Abstract
The ventral scales of most snakes feature micron-sized fibril structures with nanoscale steps oriented towards the snake’s tail. We examined these structures by microtribometry as well as atomic force microscopy (AFM) and observed that the nanoscale steps of the micro-fibrils cause a frictional anisotropy, which varies along the snake’s body in dependence of the height of the nanoscale steps. A significant frictional behavior is detected when a sharp AFM tip scans the nanoscale steps up or down. Larger friction peaks appear during upward scans (tail to head direction), while considerably lower peaks are observed for downward scans (head to tail direction). This effect causes a frictional anisotropy on the nanoscale, i.e. friction along the head to tail direction is lower than in the opposite direction. The overall effect increases linearly with the step height of the micro-fibrils. Although the step heights are different for each snake, the general step height distribution along the body of the examined snakes follows a common pattern. The frictional anisotropy, induced by the step height distribution, is largest close to the tail, intermediate in the middle, and lower close to the head. This common distribution of frictional anisotropy suggests that snakes even optimized nanoscale features like the height of micro-fibrils through evolution in order to achieve optimal friction performance for locomotion. Finally, ventral snake scales are replicated by imprinting their micro-fibril structures into a polymer. As the natural prototype, the artificial surface exhibits frictional anisotropy in dependence of the respective step height. This feature is of high interest for the design of tribological surfaces with artificial frictional anisotropy.
Highlights
Snakes (Squamata: Serpentes) have no extremities and move forward by waving their body [1,2,3,4,5]
The ventral scales of most snakes feature micron-sized fibril structures with nanoscale steps oriented towards the snake’s tail. We examined these structures by microtribometry as well as atomic force microscopy (AFM) and observed that the nanoscale steps of the micro-fibrils cause a frictional anisotropy, which varies along the snake’s body in dependence of the height of the nanoscale steps
To investigate the step height distribution along snakes’ bodies, each molted snake skin was divided along the body into eleven evenly distributed segments, and the averaged step height in these eleven sections was determined by AFM
Summary
Snakes (Squamata: Serpentes) have no extremities and move forward (or generate traction) by waving their body [1,2,3,4,5] Due to this special limbless locomotion style, their ventral scales are in direct, permanent contact with the ground. The snake’s skin acts as a protection against the environment, but serves as a tribological element that is beneficial for locomotion and has to withstand friction and wear frequently [3, 6] For this and other reasons, snakes shed their skin (ecdysis) after time spans ranging from 20 d to one year [7,8,9]. These micro-fibrils are commonly considered as an important component for the control of friction and wear during snake sliding [11, 16, 23, 24]
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