by covering the meat with translucent foils. Roughening the surface or removing water content by drying causes the structure to collapse, thus, the structural coloration to disappear.
Interference from multiple total internal reflections can occur in microscale structures, such as sessile water droplets and biphasic oil-in-water droplets as well as polymer microstructured surfaces. In this structural coloration mechanism, light rays that travel by different paths of
total internal reflection
Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflected b ...
along an interface interfere to generate iridescent colour.
Variable structures
![Variable ring patterns on mantles of the blue-ringed octopus Hapalochlaena lunulata](https://upload.wikimedia.org/wikipedia/commons/d/d6/Variable_ring_patterns_on_mantles_of_the_blue-ringed_octopus_Hapalochlaena_lunulata.png)
Some animals including
cephalopods such as squid are able to vary their colours rapidly for both
camouflage
Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the ...
and signalling. The mechanisms include reversible
proteins
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
which can be switched between two configurations. The configuration of
reflectin
Reflectins are a family of intrinsically disordered proteins evolved by a certain number of cephalopods including ''Euprymna scolopes'' and ''Doryteuthis opalescens'' to produce Iridescence, iridescent camouflage and signaling. The recently identi ...
proteins in
chromatophore
Chromatophores are cells that produce color, of which many types are Biological pigment, pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and ...
cells in the skin of the ''
Doryteuthis pealeii
The longfin inshore squid (''Doryteuthis pealeii'') is a species of squid of the family Loliginidae.
Description
This species of squid is often seen with a reddish hue, but like many types of squid can manipulate its color, varying from a deep ...
'' squid is controlled by electric charge. When charge is absent, the proteins stack together tightly, forming a thin, more reflective layer; when charge is present, the molecules stack more loosely, forming a thicker layer. Since chromatophores contain multiple reflectin layers, the switch changes the layer spacing and hence the colour of light that is reflected.
Blue-ringed octopuses spend much of their time hiding in crevices whilst displaying effective camouflage patterns with their dermal
chromatophore
Chromatophores are cells that produce color, of which many types are Biological pigment, pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and ...
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
The greater blue-ringed octopus (''Hapalochlaena lunulata'') is one of four species of highly venomous blue-ringed octopuses belonging to the family Octopodidae. This particular species of blue-ringed octopus is known as one of the most toxic ma ...
(''Hapalochlaena lunulata''), the rings contain multi-layer
iridophores
Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast, ...
. 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.
Examples
File:European bee eater.jpg, European bee-eater
The European bee-eater (''Merops apiaster'') is a near passerine bird in the bee-eater family, Meropidae. It breeds in southern and central Europe, northern and southern Africa, and western Asia. Except for the resident southern African populat ...
s owe their brilliant colours partly to diffraction grating microstructures in their feathers
File:Butterfly Morpho rhetenor helena (M) KL.jpg, In ''Morpho
The morpho butterflies comprise many species of Neotropical butterfly under the genus ''Morpho''. This genus includes more than 29 accepted species and 147 accepted subspecies, found mostly in South America, Mexico, and Central America. ''Morph ...
'' butterflies such as ''Morpho helena'' the brilliant colours are produced by intricate firtree-shaped microstructures too small for optical microscopes.
File:Parotia lawesii by Bowdler Sharpe.jpg, The male ''Parotia lawesii
Lawes's parotia (''Parotia lawesii''), is a medium-sized (up to 27 cm long) passerine of the bird-of-paradise family, Paradisaeidae. It is distributed and endemic to mountain forests of southeast and eastern Papua New Guinea. Occasionally, th ...
'' bird of paradise signals to the female with his breast feathers that switch from blue to yellow.
File:Green Swallotail (Papilio palinurus) - Relic38.jpg, Brilliant green of emerald swallowtail, '' Papilio palinurus'', is created by arrays of microscopic bowls that reflect yellow directly and blue from the sides.
File:Parides sesostris MHNT dos.jpg, Emerald-patched cattleheart butterfly, ''Parides sesostris
''Parides sesostris'', the emerald-patched cattleheart or southern cattleheart, is a species of butterfly in the family Papilionidae.
Subspecies
* ''Parides sesostris sesostris'' (Cramer, 1779) (Suriname to eastern Ecuador and south to northern ...
'', creates its brilliant green using photonic crystals.
File:Curculionidae - Lamprocyphus augustus.JPG, Iridescent scales of '' Lamprocyphus augustus'' weevil contain diamond-based crystal lattices oriented in all directions to give almost uniform green.
File:Scales covering Entimus imperialis' elytra.jpg, Iridescent scales on ''Entimus imperialis
''Entimus imperialis'', common name Brazilian diamond beetle, is a species of broad-nosed weevils belonging to the family Curculionidae, true weevil and the Entiminae subfamily.
Description
''Entimus imperialis'' can reach a length of about . T ...
'' weevil
File:Entimus imperialis' photonic crystal.jpg, Electron micrograph of the three-dimensional photonic crystal
A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of Crystal structure, natural crystals gives rise to X-ray crystallograp ...
s within the scales on ''Entimus imperialis
''Entimus imperialis'', common name Brazilian diamond beetle, is a species of broad-nosed weevils belonging to the family Curculionidae, true weevil and the Entiminae subfamily.
