  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.
