If you’ve ever wondered why some animals like butterflies, birds, and beetles shimmer in the light and change from one iridescent color to the next, it’s because they don’t rely on pigments as most of nature does. Instead, these and many other animals rely on structural colors created when light bends and reflects off of nanostructures found in wings, feathers, fur, and skin.
When nature is up to something, humans want to find a way to emulate it. To that end, scientists in China created a synthetic butterfly whose colors change as its wings flap.
In a paper published Wednesday in the journal Cell Reports Physical Science, the artificial butterfly’s wings (which are inspired by the lesser purple emperor butterfly native to Europe) are covered in photonic crystals—nanostructures that reflect certain wavelengths of light while allowing others to pass through—that change color, from purple to red, based on wing’s angle. The wings aren’t robotic or anything—they can move on their own with the help of a chemical compound that acts as an artificial muscle responsive to changes in heat and humidity.
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Don’t be fooled into believing this is just a pet project from scientists with too much time on their hands. The new study may reveal a way for us to harness structural colors to create new kinds of biosensors, help small robots move, and maybe even develop new forms of camouflage.
Trying to recreate structural colors while maintaining a material’s durability “remains a great challenge,” Mingzhu Li, a physical chemist at the University of Chinese Academy of Sciences and the study’s lead author, told the Daily Beast in an email. But Li and her team managed to find a way to overcome the obstacles, taking advantage of the way the lesser purple emperor butterfly’s wings switch from a tawny brown to purple as it beats its wings. The group’s imitation butterfly has stainless steel wings encrusted with photonic crystals which generate different colors by scattering and reflecting light. These thin wings are connected to the rest of the body through a joint made of a ceramic material called MXene, which is mixed with pDADMAC, a compound that shrinks when heated and relaxes under humidity.
When a burst of near-infrared light is directed at the joint, the wings lift and change all sorts of colors along the visible light spectrum depending on the angle. For instance, they change from steel gray to purple at an angle of 16 degrees, greenish-blue at around 22 degrees, yellow at 26 degrees, and red at 33 degrees. After the near-infrared light is turned off and water is applied to the joint, the wings rest back down and the colors disappear. The researchers found they could repeat this process at least 500 times without damaging the butterfly and precisely control how much the joint moves to get an exact color.
Aside from the butterfly, the researchers showed their technology could be adapted to a sheet of paper studded with photonic crystals that almost looks like a close-up of a bird’s feather or fabric with sequin attached to it. Li and her colleagues were able to spell out the letters “ICCAS” (for the Institute of Chemistry, Chinese Academy of Sciences) onto the paper and change up the colors up to 10,000 times doing other things without destroying it.
While the artificial butterfly’s joints are pretty quick to react to near-infrared light—it takes only about five seconds—Li says that her team wants to up the response time and sensitivity even more. This could be crucial to small movable robots as the team found contracting force behind the butterfly joint could also be used for robotic crawlers. This technology could also be handy in designing sensors or high-performance displays that change color in response to stimuli like temperature, electricity, or mechanical force. Color, far from being just pretty, could also power the world in surprising ways.
