Micro‑and nano‑structures on snake scale create ultra‑blackness by reducing light reflection.

The West African Gaboon Viper is famous for its camouflage. Geometric patterns of dark and light coloration on its skin make the snake nearly invisible against the forest floor. Moreover, the dark sections of the viper’s skin have a unique blackness called “ultra black,” which makes the snake even harder to spot. While ordinary black surfaces absorb most of the light that hits them, some light escapes and is reflected back to the viewer. This viper’s black scales, however, reflect almost no light, taking on a velvet-like appearance. Though the snake’s scales do contain pigments, the secret to this ultra-blacknesses lies in the micro- and nano-structure on each scale’s surface. By preventing reflection, these surface structures enhance the light absorption of the pigments, creating an even darker black.

The way in which this unique coloration is created is called “structural color.” Scientists have discovered this kind of color production across many animals and some plants, such as  the vibrant blue of the Morpho butterfly and the iridescent skin of berries. Most of the colors we see on a daily basis are likely caused by pigments rather than structures. Pigments are chemical molecules that absorb light of certain wavelengths. In the case of structural color, on the other hand, the shape of the surface material creates color by reflecting some but not all wavelengths of light. This leads to production of a certain coloration.

What kind of structures can create a matte black surface like that of the Gaboon Viper? The viper’s black scales feature a hierarchical micro- and nanostructured surface. They are covered with 30 μm tall ridges, which are described by researchers as “leaf-like” structures. The surface of this microstructure has even tinier crests that are about 600 nm tall. Areas between the leaf-like projections are covered with hair-like protuberances (spinules). The size of these structures is similar to that of  visible wavelengths of light (380-700nm).

This microscopic structure creates ultra blackness in the viper by reducing light reflection. If light hits a smooth surface, it is usually reflected back to the viewer. The structures on the scale, however, are thought to guide light waves toward the viper’s skin instead of the viewer. Each time that light hits one of these micro/nano-structures, some is absorbed and the rest is reflected. The newly reflected light hits other micro/nano-structures, with some more light being absorbed and the remainder being reflected. This process repeats itself with less and less light being reflected each time until no more light can be reflected. By the end, very little light is reflected to the viewer. One study shows that velvet black scales reflect four times less light than pale scales do in the UV to near IR range. This reduced light reflection results in the unique velvet appearance of the viper’s black skin. Because of its microscopic surface structure, the viper’s skin is not glossy, allowing it to effectively hunt for prey and evade predators.

 

Last Updated August 27, 2020