Journal of The Royal Society Interface
Restricted accessResearch articles

Structure and optical function of amorphous photonic nanostructures from avian feather barbs: a comparative small angle X-ray scattering (SAXS) analysis of 230 bird species

Vinodkumar Saranathan

Vinodkumar Saranathan

Department of Ecology and Evolutionary Biology and Peabody Museum of Natural History, Yale University, New Haven, CT 06520, USA

Center for Research on Interface Structures and Phenomena (CRISP), Yale University, New Haven, CT 06520, USA

[email protected] [email protected]

Google Scholar

Find this author on PubMed

,
Jason D. Forster

Jason D. Forster

Department of Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06520, USA

Center for Research on Interface Structures and Phenomena (CRISP), Yale University, New Haven, CT 06520, USA

Google Scholar

Find this author on PubMed

,
Heeso Noh

Heeso Noh

Department of Applied Physics, Yale University, New Haven, CT 06520, USA

Center for Research on Interface Structures and Phenomena (CRISP), Yale University, New Haven, CT 06520, USA

Google Scholar

Find this author on PubMed

,
Seng-Fatt Liew

Seng-Fatt Liew

Department of Applied Physics, Yale University, New Haven, CT 06520, USA

Center for Research on Interface Structures and Phenomena (CRISP), Yale University, New Haven, CT 06520, USA

Google Scholar

Find this author on PubMed

,
Simon G. J. Mochrie

Simon G. J. Mochrie

Department of Applied Physics, Yale University, New Haven, CT 06520, USA

Department of Physics, Yale University, New Haven, CT 06520, USA

Center for Research on Interface Structures and Phenomena (CRISP), Yale University, New Haven, CT 06520, USA

Google Scholar

Find this author on PubMed

,
Hui Cao

Hui Cao

Department of Applied Physics, Yale University, New Haven, CT 06520, USA

Department of Physics, Yale University, New Haven, CT 06520, USA

Center for Research on Interface Structures and Phenomena (CRISP), Yale University, New Haven, CT 06520, USA

Google Scholar

Find this author on PubMed

,
Eric R. Dufresne

Eric R. Dufresne

Department of Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06520, USA

Department of Physics, Yale University, New Haven, CT 06520, USA

Center for Research on Interface Structures and Phenomena (CRISP), Yale University, New Haven, CT 06520, USA

Google Scholar

Find this author on PubMed

and
Richard O. Prum

Richard O. Prum

Department of Ecology and Evolutionary Biology and Peabody Museum of Natural History, Yale University, New Haven, CT 06520, USA

Center for Research on Interface Structures and Phenomena (CRISP), Yale University, New Haven, CT 06520, USA

Donostia International Physics Center (DIPC), 20018 Donostia-San Sebastian, Spain

[email protected] [email protected]

Google Scholar

Find this author on PubMed

    Non-iridescent structural colours of feathers are a diverse and an important part of the phenotype of many birds. These colours are generally produced by three-dimensional, amorphous (or quasi-ordered) spongy β-keratin and air nanostructures found in the medullary cells of feather barbs. Two main classes of three-dimensional barb nanostructures are known, characterized by a tortuous network of air channels or a close packing of spheroidal air cavities. Using synchrotron small angle X-ray scattering (SAXS) and optical spectrophotometry, we characterized the nanostructure and optical function of 297 distinctly coloured feathers from 230 species belonging to 163 genera in 51 avian families. The SAXS data provided quantitative diagnoses of the channel- and sphere-type nanostructures, and confirmed the presence of a predominant, isotropic length scale of variation in refractive index that produces strong reinforcement of a narrow band of scattered wavelengths. The SAXS structural data identified a new class of rudimentary or weakly nanostructured feathers responsible for slate-grey, and blue-grey structural colours. SAXS structural data provided good predictions of the single-scattering peak of the optical reflectance of the feathers. The SAXS structural measurements of channel- and sphere-type nanostructures are also similar to experimental scattering data from synthetic soft matter systems that self-assemble by phase separation. These results further support the hypothesis that colour-producing protein and air nanostructures in feather barbs are probably self-assembled by arrested phase separation of polymerizing β-keratin from the cytoplasm of medullary cells. Such avian amorphous photonic nanostructures with isotropic optical properties may provide biomimetic inspiration for photonic technology.

    References