Beetle indicates the path to highly absorptive materials

Researchers from the Adolphe Merkle Institute’s Soft Matter Physics group have unveiled the secrets behind the extraordinary black appearance of the Euprotaetia inexpectata beetle. Its shell is one of nature’s most effective light-absorbing surfaces and could inspire ultra-black materials for various applications, from solar panels to stealth technology.

Structural absorption is a phenomenon that occurs when superficial features enhance the capacity of a material to absorb light. Surfaces that absorb more than 99 percent of incident light are commonly referred to as ‘super-black’ or ‘ultra-black.’ A notable property of these black surfaces is their lack of specular reflection, which occurs when light is reflected in a single outgoing direction, much like a mirror. The most widely recognized high-absorptivity materials are vertically aligned carbon nanotubes and etched nickel-phosphorous alloys with absorptivity values above 99.9 percent over the entire visible wavelength range. The unique properties and appearance of materials with very low reflectivities make them ideal for many applications, including solar energy conversion, stray light capture in optical instruments, and artistic displays. 

The Euprotaetia inexpectata beetle, found in various regions, including the Philippines, has a shell that absorbs up to 99.5 percent of light, making it one of the blackest natural substances recorded. The key to this phenomenon lies in the beetle's elytra — its hardened forewings —which are covered with tiny micropillars. These structures significantly enhance light absorption, reducing reflectivity to as low as 0.1 percent.

The low reflectance values are in part based on light guided through Mie scattering. This scattering is significant at dimensions comparable to the wavelength of the incident light. For example, multiple Mie scattering due to interactions with water droplets causes clouds to appear white. “Mie scattering at the micropillar of the beetle guides the light through the pillar, from where it is efficiently scattered in the forward direction, into the beetle’s cuticle,” explains Dr. Viola Vogler-Neuling. “The pillar itself and the cuticle absorb the light through integrated melanin, the same absorbing pigment that darkens our skin, hair, and eyes.”

This research underscores the independent evolution of highly absorptive surfaces in nature. The beetle’s unique construction adds to understanding how diverse organisms have evolved to minimize light reflection for survival benefits, such as camouflage or communication. Similar super-black appearances are found in butterflies, birds, and spiders, but the mechanisms can differ significantly. “One example is the bird of paradise, which exploits highly modified barbule arrays in their super-black feathers that enhance multiple light scattering,” adds Vogler-Neuling. “The light is scattered more often and is partially absorbed during each scattering event.”

The insights gained from the Euprotaetia inexpectata beetle could inspire potential bio-inspired applications for creating highly absorptive materials. By mimicking the beetle’s micropillar structures, scientists could develop ultra-black surface coatings for a range of applications, from solar panels to stealth technology. And compared to other ultra-black materials like carbon nanotubes or nickel-phosphorous alloys, these structures would be non-toxic and not require complex wastewater treatment.

Reference: Parisotto, A.; Vogler-Neuling, V. V.; Steiner, U.; Saba, M.; Wilts, B. D. Structural Light Absorption in Elytral Micropillars of Euprotaetia Inexpectata Beetles. Materials Today Advances 2023, 19, 100399.