Self Assembly of Biophotonic Nanostructures by Phase Separation
We often tend to associate the idea of color in the natural kingdom with pigmented materials. However, many colors that arise in nature come not from pigment but from ordered nanostructures that backscatter light at specific frequencies. One such example arises in certain types of birds, where the feather barbs are actually composed of <math>\beta</math>-keratin and air nanostructures which are quais-ordered and have some characteristic length scale. This paper presents two different types of feather barb nanostructures, that arising in Sialia sialis and that arising in Cotinga maynana, and hypothesizes the physical mechanism by which these nanostructures could have been created.
The plumes of the two bird species S. sialis and C. maynana, shown in the pictures above, both are composed of nanostructured material. For S. sialis, the structures are long tortuous <math>\beta</math>-keratin and air channels, while for C. maynana the structures look more like close packed spherical air structures. These two different types of patterns are seen not only for these two species but also across other bird species. The color of a bird has significance for social interactions (eg mating) , and so the mechanism by which the birds create these patterns should be robust.
This paper proposes that the method by which the nanostructures are created is through self assembly via phase separation between <math>\beta</math>-keratin and cell cytoplasm. In the case of the structures composed of channels, the mechanism must be dominated by spinodal decomposition, where the unmixed state lies within the region below the spinodal line. In the case of the spheres, the mechanism would be complete decomposition, with isolated nucleation and growth. Here the initial unmixed state must lie between the spinodal and phase boundary lines. The paper proposes that the phase diagram changes with polymerization. That is, for small molecules at a given concentration, the two materials can exist in an unmixed state. As the polymerization continues, the phase boundaries shift upwards until the unmixed state becomes thermodynamically unfavorable and a decomposition (either through spinodal or nucleation and growth) must occur.
To show some preliminary evidence for this theory, the group compared the optical spectra of the bird feathers to that of a polymer mixture undergoing spinodal decomposition. The spectrum matches up well with that of S. sialis, indicating that this may be the method by which the nanostructures are created.
This paper proposes a credible method by which the nanostructures of certain avian feathers could be created. However, direct evidence of phase separation of the <math>\beta</math>-keratin and cell cytoplasm would be needed to confirm this hypothesis.