Tuning friction with composite hierarchical surfaces

- Considerable reduction of static friction can be achieved by “smart” surface smoothing of only part of the sliding surface.
- Friction coefficients can be manipulated by means of various types of hierarchical roughness architectures.
- Stiffness variations, e.g. a linear grading of the Young's modulus, can also provide a reduction of static friction.

Macroscopic friction coefficients observed in experiments are the result of various types of complex multiscale interactions between sliding surfaces. Therefore, there are several ways to modify them depending on the physical phenomena involved. Recently, it has been demonstrated that surface structure, e.g. artificial patterning, can be used to tune frictional properties. In this paper, we show how the global friction coefficients can also be manipulated using composite surfaces with varying roughness or stiffness values, i.e. by combining geometrical features with the modification of local friction coefficients or stiffnesses. We show that a remarkable reduction of static friction can be achieved by introducing hierarchical arrangements of varying local roughness values, or by introducing controlled material stiffness variations.