Structural color
Structural colors have attracted much attention in a wide variety of disciplines. They originate from physical interactions of light with nanostructures. Recent studies have focused on ordered structures in the natural world which produce iridescent colors. However, nature extensively uses quasi-ordered structures as well to create vivid colors that are weakly iridescent, e.g., the spongy nanostructures in the feather barbs of numerous birds. The lack of pronounced iridescence led to the initial hypothesis that the colors were produced by wavelength selective scattering from single particles. Although it is now recognized that light scattering is organized to contribute effectively to coloration of quasi-ordered structures, the exact physical mechanism of color production by the quasi-ordered structures is still unclear. Is the color produced by constructive interference of scattered light from these structures with only short-range order? If so, how can it be non-iridescent? Is the color truly non-iridescent, or under what conditions is it? Does multiple scattering play a significant role in coloration? What are the differences and similarities between the color produced by an ordered structure and that by a quasi-ordered structure?

In collaboration with Prof. Richard Prum's group in the Department of Ecology and Evolutionary Biology and Peabody National History Museum, Prof. Eric Dufresne's group in the Mechanical Engineering Department, and Prof. Simon Mochrie's group in the Physics Department of Yale University, we investigate the physical mechanism for color production by isotropic nanostructures with short-range order in bird feather barbs. The color-producing quasi-ordered nanostructures within the medullary cells of bird feather barb rami belong to two morphological classes. Channel-type nanostructures consist of beta-keratin bars and air channels in elongate and tortuous forms. Sphere-type nanostructures consist of spherical air cavities in a beta-keratin matrix. Our recent studies suggest these nanostructures are self-assembled during phase separation of beta-keratin protein from the cytoplasm of the cell. The channel morphology is developed via spinodal decomposition and the sphere morphology, via nucleation and growth. To fully characterize the color, we have performed angle-resolved reflection and scattering spectrometry on feather barbs. With white light illumination of feather barbs, we measure the scattered light as a function of wavelength, sample orientation, incident light direction and viewing angle. Our results demonstrate the subtleties of non-iridescent colors from the quasi-ordered structures, namely, the colors are non-iridescent under diffusive light illumination like in nature, but iridescent under artificial directional lighting often used in the laboratory. We also show that multiple scattering of light has important contributions to coloration of quasi-ordered structures. For comparison, we have measured the color from an ordered structure and illustrate the significant difference from the color of a quasi-ordered structure. In the former, light is mostly reflected to the specular direction, while in the latter light is strongly scattered into all directions.

False color maps showing the intensity of backscattered light as function of the incident angle and wavelength from the feather barbs of (A) Sialia sialis and (B) Cotinga maynana.

(A) Male Eastern Bluebird (Sialia sialis, Turdidae). (B) Male Plum-throated Cotinga (Cotinga maynana, Cotingidae). (C) Channel-type beta-keratin and air nanostructure from back contour feather barbs of S. sialis. (D) Sphere-type beta-keratin and air nanostructure from back contour feather barbs of C. maynana. (E & F) Small-angle X-ray scattering data from the channel-type feather barb of S. sialis, and the sphere-type feather barb of C. maynana. Scale bars in (C, D) 500 nm, (E, F) 0.025 (1/nm) of spatial frequency. Photo credits: (A) Ken Thomas (image in the public domain); (B) Thomas Valqui (reproduced with permission).

Periodic biological structures have provided inspiration for groups trying to make photonic materials. Much of this work has been motivated by producing a photonic band gap (PBG). However, nature's alternative design, based on isotropic structures is just starting to be explored. Biomimetic random structures have been used to make ultra-thin mineral coatings that are brilliant white. A wide range of colors with very little angle dependence can be produced by microgel dispersions. The wavelength dependence of light scattering from quasi-ordered structures has not been taken advantage of technologically, and may have useful advantages for photonic coatings across many areas such as the production of color for paint, cosmetics and textile industries and photon management in solar cells.

In collaboration with Prof. Eric Dufresne and Prof. Corey O'Hern in the Mechanical Engineering Department of Yale University, we self-assemble the biomimetic isotropic films which display structural color that is amenable to potential applications in coatings, cosmetics, and textiles. We find that isotropic structures can produce color if there is a pronounced characteristic length-scale comparable to the wavelength of visible light and wavelength-independent scattering is suppressed.

Biomimetic films displaying structural color. (a) Photograph of a film spun-coat onto an 18 x 18 mm glass coverslip from a bidisperse suspension of polystyrene spheres with diameters of 212 nm and 254 nm in roughly equal numbers. Inset: Photograph of a dried sessile droplet. (b) Scanning electron micrograph of the top layer of the film showing there is no long range order. The field ofview is 5.2 um. (c) Small angle X-ray scattering pattern from the film: the field of view is 0.15 (1/nm) and the gray values are logarithmic in intensity. (d) The red dots are the azimuthal average of the scattering pattern in (c), and the black line is the optical reflection spectrum taken at an angle of 10 degrees from the surface normal.