Granular materials are substances made of macroscopic grains. They are ubiquitous in our everyday life and crucial for industry and geophysics. Understanding their statics and dynamics is a challenge for statistical physics, because of their intrinsic non-equilibrium nature. I will discuss a series of recent experimental results obtained within the same setup in different conditions of vibro-fluidization, density and applied external forcing. In the dilute limit, at strong vibro-fluidization, the granular media behaves as a dissipative gas: a probe (a rotator) immersed in the gas experiences random forces, performing a fascinating non-equilibrium Brownian motion, whose properties are fairly predicted by granular kinetic theory. Inelasticity of collisions and solid-on-solid friction affect the probe's dynamics and induce interesting motor ("ratchet") effects. Increasing the density and/or reducing the vibro-fluidization strength, the gas becomes a liquid: collisions become correlated and typical cage effects appear with subdiffusive transient. Surprisingly, at very low "granular temperatures", some new collective phenomena emerges inducing in the probe a pronounced superdiffusive behavior. Another important aspect of a granular media is its response properties. The application of a torque to the probe, by means of an external motor, allows us to verify linear response theory - with all its non-equilibrium effects. Finally, in the non-linear regime, we enter the domain of granular rheology, where we have discovered a discontinuous transition from thinning to thickening regime.
A Gnoli, A Lasanta, A Sarracino, A Puglisi, Scientific Reports 6, 38604 (2016)
C Scalliet, A Gnoli, A Puglisi, A Vulpiani, Physical Review Letters 114, 198001 (2015)
A Gnoli, A Puglisi, A Sarracino, A Vulpiani, PloS one 9, e93720 (2014)
A Gnoli, A Petri, F Dalton, G Pontuale, G Gradenigo, A Sarracino, A. Puglisi, Physical Review Letters 110, 120601 (2013)