with a special session devoted to
The physics of quasi-fluids and quasi-solids: the dynamics of active and granular matter
Wednesday 20 - Friday 22 June 2012, University of Parma

thursday 21 June 2012

The Physics of quasi-fluids and quasi-solids: the dynamics of active and granular matter
Julian Talbot - LPTMC-CNRS
Kinetic Theory of a Frictional Granular Motor image
We investigate the influence of dry friction on an asymmetric, granular piston of mass M composed of two materials undergoing inelastic collisions with bath particles of mass m. Numerical simulations of the Boltzmann-Lorentz equation reveal the existence of two scaling regimes depending on the friction strength. In the large friction limit, we introduce an exact model giving the asymptotic behavior of the Boltzmann-Lorentz equation. For small friction and for large mass ratio M/m, we derive a Fokker-Planck equation for which the exact solution is also obtained. Static friction attenuates the motor effect and results in a discontinuous velocity distribution.
Ferdinando Giacco, Seconda Università di Napoli
Mechanical vibrations in a spring-block model image
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.
Andrea Gnoli - CNR ISC Roma
Brownian ratchet driven by Coulomb friction image
Statistical non-equilibrium conditions, equivalent to a breakdown of time-reversal symmetry, allow the rectification of unbiased fluctuations, which is impossible in equilibrium systems. Such a mechanism, also known as ratchet effect, underlies the functioning of molecular motors which exploit non-equilibrium chemical reactions to perform work in living organisms, e.g. kinesin and myosin, or the complementary case of artificial nano-motors actuated by non-equilibrium active fluids, e.g. bacteria. Recently, it has been demonstrated that it is possible to rectify the fluctuations of macroscopic mechanical devices: these are kept in out-of-equilibrium stationary states through continuous energy dissipation balanced by random energy injection, and are realized, for instance, by suspending an asymmetric probe in a fluidized granular medium. Here, we demonstrate through a new experimental setup with a rotating device subjected to granular collisions, the existence of a net ratchet effect, originating entirely in the Coulomb friction acting on the contact surface between the rotator and its bearing. Such a friction-induced torque acts in the opposite direction with respect to the net torque provided by inelastic collisions between the ratchet and the granular fluid: the interplay between these two forces results in a resonant behavior and in a ratchet velocity inversion point. Our experimental observations are reproduced by simulations and explained by kinetic theory. This discovery paves the way to the realization of Brownian motors in the realm of micro and sub-micrometer scales purely based upon nano-friction.
11:10-11:40coffee break
Giacomo Gradenigo, CNR-ISC Roma
Out-of-equilibrium correlations and entropy production in driven granular fluids: theory, simulations and experiments image
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.
Alessandro Sarracino - Università Roma 1
Non-equilibrium fluctuations in a driven stochastic Lorentz gas image
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.
Antonio Coniglio - Università di Napoli Federico II
Dynamical heterogeneities: from gels to glasses image
13:00-15:00lunch break
Olivier Dauchot - UMR Gulliver-CNRS, ESPCI-ParisTech
Self-propelled grains : a model of active liquids. image

In many interesting situations, the interactions among self-propelled agents lead to the spontaneous emergence of self-organized collective motion. The ubiquity of the phenomenon at all scales raises the question of the existence of some underlying universal mechanisms. Recent numerical and analytical studies have confirmed the existence of a transition from a disordered state at large noise to a state with various collective properties reflecting the local symmetry of the particles and their interactions.

Though, there are still very few experimental situations where the onset of collective motion can be attributed to spontaneous symmetry breaking. Here, we report on experiments conducted with both polar self propelled and a-polar Brownian disks and by comparing the dynamics of both systems in the same experimental conditions, we demonstrate without ambiguity that collective motion emerges from the interplay of self-propulsion and hard-core repulsion only [1]. Interestingly the alignment, which has no nematic origin, is effectively induced during the collisions because of the self propulsion [2].

[1] Phys. Rev. Lett. 105, 098001 (2010)
[2] DOI: 10.1039/C2SM25186H, (2012)
Amandine Miksic, ISC-CNR Roma
Acoustic emission in a sheared granular medium image
16:00-16:30coffee break
Mario Alberto Annunziata - CNR ISC Roma
Friction law(s) in granular materials image
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.
Irene Giardina - CNR ISC Roma
Statistical Mechanics for Natural Flocks of birds image
Flocking is a typical example of emergent collective behavior, where interactions between individuals produce collective patterns on the large scale. We show that a quantitative microscopic theory for directional ordering in a flock can be derived directly from field data. We construct the minimally structured (maximum entropy) model consistent with experimental correlations in large flocks of starlings. This model shows that local, pairwise, topological (i.e. density invariant) interactions between birds are sufficient to correctly predict the propagation of order throughout entire flocks of starlings, with no free parameters.