Data from: Surface contact and design of fibrillar ‘friction pads’ in stick insects (Carausius morosus): mechanisms for large friction coefficients and negligible adhesion.

Many stick insects and mantophasmids possess tarsal ‘heel pads’ (euplantulae) covered by arrays of conical, micrometre-sized hairs (acanthae). These pads are used mainly under compression; they respond to load with increasing shear resistance, and show negligible adhesion. Reflected-light microscopy in stick insects (Carausius morosus) revealed that the contact area of ‘heel pads’ changes with normal load on three hierarchical levels. First, loading brought larger areas of the convex pads into contact. Second, loading increased the density of acanthae in contact. Third, higher loads changed the shape of individual hair contacts gradually from circular (tip contact) to elongated (side contact). The resulting increase in real contact area can explain the load dependence of friction, indicating a constant shear stress between acanthae and substrate. As the euplantula contact area is negligible for small loads (similar to hard materials), but increases sharply with load (resembling soft materials), these pads show high friction coefficients despite little adhesion. This property appears essential for the pads’ use in locomotion. Several morphological characteristics of hairy friction pads are in apparent contrast to hairy pads used for adhesion, highlighting key adaptations for both pad types. Our results are relevant for the design of fibrillar structures with high friction coefficients but small adhesion.

多数竹节虫与螳䗛（mantophasmids）均生有跗节‘跟垫’（euplantulae），其表面覆盖着阵列排布的锥形微米级棘毛（acanthae）。此类垫状物主要在受压状态下发挥作用：其抗剪切阻力随载荷增加而提升，且粘附性可忽略不计。以印度修尾竹节虫（Carausius morosus）为研究对象的反射光显微观测显示，跗节跟垫的接触面积会在三个层级上随法向载荷发生变化：其一，载荷施加使凸面垫的更大区域产生接触；其二，载荷提升会增加接触棘毛的密度；其三，更高载荷会使单根毛发的接触形态逐渐从圆形（尖端接触）变为细长状（侧面接触）。由此产生的实际接触面积增量可解释摩擦力的载荷依赖性，这表明棘毛与底物之间的剪切应力保持恒定。由于该跟垫在低载荷下接触面积可忽略（与硬质材料类似），但随载荷增加会急剧增大（类似软质材料），因此此类垫状物即便粘附性较弱，仍可实现较高的摩擦系数。这一特性对于此类垫状物在运动中的功能至关重要。毛状摩擦垫的多项形态特征与用于粘附的毛状垫存在显著差异，这凸显了两类垫状物各自的关键适应性演化特征。本研究结果可为设计高摩擦系数、低粘附性的纤维状结构提供参考。