Among the taxidermal specimens in Harvard's Museum of Comparative Zoology, past centuries-old fur coats, arises a flicker of brilliant blue. This is the spangled cotinga. Surprisingly, the cotinga is about as old as everything in the room, but its color is still as dazzling as the day it was brought to the museum. The cotinga -- or rather its feathers -- achieve this effect through structural color.
Unlike color that we usually think of, which arises from paints and dyes absorbing certain wavelengths of light and reflecting the remainder, structural color is created when an object's very nanostructure amplifies a specific wavelength. Cells in the cotinga's feathers have a series of tiny pores spaced just right so that blues (and not much of anything else) are reflected back to our eyes. Because of this, if the feathers were thoroughly pulverized, the formation of pores and therefore the color would be lost. It also means that the same color could be produced from an entirely different material, if one could recreate the same pattern made by the feathers' pores.
Researchers led by Vinothan N. Manoharan at the Harvard School of Engineering and Applied Sciences want to recreate this effect, giving human-made materials structural color. Producing structural color is not easy, though; it often requires a material's molecules to be in a very specific crystalline pattern, like the natural structure of an opal, which reflects a wide array of colors. But the pores on the cotinga's feathers lack a regular order and are therefore a prime target for imitation.
Manoharan's lab has devised a system where microcapsules are filled with a disordered solution of even smaller particles suspended in water. When the microcapsule is partly dried out, it shrinks, bringing the particles closer and closer together. Eventually the average distance between all the particles will give rise to a specific reflected color from the capsule. Shrink the capsule a bit more, and they become another color, and then another.
"There's an average distance between particles, even though there is no ordering in the particles. It's that average distance that is important in determining the color," says Manoharan, Gordon McKay Professor of Chemical Engineering and Professor of Physics at Harvard.
The findings have been published in the journal Angewandte Chemie.
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