Non-contaminating camouflage: multifunctional skin microornamentation in the West African Gaboon viper (Bitis rhinoceros)

Abstract

The West African Gaboon viper (Bitis rhinoceros) has an extraordinary coloration of pale brown and velvety black markings. The velvety black appearance is caused by a unique hierarchical surface structures which was not found on the pale brown scales. In the present study we examined the wettability of the vipeŕs scales by measuring contact angles of water droplets. Velvet black scale surfaces had high static contact angles beyond 160° and low roll-off angles below 20° indicating an outstanding superhydrophobicity. Our calculations showed that the Cassie-Baxter model describes well wettability effects for these surfaces. Self-cleaning capabilities were determined by contaminating the scales with particles and fogging them until droplets formed. Black scales were clean after fogging, while pale scales stayed contaminated. Black scales feature multifunctional structures providing not only water-repellent but also self-cleaning properties. The pattern of nanoridges can be used as a model for surface-active technical surfaces.

Figure 1. SEM-micrographs of the microornamentation (MO) of Bitis rhinoceros on different body regions.

The MO patterns coincide with the black and pale colouration. A, Pale dorsal areas show a verrucate MO pattern covered with pits in nanometer range. B, Black dorsal areas show a hierarchical microornamentation consisting of microscopic leaf-like structures covered with nanoridges. c, Ventralia have nanoridges with nanoscopic knobs (after Spinner et al., 2013).

Figure 2. Contact angle measurements on scales of B. rhinoceros.

A, Static contact angles of 1 µl droplets of water (H₂O, hatched bars), diiodomethane (CH₂I₂, white bars), and ethylene glycol (C₂H₆O₂, black bars) on ventral and dorsal pale and black scales on the exuvium of B. rhinoceros. Error bars indicate standard deviations of contact angles of ten individual measurements. B, Behavior of water droplets on the black- and white-coloured areas of the dorsal scale of B. rhinoceros. On the black area, the droplet remains in a spherical state due to the superhydrophobic properties of the underlying microstructure. In the pale area, the droplet spreads out and largely covers the area. The microstructure of this area has no superhydrophobic effect.

Figure 3. Photographic image of the skin of a living individual of B. rhinoceros after sprinkling with water.

Whereas the rest of the skin is evenly wetted and shiny on black coloured scales appear black and matt further on, because they are not covered by water. There are only droplets in the keel regions of the scales (white arrows).

Figure 4. Self-cleaning ability of the skin of B. rhinoceros.

A–C, Photographic images of an exuvium of B. rhinoceros. A, The exuvium at daylight before dusting it with the redwop. B, The exuvium under black light after dusting it with the redwop. C, The same exuvium under black light after fogging it for 30 min at an inclination of 20°. D, SEM-image of the B. rhinoceros scale with black and pale regions after contamination with the redwop dust and subsequent fogging. The (black) areas showing the characteristic micro- and nanostructure are free of the redwop particles. The (pale) areas showing the inconspicuous microstructure and no nanostructures are covered by a dense layer of the redwop particles.