Video: Color-changing material activates with water says Dutch researchers

Researchers at TU Eindhoven are developing a material that changes color when you add water to it. The researchers see numerous applications for this material, from packaging, to color changing decorations and even authenticity marks on bank notes, Eindhovens Dagblad reports.

Researchers Monali Moirangthem and Albert Schenning drew inspiration from nature. The Hercules beetle, for example, has a khaki green color, but in humid conditions its shield turns black. Using the 'trick' that light has a certain color when it reflects on microscopically layered structured, the researchers managed to imitate this technique.

Moirangthem and Schenning started with a layer of structured polymer, on which they layered different concentrations of calcium at specific locations using and ink jet printer. When it's dry, the polymer reflects only blue. But when water is added, the polymer expands and other colors develop as light reflects differently according to the level of expansion. Where no calcium was added, the polymer expands to its maximum and turns an orange color. Areas with a low concentration of calcium expand slightly and turn green. And high calcium concentration areas don't expand and stay blue. 

By adjusting the concentration of calcium, the researchers were able to create all the colors of the rainbow.

This study is still in its early phases and it will be years yet before the material can be reproduced on a large scale. 

https://www.youtube.com/watch?time_continue=31&v=M__MSKiZsOM