The best known colors result from pigments. However, the very bright colors of the blue tarantula or peacock feathers do not result from pigments, but from nanostructures that cause the reflected light waves to overlap. This produces extraordinarily dynamic color effects.
Researchers from Karlsruhe Institute of Technology (KIT), in cooperation with international colleagues, have now succeeded in replicating nanostructures that generate the same color irrespective of the viewing angle.
In contrast to pigments, structural colors are non-toxic, more vibrant and durable. In industrial production, however, they have the drawback of being strongly iridescent, which means that the color perceived depends on the viewing angle, such as the rear side of a CD. Hence, such colors cannot be used for all applications.
Bright colors of animals, by contrast, are often independent of the angle of view. Feathers of the kingfisher always appear blue, no matter from which angle we look. The reason lies in nanostructures: while regular structures are iridescent, amorphous or irregular structures always produce the same color. Until now, industries could only produce regular nanostructures in an economically efficient way.
Engineer Radwanul Hasan Siddique, in collaboration with scientists from USA and Belgium, has discovered that the blue tarantula does not exhibit iridescence in spite of periodic structures on its hairs.
Their study revealed that the hairs are multi-layered, flower-like structure, which the researchers analyzed in terms of their reflection behavior with the help of computer simulations. In parallel, they built models of these structures using nano-3D printers and optimized the models with the help of the simulations.
In the end, they produced a flower-like structure that generates the same color over a viewing angle of 160 degrees. According to the researchers, this is the largest viewing angle of any synthetic structural color reached so far.
"This could be a key first step towards a future where structural colorants replace the toxic pigments currently used in textile, packaging, and cosmetic industries," said Siddique, who now works at the California Institute of Technology. He considers short-term applications in the textile industry the most feasible.
Hendrik Hölscher of KIT thinks that the scalability of nano-3D printing is the biggest challenge on the way towards industrial use. Only few companies in the world are able to produce such prints. In his opinion, however, rapid development in this field will certainly solve this problem in the near future.
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