Butterfly wings can do remarkable things with light. Physicists have discovered how subtle differences in the tiny crystals of butterfly wings create stunningly varied patterns of colour even among closely related species. The discovery, reported today in the article “Iridescence and nano-structure differences in Papilio butterflies” by H.L. Tam, K.W. Cheah, David T.P. Goh, and Joseph K.L. Goh in the Optical Society’s (OSA) open-access journal Optical Materials Express, could lead to new coatings for manufactured materials that could change colour by design. Researchers need to figure out how to replicate the wings’ light-manipulating properties. Kok Wai Cheah, a physicist at Hong Kong Baptist University, and his colleagues are the first to investigate the colour-creating mechanisms in multiple butterfly species within a single genus.
The three tropical butterflies the researchers studied all display iridescence, a property of materials that change colour depending on the viewing angle, but they do so with different colours. Papilio ulysses, the Ulysses butterfly or blue mountain swallowtail, appears bluish green when seen from above. Its cousin Papilio peranthus, in 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.
Photo 1: The wings of the three types of butterflies under study. From left to right: front views of P. peranthus, P. blumei, and P. ulysses. The rightmost panel is a side view of P. ulysses. Credit: Optical Materials Express.
To probe the physics behind the wings’ structural colorations, the scientists examined a cross-section of each species’ wing under a scanning electron microscope. The team found that the wings contain specialised architectures in which solid flat layers, known as cuticles, alternate with thin “air” layers, known as laminae. The laminae aren’t entirely empty space, however; they also contain pillars of the cuticle material, which gives the wing a repeating crystal-like structure. This structure is similar to what is known as a Bragg reflector, which is essentially a multi-layered mirror that reflects only certain wavelengths, or colours, of light.
Photo 2: Scanning electron microscope image showing seven-layer cuticle structure of a cross-section of Papilio blumei wing scale at almost 30,000x magnification. This structure appears green when viewed from above and blue from a sharp angle. Image courtesy of H.L. Tam and K.W. Cheah, Hong Kong Baptist University.
Photo 3: Same cross-section as above, at almost 60,000x magnification. Image courtesy of H.L. Tam and K.W. Cheah, Hong Kong Baptist University.
The researchers then measured the light spectrum reflected from the wing at different angles, using a technique called angle-resolved reflection spectroscopy. They found that the varying colours 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. Cheah notes that even though these differences in wing structure are slight, they have a major effect on the butterflies’ appearance.
Cheah thinks the lessons learned from Papilio butterfly wings could lead to designer materials that wouldn’t need to be painted or dyed one specific colour. The same article of clothing, for example, could reflect a subdued colour during the workday and a more ostentatious one at night.
Next, the team plans to investigate colour-generating mechanisms in other insect body structures, such as the metallic effect produced by iridescent beetle shells.
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