Exotic bird inspires cheaper light-based camouflage design

"Structured coloration" could give you an impossible-to-chip paint job.

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Colors usually come from pigments or dyes, but are also created when light is refracted and reflected by microscopic structures. Those iridescent hues, known as "structural coloration," are often seen in nature on bird or butterfly wings. While durable and potentially useful for military and industrial applications, it's difficult and pricey to produce. However, Harvard University researchers have developed a robust and inexpensive way to build materials with structural coloring that could be used for camouflage, solar cells and optical switches.

Peacock feathers have the classic iridescent structural coloring, but the Harvard team was inspired by a different bird, the excellently-named Plum-throated Cotinga. Unlike the peacock, it gets its vibrant hues "from a disordered and porous nanonetwork of keratin that looks like a sponge or piece of coral," the team says. That pattern cancels red and yellow wavelengths out, amplifying its distinctive bright turquoise color.

The Harvard team used the relative chaos of the Cotinga's feather structure to its advantage. Using a simple etching technique, they created a "complex but random porous nanonetwork in a metallic alloy," then coated it with a thin transparent alumina layer (above). Depending on the coating thickness, the metallic alloy can create a gradient of colors ranging from blue (33-nanometers) to yellow (53 nanometers).

The team can change the color at whim just by varying the coating, which is both lightweight and scratch-proof. Just for starters, it could be used as lightweight color coatings for cars, biomimetic tissues used to test drugs and camouflage materials for the military (the research was sponsored by the US Air Force). "This system paves the way for large-scale and extremely robust metamaterials that interact with light in really interesting ways," says paper co-author Henning Galinski.