  Granular materials such as sand, powders, and food grains are ubiquitous in
civil engineering, geoscience, agriculture, and medicine. While the influence
of friction between the grains on the static structure of these systems is well
understood, its impact on the dynamics is an open problem. Here we use
particle-based simulations of a granular pack under cyclic shear and discover
that the relaxation time of the system is a non-monotonic function of friction.
By introducing the concept of dynamic constraints, we reveal that this
re-entrant dynamics is due to the competition between increasing frictional
coupling and a concurrent change in the structure of the granular pack. Our
theoretical approach, which unifies the dynamics of friction-less systems with
frictional ones, is applicable to other systems that have a complex free energy
landscape and a dynamics which involves time-dependent constraints, thus
setting the stage for a description of the dynamic behavior of a large class of
complex systems.
