Ink that changes color when exposed to light, like an octopus does to adapt to its environment, 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 colored ink particles to the surface of their skin.
NOW, Jinyao Tang at the University of Hong Kong and his colleagues have developed an ink that can also display different colors by moving colored particles in response to exposure to light. The ink is made up of titanium dioxide particles, each with different colorants and varying light responses, arranged in a solution.
When light from a standard projector is projected onto a material containing the ink, a chemical gradient causes some ink particles to rise to the surface and others to fall. “As with oil and water, [the particles] separate and float upwards, and that’s because they’re colorful,” Tang explains. “You can change their colors accordingly and they mimic the color you shoot 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, such as children’s paintings, using the ink. They found that the images remained stable for about half an hour before the ink was remixed.
With further research, this could one day be used to provide automatic camouflage. “In the forest, everything is green, so your clothes or the fabric should get that kind of green light around it, and then it turns green,” Tang explains. “Sailing in the desert, everywhere it’s yellow, so [the ink] takes on 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 mixed, 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 energy, explains James Hallette at the University of Reading, UK. “[The ink] doesn’t have electrodes, you just have an external source to change the color and set it up,” he says. “This octopus adaptive camouflage idea makes it a lot more practical than it otherwise would be.”