Copying the way butterflies create the extraordinary metallic colouring on their wings could lead to new kinds of dyes, cosmetics and even flat panel displays. Furthermore, the materials could be grown, potentially making them less expensive than manufactured alternatives.
The latest work supporting these ideas comes from a group of European and US researchers. They have, for the first time, measured the structure and optical characteristics of the photonic crystals that occur naturally in the wings of the butterfly Cyanophrys remus.
Photonic crystals are materials with a repeating structure that is similar in size to the wavelength of light. The structure forces a light to interfere with itself in a way that allows it to propagate only in certain directions and at certain frequencies.
Scientists have known for some time that the colouring on some types of butterfly wings is produced by naturally occurring photonic crystal structures in scales made of chitin, a polysaccharide widely found in insects and other animals. The scales are just a few tens of micrometres across and arranged on the surface of the wings like tiles on a roof.
The wings of the male Cyanophrys remus are bright metallic blue on one side, thought to attract mates, and a dull green on the other to act as camouflage. “The wings produce an amazing richness of colour,” says Lazlo Biro, a materials scientist at the Research Institute for Technical Physics and Materials Science in Budapest, Hungary, and one of the researchers on the team.
Now Biro and colleagues have determined the structures responsible by combining data from electron microscope images of the scales and studies of the way they reflect light. They found that each side of the wing contained different photonic structures. The metallic blue colour is produced by scales that are photonic single crystals whereas the dull green is the result of a random arrangement of photonic crystals.
This randomly arranged structure may have powerful applications. The crystals can actually produce different colours – green, yellow and blue – depending on their orientation, but the overall effect in Cyanophrys remus is a dull green. The team also found a way to make the crystals generate red reflections.
The red-green-blue palette could be used for flat-panel visual displays, says Biro, by making an array of crystals mounted on microelectromechanical arms that could change their orientation. In that way it would be possible for each “pixel” to produce red, green or blue.
The crystals could also be used in exotic cosmetics and dyes that do not rely on heavy metals for their colour. “Achieving the same effect with classical colourings would be difficult,” says Biro.
“This is really solid work,” says Andrew Parker, at the natural History Museum in London, UK. “It suggests new applications for a structure we already know about.”
Parker’s own work could help make some of these applications a reality. He has cultured the butterfly cells responsible for creating chitin scales. This allows him to grow and study the photonic crystal in his lab. “It’s possible we could see commercial applications in the next few years,” he says.
Journal reference: Physical Review E (DOI: 10.1103/PhysRevE.74.021922)