  Hierarchical structures are very common in Nature, but only recently have
they been systematically studied in materials physics, in order to understand
the specific effects they can have on the mechanical properties of various
systems. Structural hierarchy provides a way to tune and optimize macroscopic
mechanical properties starting from simple base constituents, and new materials
are nowadays designed exploiting this possibility. This can be also true in the
field of tribology. In this paper, we study the effect of hierarchical
patterned surfaces on the static and dynamic friction coefficients of an
elastic material. Our results are obtained by means of numerical simulations
using a 1-D spring-block model, which has previously been used to investigate
various aspects of friction. Despite the simplicity of the model, we highlight
some possible mechanisms that explain how hierarchical structures can
significantly modify the friction coefficients of a material, providing a means
to achieve tunability.
