Data from: Biomechanics of shear-sensitive adhesion in climbing animals: peeling, pre-tension and sliding-induced changes in interface strength

Many arthropods and small vertebrates use adhesive pads for climbing. These biological adhesives have to meet conflicting demands: attachment must be strong and reliable, yet detachment should be fast and effortless. Climbing animals can rapidly and reversibly control their pads' adhesive strength by shear forces, but the mechanisms underlying this coupling have remained unclear. Here, we show that adhesive forces of stick insect pads closely followed the predictions from tape peeling models when shear forces were small, but strongly exceeded them when shear forces were large, resulting in an approximately linear increase of adhesion with friction. Adhesion sharply increased at peel angles less than ca 30°, allowing a rapid switch between attachment and detachment. The departure from classic peeling theory coincided with the appearance of pad sliding, which dramatically increased the peel force via a combination of two mechanisms. First, partial sliding pre-stretched the pads, so that they were effectively stiffer upon detachment and peeled increasingly like inextensible tape. Second, pad sliding reduces the thickness of the fluid layer in the contact zone, thereby increasing the stress levels required for peeling. In combination, these effects can explain the coupling between adhesion and friction that is fundamental to adhesion control across all climbing animals. Our results highlight that control of adhesion is not solely achieved by direction-dependence and morphological anisotropy, suggesting promising new routes for the development of controllable bio-inspired adhesives.许多节肢动物（arthropods）与小型脊椎动物依靠粘附垫（adhesive pads）实现攀爬。这类生物粘附结构需满足相互冲突的性能要求：附着强度需足够强劲且可靠，同时脱附过程又要快速且毫不费力。攀爬动物可通过剪切力快速且可逆地调控其足垫的粘附强度，但这一粘附-摩擦力耦合现象背后的机制始终未被阐明。本研究发现，当剪切力较小时，竹节虫足垫的粘附力与胶带剥离模型的预测结果高度吻合；但当剪切力较大时，其粘附力则远超模型预测值，最终导致粘附力随摩擦力近似呈线性增长。当剥离角小于约30°时，粘附力会急剧升高，这使得附着与脱附之间的快速切换成为可能。当实验结果偏离经典剥离理论时，恰好伴随足垫滑动现象的出现，而足垫滑动通过两种机制的协同作用，极大提升了剥离力。其一，局部滑动预先拉伸了足垫，使其在脱附过程中有效刚度提升，剥离行为愈发接近不可拉伸胶带的情况。其二，足垫滑动会减小接触区域内流体层的厚度，进而提升剥离所需的应力水平。综合来看，这些效应可解释粘附与摩擦力之间的耦合机制——这一机制是所有攀爬动物实现粘附调控的核心基础。本研究结果表明，粘附调控并非仅依赖于方向依赖性与形态各向异性，这为可调控仿生粘附材料的开发提供了极具前景的全新思路。