Blue birds and red birds get their vibrant colors in very different ways, and comparing the two strategies could tell scientists something fascinating about how materials (natural or synthetic) make colors appear. The findings were published in Physical Review E last week.
The red feathers of cardinals and scarlet ibises get their brilliant color through pigments, which work by absorbing most colors of light but reflecting the ones we see. Blue pigment, however, is rare in nature. The feathers of birds like blue jays and indigo buntings have contain tiny, randomly-arranged structures that scatter light. Certain wavelengths dominate the scattering to create the color that we see.
When color arises from the structure of the material (and not the pigments), that’s called “structural coloration.” This process has been successfully mimicked using artificial materials. But while scientists have created blues using this technique, they have yet to create reds, oranges, and yellows via structural coloration -- leaving them to wonder if it’s possible to create those colors without pigments. "We thought, maybe the birds know something we don't," Harvard’s Vinothan Manoharan tells New Scientist.
To investigate, Manoharan and colleagues developed nano-sized plastic beads called “photonic glass” that act similar to the light-scattering air pockets of blue feathers. By changing the size of the beads, they could alter how light is scattered.
With small beads, the light output was dominated by blue wavelengths. Even with larger beads, red was overshadowed by a second peak at shorter wavelengths that represent the light that enters beads and reflects off their back surface. This backscattering is typically in the ultraviolet, but it moves to the visible with larger beads.
The team thinks that red, structurally colored materials can be designed by suppressing this backscattering using hollow beads. They’re working on this now.
Earlier this week, we learned about the first known case of natural iridescence in bird eggs: Great tinamous lay glossy eggs with blue shells that appear to change color depending on the angle you view them at. By contrast, when the light-scattering elements are ordered randomly the way they are in blue bird feathers and some purple beetle scales, the structural coloration can be seen from all angles.
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