Biomimetics are always high on the modern materials agenda. Nature inspires. Enough said. Well, not quite enough. There are so many strands to research attempting to grab the strength, weight for weight, of spider's silk for instance. Other research attempts to modify and improve on the engineering potential of the shells of sea creatures and often homes in on nacre, or mother of pearl, for its optical peculiarities. Then, there are the butterflies. Not least the tropical butterfly Morpho sulkowskyi.
Now, I haven't ever seen that particular species, but I have spotted quite a few other species over the last few weeks. They seem to have been in abundance in this part of the world, many with beautiful patterning and colors, perhaps not quite as vivid and absorbing as the many tropical species but intriguing nevertheless. Some of the colors of a butterfly's wings are, of course, generally not made by pigments absorbing and reflecting different wavelengths of light. They do have pigmented body parts, but much of the color character is down to photonics.
The butterfly wing has a special surface structure. This comprises an array of highly ordered microscopic scales in multiple layers, which produce color by scattering and refracting light and generating interference patterns that we see as shimmering color. This "structural coloring" relies on the self-cleaning ability of the butterfly wing as described by Osamu Sato of the Kanagawa Academy of Science and Technology in the early 2000s. Sato pointed out at the time how the Morpho's wings, for instance, are superhydrophobic, another property keenly sought in synthetic materials. In this and other species colors can change depending on angle of incident light or viewing angle.
Recent work in the journal Interface Focus, shows how a reflected-light microscope and a digital SLR camera can be used to image optical scattering from butterfly wing scales and similarly be used to investigate the reflectance of other structurally colored objects, such as cholesteric liquid crystals. Angle-dependent reflectance spectra can thus be obtained from the RAW image recorded by the camera. In the same journal more insights into how butterfly wing nanostructures selectively reflect parts of the visible spectrum to give characteristic colors that are non-iridescent, matte-green in Lycaenid species. Intriguingly, these wings have their distinctive appearance despite theory suggesting that they should display blue-green-yellow iridescence at the individual crystals; we have much to learn. The orientation of individual crystals within the wing surface are preferentially aligned although not always uniformly although orientation was correlated directly with color at low angle of view and illumination. Such insights could inform the creation of photonic crystals and meta materials. Indeed, I wonder whether researchers have pondered the idea of "macroscopic" versions of butterfly wing type structures that would modulate longer wavelengths of electromagnetic radiation rather than visible light and ultraviolet.
Other work in Polyommatus icarus butterflies suggests that colors can change in response to cold stress in these butterflies in their structural colors. That said, a more obvious effect is seen with the pigmented parts of the species, which could take research in another direction. In yet another species, Rajah Brooke's bird wing, Trogonoptera brookiana, the dazzling colors of this Malaysian species of butterfly are generated by a range of types of wing scale ultra-structures. The absence of color, whiteness, is also common on the wings, legs, antennae, or bodies of many species of Lepidoptera but is not always the most striking nor intriguing property. The white spots on the wings of Carystoides escalantei, which is a species active at dusk in the Costa Rican rainforest, has angle dependency and is dull or bright depending on angle of observation, one might imagine that such a system could be mimicked and exploited in smart windows that control light and heat levels in a building.
It is not only about color, not only are many butterfly wings superhydrophobic and self-cleaning and highly colored, but some exhibit different optical responses to various vapors. New insights into how that works could lead to novel sensors.
Thus, for millions of years, one has to assume that nature has been manipulating the flow of light using synthetic nano and micro structures. Millions of years of evolution is a lot of optimization and so the urge to create biomimetic materials that emulate some of these properties is strong partly because it could save us a lot of time and effort in attempting to start from scratch.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase.