An ink that changes color when exposed to light, like an octopus does to adapt to its surroundings, could one day be used for automatic camouflage.
Most color-changing inks and materials use chemical reactions, but these can be unstable and difficult to control. Instead, octopuses use special muscles to push the colored ink particles toward the surface of their skin.
Now, jinyao tang at the University of Hong Kong and colleagues have developed an ink that can display different colors in a similar way by moving dyed particles in response to exposure to light. The ink is made up of titanium dioxide particles, each with different tints and varying light responses, arranged in a solution.
When light from a standard projector shines on the ink-containing material, a chemical gradient causes some ink particles to rise to the surface and others to fall. “As with oil and water, [the particles] they separate and float to the top, and that’s because they’re colored,” Tang says. “You can change their colors accordingly and they mimic whatever color you’re actually shooting at them.”
Tang and his team formulated their ink with three colors (cyan, magenta, and yellow) used in the common CMY color scheme. They then used a modified projector to display semi-permanent images, like children’s paintings, using the ink. They found that the images were stable for about half an hour before the ink was remixed.
With more research, this could one day be used to provide automatic camouflage. “In the forest, everything is green, so your clothing or material needs to get that kind of green colored light and then it turns green,” Tang says. “Sailing through the desert, everything is yellow, so [the ink] acquires a yellowish color.
However, to be used in such a setting, the ink will need to hold the desired color for more than half an hour before being remixed, Tang says.
Because the ink rearranges itself without electricity, it could be used in a wide range of applications, such as updating signs without requiring large amounts of power, he says. james hallett at the University of Reading, UK. “[The ink] it doesn’t have electrodes, it just has an external source to change the color and put it in place,” he says. “That adaptive octopus camouflage idea makes it much more practical than it would be otherwise.”