If you’ve ever wondered why some animals like butterflies, birds, and beetles shimmer in the light and change from one iridescent color to another, it’s because they don’t depend on pigments like the most of nature. Instead, these animals and many others rely on structural colors created when light bends and reflects off nanostructures found in wings, feathers, fur and skin.
When nature cooks up something, humans want to find a way to imitate it. To this end, Chinese scientists have created a synthetic butterfly whose colors change as its wings flap.
In a article published on Wednesday in the magazine Physical Sciences Cell Reports, the wings of the artificial butterfly (which is inspired by the small purple emperor butterfly native to Europe) are covered in photonic crystals – nanostructures that reflect certain wavelengths of light while letting others pass – which change color , from purple to red, based on wing angle. The wings aren’t robotic or anything – they can move on their own using a chemical compound that acts like an artificial muscle sensitive to changes in heat and humidity.
Don’t be fooled into thinking this is just a pet project of scientists with too much free time. The new study could tell us a way to harness structural colors to create new types of biosensors, help small robots move around, and perhaps even develop new forms of camouflage.
Trying to recreate structural colors while maintaining the durability of a material “remains a big challenge,” said Mingzhu Li, a physical chemist at the University of the Chinese Academy of Sciences and lead author of the study, at 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 little purple emperor butterfly’s wings turn from tawny-brown to purple when it flaps its wings. The band’s imitation butterfly has stainless steel wings encrusted with photonic crystals that generate different colors by scattering and reflecting light. These thin wings are joined to the rest of the body by a joint made of a ceramic material called MXene, which is mixed with pDADMAC, a compound that shrinks when heated and expands 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 example, they change from steel gray to purple at an angle of 16 degrees, to greenish blue at about 22 degrees, to yellow at 26 degrees, and to red at 33 degrees. Once the near infrared light is turned off and water is applied to the seal, the wings rest and the colors disappear. The researchers found they could repeat this process at least 500 times without damaging the butterfly and precisely controlling the movement of the joint to achieve an exact color.
In addition to the butterfly, the researchers showed that 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 sequins attached. Li and his colleagues were able to spell the letters “ICCAS” (for Institute of Chemistry, Chinese Academy of Sciences) on the paper and change the colors up to 10,000 times by doing something else without destroying it.
Although the artificial butterfly’s joints react fairly quickly to near-infrared light (it only takes about five seconds), Li says his team wants to increase response time and sensitivity even more. This could be crucial for small mobile robots, as the team discovered that the contraction force behind the butterfly joint could also be used for robotic crawlers. This technology could also be useful for designing high-performance sensors or displays that change color in response to stimuli such as temperature, electricity or mechanical force. Color, far from just being pretty, could also power the world in surprising ways.