How non-iridescent colors are generated by quasi-ordered structures of bird feathers
Most colours in nature are inherent due to the chemical nature of the material - chlorophyll gives the green colour in leaves, anthocyanin the colour red in petals and carotenoids renders the rosy pink colour of flamingo's plumage. But it has been known for a long time that there are many colouring that has a more physical origin, such as diffraction and intereference of light. Newton explained the colour of the peacock feather in terms of thin-film intereference, and the colours of the butterfly wing can be understood due to intereference between lamellar layers, just as intereference between periodic groovings on a disc can give rise to colours.
Left: TEM of butterfly wing, right: schematic of how structural colour arises due to interference between the arrays of structures evenly spaced out.
But it is often thought that such structural colours are synonym with iridescence* - the phenomenon where the colour changes with viewing angle. This is because structural colours often arises due to highly-ordered, periodic structures which breaks the isotropy of space. The authors here demonstrates how conventional wisdom is not always true and how quasi-ordered structures in bird feathers can result in non-iridescent colours.
* Strictly speaking, it has been known that there are many scattering phenomenon that are non-iridescent, such as the scattering light of fat emulsions in milk, but this involves scattering of the whole spectrum of visible light giving the white colour in milk, whereas in peacock feather and butterfly wing, only light with a particular wavelengths are reflected.
It was found that the structural colour in bird feathers (C. continga, I. Puella) are due to quasi-ordered structures of nano-sized air pockets trapped in a mesh of keratin. There is some short ordered correlation between the positions of the air pocket in the scale of the wavelength of light, but they are isotropic at longer order. The air pockets are shown in the figure below.
The amount of light reflected specularly and scattered by the feather is studied in details both by uni-directional light, i.e. light that is shone at an angle, as well as by a light-source that is omni-directional which is more representative of what is encountered in nature. An elliptical mirror is used to focus light to the feather which was placed in one of the focii as shown below.
The reflection and scattering spectrum can be understood essentially by light scattered once by the air pockets and constructively interefering with one another. The reason why most light is only scattered once is complex and not well understood, but it was thought that this could be due to near field coupling. Assuming single scattering however explains why feather looks non-iridescent in the presence of omni-directional light, though in the presence of uni-directional light, there is a slight dependence of colour on viewing angle. There is also good fit between experimental results and theoretical predictions for the amount of scattering as shown below.
It is very interesting how men's ingenuity and nature sometimes converges to the same solution or diverges completely when faced with the same problem. For locomotion, nature came up with legs, while men decided to settle on wheels. On the other hand, by sheer coincidence, there is a striking resemblence in the Yagi-Uda antenna that is still used for transmission of TV signal and the array structure that we see in butterfly wings. Both work using similar principles - constructive interference to amplify signal - but one is shaped by evolutionary forces, while the other sheer brilliance of men.
Structural colour is interesting study on its own merit, but there is no question of the technological applications that it has. The latest LED screen requires a backlit illumination, which works well inside a room, but hopeless in the sun. Nature came up with a better solution clearly. The feathers of birds are clearly visible in the sun. Quasi-ordered structures based screen can be the next LED screen requiring no backlighting and perfectly visible outdoors. It is also really awe-inspiring how nature come up with beautiful solutions to render colour. Just look at the cephalopods changing their colours and textures at will !
1. "How non-iridescent colors are generated by quasi-ordered structures of bird feathers", Noh et al, Advanced Materials, 2010
2. "Encoding Complex Wettability Patterns in Chemically Functionalized 3D Photonic Crystals", Burgess et al, JACS, 2011
3. "Photophysics of Structural Color in the Morpho Butterflies", Kinoshita et al, Forma, 2002
4. "How cephalopods change color", Wood and Jackson