Spatially modulated structural colour in bird feathers

Abstract

Eurasian Jay (Garrulus glandarius) feathers display periodic variations in the reflected colour from white through light blue, dark blue and black. We find the structures responsible for the colour are continuous in their size and spatially controlled by the degree of spinodal phase separation in the corresponding region of the feather barb. Blue structures have a well-defined broadband ultra-violet (UV) to blue wavelength distribution; the corresponding nanostructure has characteristic spinodal morphology with a lengthscale of order 150 nm. White regions have a larger 200 nm nanostructure, consistent with a spinodal process that has coarsened further, yielding broader wavelength white reflectance. Our analysis shows that nanostructure in single bird feather barbs can be varied continuously by controlling the time the keratin network is allowed to phase separate before mobility in the system is arrested. Dynamic scaling analysis of the single barb scattering data implies that the phase separation arrest mechanism is rapid and also distinct from the spinodal phase separation mechanism i.e. it is not gelation or intermolecular re-association. Any growing lengthscale using this spinodal phase separation approach must first traverse the UV and blue wavelength regions, growing the structure by coarsening, resulting in a broad distribution of domain sizes.

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

“…High-resolution studies of butterfly single scales revealed the 3D photonic nanostructure as a single network of gyroid morphology formed by chitin and air which is reminiscent of cubic phases observed in amphiphilic soft matter systems such as surfactants, block copolymers and lipids in water or block selective solvents. Results supported the hypothesis that color-producing protein and air nanostructures in feather barbs are self-assembled by arrested phase separation of polymerizing β-keratin [56], and the nanostructure can be varied continuously by regulating the time the keratin network is allowed to phase separate before mobility in the system is arrested [131]. In arthropod scales and setae, a richer nanostructural diversity, including triply periodic bicontinuous networks, close-packed spheres, inverse columnar, perforated lamellar, and disordered spongelike morphologies have been reported [130].…”

“…An important motivation for studying long-range ordered colloidal systems has been from the point of view of potential photonic materials [8]. More recent investigations employed high-resolution SAXS for elucidating the structural basis of colors in biophotonic specimens [57,130,131]. High-resolution studies of butterfly single scales revealed the 3D photonic nanostructure as a single network of gyroid morphology formed by chitin and air which is reminiscent of cubic phases observed in amphiphilic soft matter systems such as surfactants, block copolymers and lipids in water or block selective solvents.…”

Interactions between colour-producing mechanisms and their effects on the integumentary colour palette

“…Production of non-iridescent blue colour by birds also involves co-localized melanosomes ( figure 2III). These melanin-filled organelles are typically found basal to the spongy layer and serve the critical function of absorbing incoherently scattered white light [36,39,49]. Only a certain percentage of light is scattered by the spongy layer to produce blue, and the remainder is scattered incoherently.…”

Solution‐Processable Photonic Inks of Mie‐Resonant Hollow Carbon–Silica Nanospheres

“…However, ordered nanostructures lead to structural color that depends strongly on the viewing angle, which limits its practical applications. By contrast, materials with short‐range order, such as colloidal glasses or blue bird feathers, show structural color that is independent of angle. While these materials could in principle be used in applications that require structural color that is indistinguishable from pigment, they also tend to diffuse or multiply scatter light, leading to colors that are not saturated.…”