Scientists at the University of California (UC), Riverside have created a new type of ultra-thin film with an incredible ability to change color as it is twisted or bent. Such a material, the researchers believe, could one day make its way onto robots to give them chameleon-like skin, or into cash as a means of verifying its authenticity.
The new film functions in a similar way to some color-changing materials we’ve looked at in the past, which have involved charged microparticles that change color in response to light, and very fine ridges that reflect different wavelengths of light as they are twisted or bent. We also looked at an earlier iteration of the same UC Riverside team’s film back in 2014, which changed color in response to pressure as a way of monitoring mechanical stress.
But the latest version is different to these previous films, according to the team, as it can be programmed to display uniquely complex patterns. The team achieved this by making the film out of gold nanoparticles shaped as tiny rods, which present as different colors depending on their size, shape and orientation.
“In our case, we reduced gold to nano-sized rods,” says chemistry professor Yadong Yin. “We knew that if we could make the rods point in a particular direction, we could control their color. Facing one way, they might appear red. Move them 45 degrees, and they change to green.”
To make millions of these gold nanorods face the same way, the team first fused them to smaller magnetic rods and encapsulated the pair in a polymer shield, locking them in place side by side and allowing their orientation to be dictated by magnets.
“Just like if you hold a magnet over a pile of needles, they all point in the same direction,” says Yin said. “That’s how we control the color.”
The nanorods are then dried onto a thin film and their orientation is locked in place.
“But, if the film is flexible, you can bend and rotate it, and will still see different colors as the orientation changes,” Yin explains.
According to the researchers, the film can be applied to the surface of objects as easily as spraying paint on a house, which opens up some interesting potential. It could be put to use on robots so they change color in response to changes in their environment, or could be used as an authentication technology for money. They even see futuristic art as a possibility.
“Artists could use this technology to create fascinating paintings that are wildly different depending on the angle from which they are viewed,” Li says. “It would be wonderful to see how the science in our work could be combined with the beauty of art.”
The research was published in the journal Nature Communications, and the film is demonstrated in the video below.
Color-changing gold nanorod film
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