July 18, 2013
Butterflies Could Inspire New Hue-Changing Materials
April Flowers for redOrbit.com - Your Universe Online
Butterflies have beautiful wings full of colorful patterns that do remarkable things with light. A new study from a Hong Kong Baptist University team of physicists has uncovered how subtle differences in tiny crystals of butterfly wings create stunningly varied patterns of color, even among closely related species. If the team can figure out how to replicate the wings' light-manipulating properties, the discovery could lead to new coatings for manufactured materials that could change color by design.
"It was very exciting to see how nature can create a nanostructure that's not easy to replicate by humans," says Kok Wai Cheah, a physicist at Hong Kong Baptist University. Cheah and his team are the first to investigate the color-creating properties in multiple butterfly species within a single genus.
The research team studied three tropical butterfly species that display iridescence - a property of materials that change color depending on the viewing angle. All three, however, displayed different colors. When seen from above, Papilio ulysses, the Ulysses butterfly or blue mountain swallowtail, appears bluish green, while its cousin Papilio peranthus, by contrast, looks yellowish green, and a third relative, Papilio blumei, the green swallowtail, is more of a deep green. All three species shift toward a deep blue color when viewed from a sharp angle.
To understand the physics behind the wings' structural colorations, the team looked at a cross-section of each species' wing under a scanning electron microscope. The wings contain specialized architectures, with solid flat layers known as cuticles alternating with thin "air" layers known as laminae. The laminae aren't entirely air, however. They also contain pillars of cuticle material, giving the wing a repeating crystal-like structure, which is similar to a Bragg reflector - essentially a multi-layered mirror that reflects only certain wavelengths, or colors, of light.
The light spectrum reflected from the wing at different angles was then measured using a technique called angle-resolved reflection spectroscopy. The researchers found that the varying colors between the three species' wings are because of slight differences in crystal parameters. For example, P. ulysses has seven cuticle layers, while P. peranthus has eight. There is a variation between thicknesses of cuticles and air layers between the species as well. Even though these differences are slight, Cheah notes, they have a major effect on the butterflies' appearance. "It all comes from the fact the wing structure has subtle differences between these three types of butterfly," he says.
The lessons learned from butterfly wings could lead to designer materials, Cheah thinks. These materials wouldn't need to be painted or dyed one specific color. An article of clothing could reflect a subdued color during the workday, and then display a more ostentatious one at night. "You would just tune your structure to produce the color you want," says Cheah.
The team is planning further research to investigate color-generating mechanisms in other insect body structures, such as the metallic effect produced by iridescent beetle shells.
The findings of this study were reported in the open-access journal, Optical Materials Express.