Humidity changes material of color

It looks like a simple blue surface. But add water or breathe and suddenly a brightly colored flower appears (see video). Researchers from Eindhoven University of Technology developed this 'painting' that changes color under the influence of moisture. The 'trick' is that light has a certain color when it reflects on microscopically layered structures. The moisture causes the layered material to expand and create a different color. Applications are numerous, varying from packaging, houses, cars or color-changing decorations, but also, for example, security features on banknotes.

The principle that colors can arise from the refraction of light on invisible microscopic structures, in addition to pigments, is something we know from nature. The Hercules beetle, for example, has a khaki green color, but in humid conditions the shield of the beetle turns black. TU/e researchers Monali Moirangthem and Albert Schenning wanted to imitate this technique.

All colors of the rainbow

Until now, it was only possible to change the basic color blue to green, thanks to a technique that PhD student Moirangthem already developed during her doctoral research, in which she was made a test strip that was able to reveal anomalous calcium concentrations in the blood. But now, by experimenting with the degree of expulsion, Moirangthem and Schenning know how to make all the colors of the rainbow.

Calcium concentration

The academics started their research with a layer of structured polymer, where an inkjet printer was used to deposit different concentrations of calcium at specific spots. In the dry state this only reflects blue light, but after the addition of water the polymer expands and other colors develop. In spots where no calcium is present, the polymer expands to a maximum, these areas bounce the light back differently and therefore take on an orange color. Spots with low concentrations of calcium expand slightly, causing a green color to appear. At high concentrations of calcium, the polymer does not expand at all, so these areas retain their blue color. By adjusting the concentration of calcium nitrate, Moirangthem and Schenning have succeeded in creating all visible colors.