Whilst our understanding of coloured objects is dominated by the role of pigments and dyes, the natural world makes extensive use of nanotechnology to yield vivid hues, often with added iridescence. The mechanisms involve constructive and destructive interference of light - this is referred to as structural colour. This is the best way to get a brilliant white or an ultrablack. Structural colours can be produced by highly periodic photonic crystals (as in butterfly wings), and also by "amorphous, or quasi-ordered, dielectric nanostructures where there are local correlations but little long-range order" (as in the feathers of some birds).
Nanostructures that produce some birds' brightly colored plumage, such as the blue feathers of the male Eastern Bluebird, have a sponge-like structure. (Photo: Ken Thomas, source here)
Some feather colours result from the presence of a spongy material with nano-sized air bubbles. In technical language, the visual effects are created by "light scattering from spongy beta-keratin and air nanostructures within the medullary cells of avian feather barb rami". An interdisciplinary research team have analysed the feathers from different birds and tested out various hypotheses for the formation of colour. They favour a phase-separation model:
"They compared the nanostructures to examples of materials undergoing phase separation, in which mixtures of different substances become unstable and separate from one another, such as the carbon-dioxide bubbles that form when the top is popped off a bubbly drink. They found that the color-producing structures in feathers appear to self-assemble in much the same manner. Bubbles of water form in a protein-rich soup inside the living cell and are replaced with air as the feather grows."
They identified two distinct classes of nanostructure: channel morphologies and sphere morphologies. From previous work, they find that these nanostructures are unrelated to phylogeny and they conclude that the "channel and sphere nanostructures have evolved independently in many lineages of birds".
In order to form recognisable, reproducible plumage patterns, it is necessary for the nanostructure development to be precisely controlled during feather development. This is no mean task. Based on their phase separation model, they are able to spell out the implications for managing the process.
"In this model, birds develop color producing nanostructures by controlling the combined process of phase separation and kinetic arrest with three variables: the rates of beta-keratin expression, beta-keratin polymerization, and filament crosslinking."
There are two points worthy of note here. The first concerns the complexity of the process and the potential for biomimetics. The researchers speak of elegance and intricacy. Self assembly does occur - but only under closely engineered conditions. Thus, the context for self-assembly is intelligent design.
"We have found that nature elegantly self assembles intricate optical structures in bird feathers. We are now mimicking this approach to make a new generation of optical materials in the lab."
The other point relates to Darwinian storytelling regarding bird plumage colours. Many have suggested that feather plumage colours are pointers to fitness - however that is defined. The new research effectively drives a wedge between Darwinian adaptation theory and the empirical data.
"Many biologists think that plumage color can encode information about quality - basically, that a bluer male is a better mate," said Richard Prum, chair of the Department of Ecology and Evolutionary Biology and one of the paper's authors. "Such information would have to be encoded in the feather as the bubbles grow. I think our hypothesis that phase separation is involved provides less opportunity for encoding information about quality than most biologists thought. At the same time, it's exciting to think about other ways birds might be using phase separation."
Self-assembly of amorphous biophotonic nanostructures by phase separation
Eric R. Dufresne, Heeso Noh, Vinodkumar Saranathan, Simon G. J. Mochrie, Hui Cao and Richard O. Prum
Soft Matter, Online 30 March 2009, DOI: 10.1039/b902775k
Some of the most vivid colors in the animal kingdom are created not by pigments, but by wavelength-selective scattering of light from nanostructures. Here we investigate quasi-ordered nanostructures of avian feather barbs which produce vivid non-iridescent colors. These beta-keratin and air nanostructures are found in two basic morphologies: tortuous channels and amorphous packings of spheres. Each class of nanostructure is isotropic and has a pronounced characteristic length scale of variation in composition. These local structural correlations lead to strong backscattering over a narrow range of optical frequencies and little variation with angle of incidence. Such optical properties play important roles in social and sexual communication. To be effective, birds need to precisely control the development of these nanoscale structures, yet little is known about how they grow. We hypothesize that multiple lineages of birds have convergently evolved to exploit phase separation and kinetic arrest to self-assemble spongy color-producing nanostructures in feather barbs. Observed avian nanostructures are strikingly similar to those self-assembled during the phase separation of fluid mixtures; the channel and sphere morphologies are characteristic of phase separation by spinodal decomposition and nucleation and growth, respectively. These unstable structures are locked-in by the kinetic arrest of the beta-keratin matrix, likely through the entanglement or cross-linking of supermolecular beta-keratin fibers. Using the power of self-assembly, birds can robustly realize a diverse range of nanoscopic morphologies with relatively small physical and chemical changes during feather development.
Bird Feathers Produce Color Through Structure Similar to Beer Foam, Yale University Office of Public Affairs (April 2, 2009)
Tyler, D. Optimal design for brilliant whiteness in a beetle, (ARN Literature blog, 20 January 2007)
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