Butterfly Wing Nanostructures Achieve Ultrablack Coloration

A team at Duke University is studying butterflies with wings that are 10 to 100 times darker than everyday black objects.

As little as 0.06% of the light that hits the ultrablack butterfly wings is reflected back to the eye. This percentage is close to the blackest black coatings made by humans to help solar panels absorb more energy or enable line telescopes to reduce stray light. The butterflies achieve this light-trapping effect using wing scales that are only a few μm deep — just a fraction as thick as the blackest synthetic coatings.

The black patches on the wings of some butterflies are 10 to 100 times darker than everyday black objects. Clockwise from top left: Catonephele numilia, Parides iphidamas, Heliconius doris, Parides sp. Courtesy of Richard Stickney, Museum of Life and Science.

The researchers used high-resolution scanning electron microscopy and computer simulations to examine the microscopic structures on the wings of ultrablack and “regular” black or dark brown butterflies from Central and South America and Asia. They found that both ultrablack and regular black wings have scales with parallel ridges on their surfaces and pillars within, but the ridges and pillars are deeper and thicker in ultrablack scales.

Butterfly wings look smooth to the naked eye, but magnification shows that they are covered in scales with a mesh-like surface of ridges and holes that channel light into the scale’s spongy interior. Inside the scales, pillar-like beams of tissue scatter light until it is absorbed. When the researchers mimicked different wing scales in computer simulations, scales lacking either the ridged surface or interior pillars reflected up to 16 times more light.

The 3D structure of the ultrablack butterfly wings allowed light to enter the wing scales, but very little of it bounced back. This 3D architecture is so good at swallowing light that the ultrablack scales still looked black even when they were coated with gold, the researchers said.

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. Courtesy of Alex Davis, Duke University.

None of the butterflies studied 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 μm high. What makes the butterflies interesting, the researchers said, is they rival the best light-trapping nanotechnology using natural structures that are only a fraction as thick.

Ultimately, the findings could help engineers design thinner ultrablack coatings that reduce stray light without weighing things down for applications ranging from military camouflage to lining space telescopes aimed at faint, distant stars.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-020-15033-1).

Some butterflies have ultrablack wings that rival the blackest materials made by humans, using wing scales that are only a fraction as thick. Duke researchers have discovered how they make ultrathin substances that soak up all the light. Courtesy of Alex Davis, Duke University, and Richard Stickney, Museum of Life and Science.