Previous studies have demonstrated the nano-holes in the upper lamina of papilionid butterfly scales allow light to enter the interior of the scale where it is absorbed by melanin that is bound to chitin in the cuticle 8, 9, 13. Butterflies, in particular, offer a versatile study system for investigating natural ultra-black surfaces because: (i) their scales have evolved a number of optical specializations, including multilayer reflectors and thin fims 12, (ii) the scales are several times thinner than other naturally occuring ultra-black materials or synthetic alternatives, and (iii) butterfly scales are under constraint to be both light and robust for use in flight. Lastly, certain papilionid butterflies produce ultra-black wing patches with two layers of thin (~2.5 µm), overlapping scales that have an upper lamina patterned with a quasi-honeycomb structure made of crossribs connecting ridges 8, 9. Similarly, certain peacock jumping spiders have evolved a cuticular micro-lens array that limits surface reflection with multiple scattering between adjacent lenses and allows the light to pass further into the cuticle where it is absorbed 11. Several species of birds of paradise have evolved complex barbule microstructures that increase light scattering and, consequently, the number of opportunities for light absorption by melanin embedded within the feather 10. Recently, it has been shown that several animals have evolved micro- or nanostructures that reflect as little as 0.05% of visible light, even at normal incidence 7, 8, 9, 10, 11.
Naturally occurring ultra-black materials may offer insight into more robust absorbers for future replication. The nanotubes, however, must be fabricated at high temperatures and are extremely susceptible to abrasion and other forms of damage, making them unsuitable for many uses. The blackest of these synthetic ultra-black materials (Vantablack) is made from a sparse array of vertically aligned carbon nanotubes 5. Light that is not reflected from the surface is scattered within the material until it is absorbed. In both materials specular reflection is reduced by nano-scale surface roughness caused by either acid etching in the case of nickel-phosphorous alloys or nanotube deposition in carbon arrays.
Currently, most synthetic ultra-black materials are made from nano-patterned metals or carbon nanotubes 5, 6. Ultra-black materials, those with both exceptionally low reflectance and high absorbance, have the potential to increase photovoltaic cell efficiency, improve stray light capture in telescopes, and inform the design of infrared or radar camouflage, among other applications 1, 2, 3, 4. We hypothesize that butterflies use ultra-black to increase the contrast of color signals. Our results demonstrate that butterflies produce ultra-black by creating a sparse material with high surface area to increase absorption and minimize surface reflection. Using scanning electron microscopy, we find considerable interspecific variation in the geometry of the holes in the structures, and verify with finite-difference time-domain modeling that expanded trabeculae and ridges, found across ultra-black butterflies, reduce reflectance up to 16-fold. Here, we examine a phylogenetically diverse set of butterflies and demonstrate that other butterflies employ simpler nanostructures that achieve ultra-black coloration in scales thinner than synthetic alternatives. It is unknown if other ultra-black butterflies use this mechanism. Of these, certain papilionid butterflies have reflectances approaching 0.2%, resulting from a polydisperse honeycomb structure. Recently, it has been shown that animals such as jumping spiders, birds, and butterflies have evolved ultra-black coloration comparable to the blackest synthetic materials.
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