Friction Reduction by Deformation and Contact-Area Reduction in Elastic Deformation Materials That Mimic the Hierarchical Surface Microstructure of Carrion Beetles.

We have newly demonstrated that the combination of the submicron-wrinkled dome microstructure of carrion beetles and elastic deformation materials has a friction-reducing effect that is difficult to achieve with conventional single-layer surface microstructures. Elastic deformation materials, which have excellent material properties such as flexibility, also have the disadvantage of high friction due to a large contact area, hysteresis friction, and digging friction. In biomimetic microstructures, the contact area decreases as the structure size increases, but not infinitely, and structure deformation is suppressed to reduce hysteresis and digging friction. This suggests that there is a limit to the friction reduction achieved by a single-layer structure. Friction force measurements and FEM calculation results showed that the reduction in friction force and contact area was limited in a single-layer structure consisting only of wrinkles and domes, but the hierarchical structure reduced friction by reducing the contact area more than in a single-layer structure. The anisotropy of the sliding direction, the difference between static and dynamic friction coefficients, and differences in frictional force due to structural shape at the same contact area suggest that structural deformation suppresses the increase in hysteresis friction. In large single-layer structures, the friction-reducing effect due to deformation was lost, but in hierarchical structures, the friction reduction due to deformation occurred because the contact points were small. The deformation was supported by FEM calculations. Combining the properties of elastic deformation material with a carrion beetle-mimicked hierarchical structure enables friction reduction by surface microstructure deformation while reducing contact area using a single material, differing from previous reports. The hierarchical structure maintains its friction-reducing effect even after 100 sliding cycles and can be transferred as a single material from molds formed through self-organization, offering various advantages for practical application. These findings contribute to the design of microstructure-based lubricant-free and low-friction surfaces.