Department of Zoology, University of Cambridge, , Cambridge CB2 1TN, UK.
J R Soc Interface. 2014 Feb 19;11(94):20140034. doi: 10.1098/rsif.2014.0034. Print 2014 May 6.
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.
许多竹节虫和螳䗛目昆虫拥有覆盖着锥形微毛（刚毛）阵列的跗节“后跟垫”（euplantulae）。这些垫主要用于压缩；它们对负载的响应表现出逐渐增加的抗剪切阻力，且显示出可忽略不计的粘附力。竹节虫（Carausius morosus）的反射光显微镜显示，“后跟垫”的接触面积在三个层次上随正常负载而变化。首先，加载会使更多的凸垫接触面积参与接触。其次，加载会增加接触的刚毛密度。第三，更高的负载会逐渐改变单个毛发接触的形状，从圆形（尖端接触）变为拉长的（侧面接触）。由此产生的实际接触面积的增加可以解释摩擦的负载依赖性，表明刚毛和基质之间存在恒定的剪切应力。由于在小负载下，后跟垫的接触面积可以忽略不计（类似于硬材料），但随着负载的增加而急剧增加（类似于软材料），因此这些垫尽管粘附力很小，但仍表现出高摩擦系数。这种特性似乎对于垫在运动中的使用至关重要。毛发摩擦垫的几个形态特征与用于粘附的毛发垫明显相反，突出了两种垫类型的关键适应。我们的研究结果对于设计具有高摩擦系数但粘附力小的纤维状结构具有重要意义。
