On Surface Structure and Friction Regulation in Reptilian Limbless Locomotion

TL;DR

This study investigates how the hierarchical surface structure of Python regius skin influences its anisotropic frictional behavior, revealing natural surface topography's role in friction regulation and energy efficiency.

Contribution

It provides a detailed analysis of snakeskin surface topography and its impact on friction, introducing a novel in vivo tribo-acoustic measurement method for reptilian skin.

Findings

Frictional response is direction-dependent, with lower friction in forward motion.

Surface micro-features and their asymmetry control frictional anisotropy.

Hierarchical surface topology enables friction optimization in snakeskin.

Abstract

One way of controlling friction and associated energy losses is to engineer a deterministic structural pattern on the surface of the rubbing parts (i.e., texture engineering). Custom texturing enhances the quality of lubrication, reduces friction, and allows the use of lubricants of lower viscosity. To date, a standardized procedure to generate deterministic texture constructs is virtually non-existent. Many engineers, therefore, study natural species to explore surface construction and to probe the role surface topography assumes in friction control. Snakes offer rich examples of surfaces where topological features allow the optimization and control of frictional behavior. In this paper, we investigate the frictional behavior of a constrictor type reptile, Python regius. The study employed a specially designed tribo-acoustic probe capable of measuring the coefficient of friction and detecting the acoustical behavior of the skin in vivo. The results confirm the anisotropy of the frictional response of snakeskin. The coefficient of friction depends on the direction of sliding: the value in forward motion is lower than that in the converse direction. Detailed analysis of the surface metrological feature reveal that tuning frictional response in snakes originates from the hierarchical nature of surface topology combined to the profile asymmetry of surface micro-features, and the variation of the curvature of the contacting scales at different body regions. Such a combination affords the reptile the ability to optimize the frictional response.