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Short and long-range order in the velocity field of dense granular systems
I will start with an introduction to the physics of granular and active systems, discussing interesting phenomena and applications, and suggesting qualitative analogies between these two families of systems. Then I will describe a recent experiment where a rotating probe, immersed in a vibro-fluidized dense 3d granular system, displays persistent angular motion in the form of superdiffusion at very long times, notwithstanding the absence of any geometrical asymmetry or polarity in the probe or in the grains. Discrete-element molecular dynamics simulations of the same setup reveal that the probe’s persistent motion is sustained by persistent motion of a macroscopic fraction of the granular medium, which is realised by spontaneous order in the granular velocity field. Inspired by this observation, I will sketch a brief overview of old and new simple dynamical models which lead to velocity ordering. My overview will include both granular and active kinetic theories, with a focus on some useful lattice models where the continuum (hydrodynamic-like) limit can be performed analytically. I will insist in finding analogies in the minimal ingredients underlying the phenomenon of velocity ordering in the two categories of systems.