Description
''Entimus imperialis'' can reach a length of about . T ...
'' weevil
File:Aphrodita aculeata (Sea mouse).jpg, Hollow nanofibre bristles of ''Aphrodita aculeata
''Aphrodita aculeata'', the sea mouse, is a marine polychaete worm found in the North Atlantic, the North Sea, the Baltic Sea and the Mediterranean. The sea mouse normally lies buried head-first in the sand. It has been found at depths of ove ...
'' (a species of sea mouse) reflect light in yellows, reds and greens to warn off predators.
File:Loligo pealeii.jpg, Longfin inshore squid, ''Doryteuthis pealeii
The longfin inshore squid (''Doryteuthis pealeii'') is a species of squid of the family Loliginidae.
Description
This species of squid is often seen with a reddish hue, but like many types of squid can manipulate its color, varying from a deep ...
'', has been studied for its ability to change colour.
File:Thinfilmbubble.jpg, Thin-film interference
Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another, either enhancing or reducing the reflected light. When the thickness of the film is an ...
in a soap bubble. Colour varies with film thickness.
In technology
Gabriel Lippmann
Jonas Ferdinand Gabriel Lippmann (16 August 1845 – 13 July 1921) was a Franco-Luxembourgish physicist and inventor, and Nobel laureate in physics for his method of reproducing colours photographically based on the phenomenon of interference. ...
won the Nobel Prize in Physics in 1908 for his work on a structural coloration method of colour photography, the
Lippmann plate
Gabriel Lippmann conceived a two-step method to record and reproduce colours, variously known as direct photochromes, interference photochromes, Lippmann photochromes, Photography in natural colours by direct exposure in the camera or the Lippman ...
. 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.
In 2010, the dressmaker Donna Sgro made a dress from
Teijin
is a Japanese chemical, pharmaceutical and information technology company. Its main fields of operation are high-performance fibers such as aramid, carbon fibers & composites, healthcare, films, resin & plastic processing, polyester fibers, produc ...
Fibers' Morphotex, an undyed fabric woven from structurally coloured fibres, mimicking the microstructure of ''Morpho'' butterfly wing scales. The fibres are composed of 61 flat alternating layers, between 70 and 100 nanometres thick, of two plastics with different refractive indices,
nylon
Nylon is a generic designation for a family of synthetic polymers composed of polyamides ( repeating units linked by amide links).The polyamides may be aliphatic or semi-aromatic.
Nylon is a silk-like thermoplastic, generally made from petro ...
and
polyester
Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
, in a transparent nylon sheath with an oval cross-section. The materials are arranged so that the colour does not vary with angle. The fibres have been produced in red, green, blue, and violet.
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
Military camouflage is the use of camouflage by an armed force to protect personnel and equipment from observation by enemy forces. In practice, this means applying colour and materials to military equipment of all kinds, including vehicles, ...
fabrics that vary their colours and
pattern
A pattern is a regularity in the world, in human-made design, or in abstract ideas. As such, the elements of a pattern repeat in a predictable manner. A geometric pattern is a kind of pattern formed of geometric shapes and typically repeated l ...
s to match their environments, just as
chameleon
Chameleons or chamaeleons (family Chamaeleonidae) are a distinctive and highly specialized clade of Old World lizards with 202 species described as of June 2015. The members of this family are best known for their distinct range of colors, bein ...
s and
cephalopod
A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
s do. The ability to vary reflectivity to different wavelengths of light could also lead to efficient optical switches that could function like
transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
s, enabling engineers to make fast optical computers and routers.
The surface of the
compound eye
A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which disti ...
of the
housefly
The housefly (''Musca domestica'') is a fly of the suborder Cyclorrhapha. It is believed to have evolved in the Cenozoic Era, possibly in the Middle East, and has spread all over the world as a commensal of humans. It is the most common fl ...
is densely packed with microscopic projections that have the effect of reducing reflection and hence increasing transmission of incident light.
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.
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
Reactive-ion etching (RIE) is an etching technology used in microfabrication. RIE is a type of dry etching which has different characteristics than wet etching. RIE uses chemically reactive plasma to remove material deposited on wafers. The pl ...
.
See also
*
Animal coloration
*
Camouflage
Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the ...
*
Patterns in nature
Patterns in nature are visible regularities of form found in the natural world. These patterns recur in different contexts and can sometimes be modelled mathematically. Natural patterns include symmetries, trees, spirals, meanders, waves, foa ...
Bibliography
Pioneering books
*
Beddard, Frank Evers (1892). ''
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
''Micrographia: or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses. With Observations and Inquiries Thereupon.'' is a historically significant book by Robert Hooke about his observations through various lenses. It ...
'', John Martyn and James Allestry, London.
*
Newton, Isaac (1704). ''
Opticks
''Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light'' is a book by English natural philosopher Isaac Newton that was published in English in 1704 (a scholarly Latin translation appeared in 1706). (''Opti ...
'', 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
Notes
References
External links
National Geographic News: Peacock Plumage Secrets Uncovered*
Butterflies and Gyroids – Numberphile
{{colour topics
Color
Animal coat colors
Optical materials
Nanotechnology