Anew film made of gold nanoparticles changes color in response to any type of movement. Its qualities could allow robots to mimic chameleons and octopi — among other futuristic applications.
Unlike other materials that try to emulate nature’s color changers, this one can respond to any type of movement like bending or twisting. Robots coated in it could enter spaces that might be dangerous or impossible for humans and offer information just based on the way they look; for example, a camouflaged robot could enter tough-to-access underwater crevices. If the robot changes color, biologists could learn about the pressures facing animals that live in these environments. Although some other color-changing materials can also respond to motion, this one can be printed and programmed to display different, complex patterns that are difficult to replicate.
Nanomaterials are simply materials that have been reduced to an extremely small scale — tens of nanometers in width and length or about the size of a virus. When materials like silver or gold become smaller, their colors will change depending on their size, shape, and the direction they face.
In this case, the researchers reduced gold to nano-sized rods. By pointing the rods in a particular direction, their color could be controlled. Facing one way, they might appear red but when moved 45 degrees, they change to green.
The problem facing the team was how to take millions of gold nanorods floating in a liquid solution and get them all to point in the same direction to display a uniform color. Their solution was to fuse smaller magnetic nanorods onto the larger gold ones. The two different-sized rods were encapsulated in a polymer shield, so that they would remain side by side. That way, the orientation of both rods could be controlled by magnets.
Once the nanorods are dried into a thin film, their orientation is fixed in place and they no longer respond to magnets. But if the film is flexible, it can be bent and rotated and will still see different colors as the orientation changes
Other materials, like butterfly wings, are shiny and colorful at certain angles and can also change color when viewed at other angles; however, those materials rely on precisely ordered microstructures, which are difficult and expensive to make for large areas. The new film can be made to coat the surface of any sized object just as easily as applying spray paint on a house.
Though futuristic robots are an ultimate application of this film, it can be used in many other ways; for example, the film can be incorporated into checks or cash as an authentication feature. Under normal lighting, the film is gray but when when viewed through polarized sunglass lenses, elaborate patterns can be seen. In addition, the color contrast of the film may change dramatically if the film is twisted.