Analyzing the Tribological Combination of Microstructure and Lubricant in Beetle Joints for the Development of Environmentally Friendly Lubricants ^†

Insects optimize friction in their joints by combining microstructures with a—so far unknown—lubricant. To develop environmentally friendly lubricants, we research the sophisticated tribological system found in the joints of beetles. We characterize the lubricant as well as the microstructure of the joints to gain inspiration for the development of a degradable and—hopefully—superior alternative to mineral-oil-based lubricants. However, restrained by the tiny quantities of beetle lubricant and the compactness of their joints, this tribological analysis is challenging. Therefore, we apply atomic force microscopy (AFM) to record the joints' microstructures and the lubricant's frictional properties. Furthermore, we research the inner structure of the bearing surface in beetle joints by focused ion beam (FIB) tomography. With this approach, we discover a network of channels supplying the lubricant to pores which represent the inlets of the hinged joint system. As a subsequent step, we analyze different types of presently available plant mucilage using AFM friction measurements to compare the suitability of plant mucilage as an alternative lubricant to the tiny quantities of beetle lubricant. Finally, we develop an artificial surface mimicking the microstructure of beetle joints. We determine its frictional properties utilizing colloidal AFM probes in the dry state as well as the lubricated state with plant mucilage as the lubricant.