Anisotropic Friction in Biological Systems

Biological surfaces covered with micro- and nanostructures, oriented at some angle to the plain may cause strong mechanical anisotropy. Some of them also exhibit pronounced flexibility due to the material of the supporting layer or due to flexible connecting joints. Flexible systems have a wide range of functions including the transport of particles in insect cleaning devices and the propulsion generation during slithering locomotion of snakes. In this chapter, we study the dependence of the anisotropic friction on the slope of the structures, rigidity of their joints, and sliding speed. A system of this kind is the snake skin consisting of stiff scales embedded in a flexible supporting layer. Additionally, there is also microstructure with strongly anisotropic orientation on these scales, which provides frictional anisotropy of the skin. The main function of such hierarchical anisotropic structures is to generate low sliding friction in the forward sliding direction, and high propulsive force along the substrate. Snakes are also able to dynamically adapt their friction interactions by redistributing their local pressures and changing their winding angles, when either friction anisotropy is suppressed by the low friction substrate, or when the external force displacing snake overcomes friction resistance on inclines. In order to understand these biotribology problems, we develop a set of corresponding numerical models.