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Black butterfly wing structure inspires ultra-dark, lightweight materials

Image credit: Dreamstime

The 3D structures that make the wings of black butterflies so dark could help engineers to design thin, lightweight coatings that provide extreme light-absorbing qualities.

Progressively darker materials have been developed by scientists over the last decade, such as the accidental discovery of Vantablack in 2014 and 2019’s carbon nanotube-based material, which is 10 times blacker than anything made before it.

Scientists at Duke University in North Carolina have been studying butterflies that are naturally 10 to 100 times darker than charcoal, fresh asphalt, black velvet and other everyday black objects. As little as 0.06 per cent of the light that hits them is reflected back to the eye.

This extreme light-trapping effect is achieved using wing scales that are only a few microns deep, which is a fraction as thick as the blackest synthetic coatings.

The secret to this dark coating is due to an optical illusion created by the 3D structure of the butterflies’ wing scales.

Using a high-resolution scanning electron microscopy and computer simulations, the team found that while butterfly wings may look smooth to the naked eye, magnifying them by thousands of times reveals that they are in fact covered in scales, with a mesh-like surface of ridges and holes that channel light into the scale’s spongy interior. There, pillar-like beams of tissue scatter light until it is absorbed.

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 per cent of visible light.

None of these naturally occurring blacks are as dark as the blackest synthetic blacks on record, but butterfly wings come close enough and use structures that are only a fraction as thick.

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 - e.g. for stealth aircraft that can’t be seen at night or detected by radar - to lining space telescopes aimed at faint, distant stars.

“Some animals have taken black to an extreme,” said Alex Davis, a graduate student in the lab of Duke University biologist Sönke Johnsen.

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.

When the team mimicked different wing scales in computer simulations, scales lacking either the ridged surface or interior pillars reflected up to 16 times more light. That would be like going from ultra-black to dark brown, Davis said.

This 3D architecture is so good at swallowing light that the ultra-black scales still looked black even when coated with gold. “You almost can’t make them shiny,” Davis said.

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, coloured or iridescent patches, so the idea is they might work like a dark picture frame to make the brighter blotches pop.

“Why be so black?” Davis said. “We think it’s likely some sort of signal to mates or maybe a predator. There’s a host of other possibilities and we’re hoping to clear that up.”

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