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Researchers have used high resolution scanning electron microscopy and computer simulations to examine the microscopic structures on the wings of 10 species of ultra-black butterflies and four regular black or dark brown butterflies from Central and South America and Asia.
Design Thin, Lightweight Coatings
Ultimately, the findings could help engineers design thinner ultra-black coatings that reduce stray light without weighing things down, for applications ranging from military camouflage -- for stealth aircraft that can’t be seen at night or detected by radar -- to lining space telescopes aimed at faint, distant stars.
It turns out the key differences between ultra-black and regular black scales lie elsewhere. When they looked at the butterflies’ wings under an electron microscope, they found that both ultra-black and regular black scales have parallel ridges on their surface and pillars within. But the ridges and pillars are deeper and thicker in ultra-black scales compared to “normal” black scales.
A close-up look at the wings of the Rajah Brooke's birdwing butterfly with a scanning electron microscope reveals tiny structures in their wing scales that trap light so that virtually none escapes.
When the team mimicked different wing scales in computer simulations, scales lacking either the ridged surface or interior pillars reflected up to 16 times lighter. That would be like going from ultra-black to dark brown, Davis said.
Similar deep blacks have popped up in other animals, such as peacock spiders and birds of paradise, which are known to reflect as little as 0.05% of visible light.
Best Light-trapping Nanotechnology
None of these natural beauties is quite as dark as the blackest synthetic blacks on record, which absorb more than 99.99% of incoming light using tightly packed “forests” of carbon nanotubes around 10 to 50 microns high. But what makes butterflies interesting, the researchers say, is they rival the best light-trapping nanotechnology, using structures that are only a fraction as thick.
The blackness on the wings of many male butterflies is darker than it is on their female counterparts, so one theory is it helps them show off to potential mates. The black regions always border white, colored, or iridescent patches, so the idea is they might work like a dark picture frame to make the brighter blotches pop.
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Artists have known for a long time that the same color can look very different on different backgrounds,” Johnsen said.
The next step, Davis said, is to figure out how many times butterflies have evolved ultra-black wings, and determine whether those species have anything in common that might help explain what favored the change.
Source: Duke University