Characterization of the microscopic tribological properties of sandfish (Scincus scincus) scales by atomic force microscopy

Abstract

Lizards of the genus Scincus are widely known under the common name sandfish due to their ability to swim in loose, aeolian sand. Some studies report that this fascinating property of sandfish is accompanied by unique tribological properties of their skin such as ultra-low adhesion, friction and wear. The majority of these reports, however, is based on experiments conducted with a non-standard granular tribometer. Here, we characterise microscopic adhesion, friction and wear of single sandfish scales by atomic force microscopy. The analysis of frictional properties with different types of probes (sharp silicon tips, spherical glass tips and sand debris) demonstrates that the tribological properties of sandfish scales on the microscale are not exceptional if compared to snake scales or technical surfaces such as aluminium, Teflon, or highly oriented pyrolytic graphite.

Keywords: Scincus scincus; biotribology; frictional properties of reptile scales; sandfish.

Figure 1

(a) Photograph of a sandfish (S. scincus) in its natural habitat (copyright Gerrit Jan Verspui). (b) Photograph of scales from moulted sandfish skin (S. scincus) examined in this study. Cut parts of the moulted skin or singled scales were used for all measurements. (c) The typical contact angle of a single sandfish scale is about 100° (droplet volume 1 µL).

Figure 2

SEM images of some probes used in this study. (a) Sharp tip of a conventional AFM cantilever made from silicon. (b) Sand particle glued to the end of a tipless cantilever (“sand probe”). The inset is a side view. (c) Glass sphere glued to the cantilever end (“spherical probe”). (d) Spherical glass probe coated with copper (“spherical probe with Cu coating”).

Figure 3

Structure of the analysed S. scincus scales. (a) The topography of a dorsal scale measured by atomic force microscopy reveals a structure of steps, which have a saw-tooth like shape magnified in the inset. (b) The topography of a ventral scale does not reveal steps. However, tiny scratches are sometimes visible. (c) A cross section of a dorsal scale recorded by scanning electron microscopy suggests that sandfish scales have a layered internal structure.

Figure 4

(a) Typical force–distance curves obtained with sand probe, spherical glass probe and sharp silicon tip, respectively. (b) Adhesion force obtained with four probes at 15 arbitrarily chosen positions on a sandfish scale. The adhesion force was measured ten times at each position and the error bars correspond to the statistical error. The dashed lines represent the overall average adhesion of all 150 measurements obtained with each probe, respectively. (c) The same experiment with spherical glass probes with a diameter of 20 µm without or with Cu or W coating reveals no significant dependence of adhesion on the coating. (d) Adhesion forces measured on different types of samples with a sharp tip reveal that the adhesion of the analysed sandfish scales is not significantly lower as that of other materials such as Clifford’s diadem snake (S. diadema) or technical surfaces such as Teflon.

Figure 5

Direct comparison of the adhesion force measured with a sand probe on the scales of four species (P. guttatus, E. pyramidum, S. diadema, and N. atra) and dorsal scales of sandfish (S. scincus). Each bar corresponds to five force-vs-distance curves on fourteen different positions, i.e., n = 70 measurements.

Figure 6

(a) Frictional force as a function of the normal load measured with a sharp silicon tip on a dorsal sandfish scale and five other sample surfaces. All data was obtained with the same sharp silicon tip. (b) Frictional force as a function of the normal load measured on a sandfish scale with a sharp silicon tip, a sand probe and a spherical glass probe. The dashed lines in the plots represent linear fits to the respective data sets of each material. The resulting gradient represents the microscopic frictional coefficient µ given in the legends.

Figure 7

(a) Wear experiments recorded on five different materials with hard cantilevers (spring constant of approx. 40 N/m) and sharp silicon tips. Nine areas (5 µm × 5 µm) were scratched on each sample with increasing load or fixed load and increasing time. Every experiment was started with a pristine sharp tip cantilever. The red rectangles mark areas where no wear was observed. The top line shows the wear pattern with increasing normal load in steps (left to right and top to bottom). In this way the loading force increased to 35–60 µN in the lower right corner while the other scratching conditions were fixed. For the scratch test in the bottom line the normal load was fixed to 19.6 µN but the scratching time was increased stepwise by 2.5 min. (b, c) Scratching depth plotted as a function of normal load and scratching distance extracted from the wear patterns in a). The overall wear resistance of sandfish scale (S. scincus) against a sharp silicon tip is not superior to technical surfaces or to S. diadema.

Figure 8

Comparison of the friction coefficients as measured by AFM and microtribometry in a sphere-on-plate reciprocating configuration. The diameter of the sapphire sphere was 1 mm. The materials tested were sandfish (S. scincus) and snake (S. diadema) scales as well as technical surfaces. For the latter PMMA, silicon, Teflon, HOPG, 100Cr6 bearing steel and PEEK were chosen as representatives.