Institute of Microstructure Technology (IMT), Karlsruhe Institute of Technology (KIT), H.-v.-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
Bioinspir Biomim. 2020 Aug 12;15(5):056014. doi: 10.1088/1748-3190/ab9e51.
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.
大多数蛇的腹鳞具有微米级的原纤结构，纳米级的台阶朝向蛇的尾部。我们通过微摩擦学以及原子力显微镜（AFM）来研究这些结构，观察到微原纤的纳米级台阶导致摩擦各向异性，这种各向异性沿蛇的身体变化，取决于纳米级台阶的高度。当尖锐的 AFM 针尖沿着纳米级台阶向上或向下扫描时，会检测到显著的摩擦行为。向上扫描（从尾部到头部方向）时会出现较大的摩擦峰，而向下扫描（从头部到尾部方向）时则会观察到相当低的峰。这种效应在纳米尺度上产生摩擦各向异性，即沿头部到尾部方向的摩擦低于相反方向。整体效应随微原纤台阶高度的增加呈线性增加。尽管每个蛇的台阶高度不同，但沿所研究蛇体的台阶高度分布遵循共同的模式。由台阶高度分布引起的摩擦各向异性在靠近尾部处最大，在中间处居中，在靠近头部处最小。这种摩擦各向异性的共同分布表明，蛇甚至通过进化优化了纳米级特征，如微原纤的高度，以实现最佳的运动摩擦性能。最后，通过将腹鳞的微原纤结构压印到聚合物中来复制蛇的腹鳞。作为天然原型，人工表面的摩擦各向异性取决于相应的台阶高度。这种特性对于具有人工摩擦各向异性的摩擦学表面的设计具有重要意义。
