Unlocking the Nanoscale Secrets of Bird-Feather Colors
MAY 18, 2012
For millennia birds have been prized, even hunted, for their vivid, beautiful plumage. But what makes their feathers so colorful? A new study of feathers carried out at the U.S. Department of Energy Office of Science’s Advanced Photon Source (APS) at Argonne National Laboratory has revealed the complex three-dimensional nanostructures responsible for non-iridescent or angle-independent colors in bird feathers. These biological photonic nanostructures could serve as a source of biomimetic inspiration for new versions of the ubiquitous photonic devices, such as CD/DVD/Blu-ray and remote-control devices, optical data recorders, fiber optic telecommunications, video displays, and optical pumping of high-power lasers.
While many birds produce colors using pigments or dyes, there are apparently no blue pigments in birds, while green pigments are exceedingly rare. Birds have had to evolve to structurally produce these shorter- and middle-wavelength colors instead, using nanoscale surface structural features on feathers, which are repeating modulations in material composition or refractive index on the order of a few hundred nanometers, comparable to certain wavelengths of visible light that get constructively reinforced.
“The amorphous or quasi-ordered nanostructures that ultimately give feathers their color look strikingly similar to porous forms and structures seen in beer foam, corroding metal alloys, and polymer melts, structures that are all self-assembled by the process of phase separation, where the whole breaks into its constituent parts,” said Vinod Saranathan, lead author on the paper that appeared online in the Journal of the Royal Society Interface.
Using the pinhole small-angle x-ray scattering technique at the X-ray Science Division 8-ID-I beamline at the APS, Saranathan and his coworkers from Yale University, Oxford University, and the Donostia International Physics Center accurately determined the three-dimensional, nanoscale internal structure of feathers from 230 species of birds of the world.
The study finds that in feather barbs (barbs and barbules are respectively the primary and secondary branches of a feather), the complex biophotonic nanostructures made up of the protein beta-keratin and air occur in one of two fundamental forms – either as a tortuous network of air channels in keratin (like a porous sponge) or as an array of spherical air bubbles in keratin (like Swiss cheese), but sometimes as more disordered and highly variable versions of these two forms.
Despite this apparently chaotic arrangement, the team’s x-ray scattering experiments found a kind of “quasi-order” in the material variation of the nanostructures, but in multiple directions that accounts for their unusual isotropic (non-iridescent) optical qualities.
Because the quasi-ordered architecture of barbs interacts strongly with light, they often produce double peaks even in the ultraviolet range, which, unlike humans, birds can see.
Indeed, the strongly interacting nature of the quasi-ordered nanostructures in bird feathers has inspired the optics experts of the team to design a novel random laser without the need of an optical resonant cavity to trap the photons.
The study finds that very similar barb nanostructures have evolved independently in many families of birds (at least 44), but these look very much like other nanostructures seen in the physical world, such as beer foam, corroding metal alloys, and oil-in-water.
“This suggests that many lineages of birds have independently evolved to utilize the self-assembling properties of a polymerizing protein in solution (phase separation of keratin from the cytoplasm of barb cells) to create optical nanostructures,” Saranathan said.
Yet the secrets of structural colors aren’t just for the birds. They could also help to develop new materials for photonic devices that would not allow the passage of a certain band of wavelengths in any direction; such istropic photonic crystals are currently hard to fabricate defect-free and on an industrial scale.
“The nanostructures in bird feather barbs, that are likely self-assembled and have evolved over millions of years of selection for a consistent optical function, could be used to inspire novel photonic devices. They could be used as biotemplates for the fabrication of photonic materials using better technological raw materials (such as titania or silica), or we can try and mimic their process of self-assembly using synthetic polymers for color-tuneable applications,” Saranathan said.
So, in the not too distant future, we could be growing our own artificial feathers not just to dazzle and amaze but to harness the power of light.