Mechanical vibrations may influence the frictional force between sliding surfaces, affecting their relative motion and the associated stick-slip dynamics. This effe€ect is relevant for phenomena occurring at very different length scales, from atomic to mesoscopic systems, as the physics responsible for friction is expected to be largely the same [1, 2, 3]. The study of mechanical perturbated systems is frequently connected to the geophysical scale, where it is possible that earthquakes, a stick-slip frictional instability [4], may be actually triggered by incoming seismic waves, a phenomenon regularly observed in numerical simulations of seismic fault models [5]. The role of external perturbations has also been investigated via simulations of vibrated and sheared Lennard-Jones particles at zero temperature [6]. This work revealed the possibility to suppress the friction coefficientnt by applying perturbations in a well defined range of frequencies. However, it is not clear whereas the presence of the particles in between the sliding surfaces is essential to reproduce friction suppression. Via the analytical and numerical study of three variants of the usual spring-block model in the presence of an history dependent frictional force, we identify the conditions under which friction is suppressed and/or recovered [7]. In all the cases the block moves along a surface which is vibrated along the vertical direction, and the role of both the amplitude and the frequency of vibration is explored. An order parameter is introduced to differentiate the stick-slip and the flowing phases, and a phase diagram is proposed for each model. Results show that by incresing the intensity of the perturbation we observe a transition from the stick-slip to the sliding phase. Only in the presence of a modulated surface, and of a block confined by a force which is always normal to this surface, a further increase of the oscillation frequency leads to a second friction recovery transition, in which the system transients from the sliding to the stickslip phase. This result clarify that the friction recovery transition is not a peculiarity of many particle systems but rather a phenomenon linked to the modulation of the surface over which the block slides.

[1] M. Urbakh, J. Klafter, D. Gourdon, and J. Israelachvili, Nature 430, 29 (2004)
[2] A. Socoliuc, E. Gnecco, S. Maier, O. Pfeiffer, A. Batoff,ff, Bennewitz and E. Meyer, Science 313, 207 (2006).
[3] P.A. Johnson and X. Jia, Nature 437, 871 (2005).
[4] C. Marone, Nature 391, 69 (1998).
[5] M. P. Ciamarra, E. Lippiello, C. Godano and L. de Arcangelis, Phys. Rev. Lett. 104, 238001 (2010).
[6] R. Capozza, A. Vanossi, A. Vezzani, and S. Zapperi, Phys. Rev. Lett 103, 085502 (2009).
[7] F. Giacco , E. Lippiello, M. Pica Ciamarra. Submitted to Phys. Rev. E.

In a driven granular fluid energy is continuously gained from a thermal bath and lost through inelastic collisions. The irreversible nature of this dynamics produces some correlations between the hydrodynamics fields, that are absent at equilibrium. In this talk we summarize some results on the spectrum of velocity structure factors in a driven granular fluid. This spectrum can be calculated analytically from a standard fluctuating hydrodynamic theory and theoretical predictions are found in good agreement with the results of both event-driven molecular dynamic simulation and real experiments, realized with a monolayer of inelastic beads fluidized with a vertical shaking. We also derive a coarse-grained entropy production formula for granular fluid models, making explicit the dependence of this observable from the out-of-equilibrium correlations between hydrodynamics fields and showing that entropy production strongly depends on the kind of thermostat coupled to the granular fluid.

We study the stationary state of a one-dimensional kinetic model where a probe particle is driven by an external field E and collides, elastically or inelastically, with a bath of particles at temperature T. In particular, we focus on the stationary distribution of the velocity of the particle, and on the study of the fluctuations of the stochastic entropy and of the work done by the field.

TBA

Granular materials are present in many aspect of our lives. Let us think of sand, pills, nuts, rice, powder, builiding materials, cereals for breakfast just to make some examples. All of them borrow some features from solid, liquid and gaseous state so that they act as they were a 'fourth' state of matter. Furthermore, they can exhibit collective phenomena and their phase diagrams are often phenomenologically rich and non-easily predictable. The knowledge of the friction law of sheared granular materials is thus important for speculative and also practical reasons, like packing or transport problems. We did extensive Molecular Dynamics simulations of mono- and bi-disperse granular mixtures under different types of shear and found that friction law has some universal features which depend only on the properties of granular constituents, while other dynamical features depend on the way the shear is done.