Subtle differences in the small crystal structures responsible for the varied patterns of color in butterfly wings could lead to new material coatings that change color by design. Butterfly wings can do remarkable things with light, but much research still needs to be done for humans to replicate the wings’ light-manipulating properties. “It was very exciting to see how nature can create a nanostructure that’s not easy to replicate by humans,” said Kok Wai Cheah, a physicist at Hong Kong Baptist University, who has been working with colleagues to investigate the color-creating mechanisms in multiple butterfly species within a single genus. The three tropical butterflies under investigation all display iridescence, a property of materials that change color depending on the viewing angle, but they do so with different colors. Papilio ulysses, the Ulysses butterfly or blue mountain swallowtail, appears bluish-green when seen from above. Its cousin Papilio peranthus, by contrast, looks yellowish-green from above, and a third relative, Papilio blumei, the green swallowtail, is more of a deep green. All three shift toward deep blue when viewed from a sharp angle. The wings of the three types of butterflies under study at Hong Kong Baptist University. From left: front views of P. peranthus, P. blumei and P. ulysses. The rightmost panel is a side view of the P. ulysses. Courtesy of Optical Materials Express. To probe the physics behind the wings’ structural colorations, the investigators examined a cross-section of each species’ wing under a scanning electron microscope. They discovered that the wings contain specialized architectures in which solid flat layers known as cuticles alternate with thin “air” layers known as laminae. The laminae are not entirely empty space, however; they also contain pillars of the cuticle material, which gives the wing a repeating crystal-like structure. Scanning electron microscope image showing seven-layer cuticle structure of a cross-section of Papilio blumei wing scale at almost 30,000× magnification. This structure appears green when viewed from above, and blue from a sharp angle. Courtesy of H.L. Tam and K.W. Cheah, Hong Kong Baptist University. This structure is similar to what is known as the Bragg reflector — essentially a multilayered mirror that reflects only certain wavelengths of light. Light reflected from the wing was measured using angle-resolved reflection spectroscopy, revealing that the varying colors of the three species’ wings arise from slight differences in crystal parameters. P. ulysses has seven cuticle layers, for example, while P. peranthus has eight. The thicknesses of the cuticles and air layers also vary between species. Even though these differences are slight, they have a major effect on the butterflies’ appearance, Cheah said. “It all comes from the fact the wing structure has subtle differences between these three types of butterfly,” he said. Same cross-section, at almost 60,000× magnification. Courtesy of H.L. Tam and K.W. Cheah, Hong Kong Baptist University. Information from Papilio butterfly wings could lead to designer materials that wouldn’t need to be painted or dyed one specific color, Cheah said. The same article of clothing, for example, could reflect a subdued color for the workday, and a more ostentatious one for the evening. “You would just tune your structure to produce the color you want,” Cheah said. The researchers are now investigating the color-generating mechanisms of other insects, including the metallic effect produced by iridescent beetle shells. The study was published in Optical Materials Express (doi: 10.1364/OME.3.001087). For more information, visit: www.hkbu.edu.hk