Exceptional white
Applying a varnish that thinly would be pointless - the scales on Cyphochilus' shell are only microns thick, but still shine as white as paper. The finest nanostructures are the solution to the riddle. Recreating it once might be worthwhile.
Hidden behind mushrooms generally doesn't require great skill - provided you're not too tall, of course. However, if the targeted mushrooms are white, while you shimmer in all the colors of the rainbow, even small enough will not help you any further. Beetles of the Southeast Asian genus Cyphochilus therefore took the only logical step: They also turned pale.
This is by no means trivial. In order to become really white, the usual coloring mechanisms in the animal kingdom - neither the pigmentation nor the iridescent surface structure of many insect shells - are usually not sufficient. This is because they always only reflect light of individual wavelengths. However, the color white usually requires a material that scatters all wavelengths at once.
Cyphochilus has mastered this problem brilliantly. It not only shines in a white that can compete with all artificial dyes - and even surpasses most of them - it also manages this feat with an extremely thin structure of only five micrometers (thus thousandths of a millimeter) thick. Artificial dye lakes and other materials would have to be two orders of magnitude thicker to reflect light comparably well, says Peter Vukusic, a physicist at the University of Exeter.
Together with his colleagues, he has now found out how the beetle's trick works: The reason for the white color lies in the wafer-thin scales that cover the insect's body and whose surface consists entirely of a network of fine fibers. These so-called filaments, each only around 250 nanometers in cross-section, have the property of reflecting light and make the scales a photonic material that can selectively influence the propagation of incident light waves.
Such photonic substances are also known from butterflies and other beetles. In contrast to those of Cyphochilus, however, they have a regular, crystal-like structure. The iridescent effect of such photonic crystals arises precisely from the fact that their regular three-dimensional arrangement reflects different wavelengths from different viewing angles. In Cyphochilus, on the other hand, the filaments are highly disordered. Only this random arrangement creates the scattering of all wavelengths and makes the crawler white.
In both cases, the space between the light-active structural components also plays a role. The light is captured there and reflected back and forth until it finds its way out of the material again. How often this happens and which angles occur then determines the visual impression.
If Vukusic has his way, it's up to the bionics experts to use the special color properties of the beetle scales for technical purposes. New developments in mobile phone displays and e-paper, i.e. electronic "newspaper", already use so-called organic light-emitting diodes (OLEDs). Their advantage, compared to liquid crystal displays, for example, lies in the flexibility and small thickness of the display surface. A material that mimics beetle scales could also be used here, the researcher speculates.
Vukusic himself took the optical properties of butterfly wings as a model a few years ago and, together with a cosmetics company, developed a make-up that works entirely without pigments. Since the beetle is many times whiter than the enamel of teeth, as the scientists discovered, they are already promising a bright future: just a tiny additional layer would be enough and the term "bright smile" would have a whole new meaning.
