We performed the statistical analysis of acoustic emission time series in the ultrasonic frequency range, obtained experimentally from laboratory samples subjected to external uniaxial elastic stress. We found a power law scaling behavior in both the acoustic emission amplitude distribution and time correlation function, with exponents very close to those found in fracturing processes occurring at different time and space scales. These facts strongly suggest the existence of a critical dynamics underlying the process, which might be related to the idea of a self-organized critical state based on the energy dissipation through all the length scales.PACS numbers: 62.20.Mk, 05.40.+j, 91.60.Lj Power law behavior in physical phenomena is usually the fingerprint of temporal and spatial critical fluctuations of which well known examples are Ising-like systems, fractal growth phenomena, turbulence, etc. Unlike the usual second order phase transitions, some of the previous examples exhibit a critical behavior without the need to fine tune any control parameter; i.e., the critical state is an attractor of the dynamics. A few years ago Bak, Tang, and Wiesenfeld [1] termed this kind of situation "self-organized criticality" (SOC) and introduced a simple model of a dynamically driven system, inspired by the dynamics of sandpiles, that evolves spontaneously to a stationary critical state. This model is an example of SOC phenomenon in which a system with short range coupling self-stabilizes in a stationary state characterized by avalanches (activity) with power law distribution functions. Hence, the system has no characteristic length (and is therefore self-similar) and is in this sense critical. The SOC concept has been proposed also as a possible mechanism for the generation of the so-called 1/ f noise; however, it has been shown successively [2,3] that the spatialtemporal scaling in the SOC state does not necessarily manifest itself in nontrivial exponents for the power spectrum.Because of the importance of the SOC concept as a possible unifying framework for a wide range of physical phenomena, a lot of work has been devoted to studying these systems through computer simulations, theoretical approaches, and experimental findings [4]. In particular, the SOC framework has been proposed as a possible interpretation for the empirical observation of the energy release in earthquakes [5]. In fact, existence of statistical self-similarity in seismic processes is a well established fact, which has its strongest evidence in the power law behavior of the well known Gutenberg'S [6] and Omori's [7] empirical laws. Power law behavior was observed by Mogi [8] in the distribution of the maximum trace amplitude of audio signals emitted from samples subjected to various forms of stress, in analogy with the Ishimoto Iida 0031-9007/94/73(25)/3423(4)$06.00 empirical relation [9]. Hirata [10] observed self-similarity in the time frequency distribution of aftershock signals due to fracturation of basalt under constant stress. More recently, ...

We investigate the properties of a model of granular matter consisting of N Brownian particles on a line subject to inelastic mutual collisions. This model displays a genuine thermodynamic limit for the mean values of the energy and the energy dissipation. When the typical relaxation time τ associated with the Brownian process is small compared with the mean collision time τc the spatial density is nearly homogeneous and the velocity probability distribution is gaussian. In the opposite limit τ ≫ τc one has strong spatial clustering, with a fractal distribution of particles, and the velocity probability distribution strongly deviates from the gaussian one.PACS: 81.05. Rm, 05.20.Dd, 05.40.+j In the past few years granular materials have become an intriguing subject of research [1] - [7], since they pose novel questions and challenges to the theorists and experimentalists. The constituting elements of such materials are solid particles, whose size may range from few microns to few centimeters, and which are subject to nonconservative contact forces such as friction and cohesion.Their collective behavior is peculiar and different from other forms of matter, such as solids, liquids or gases, and the ordinary statistical mechanical approach, which successfully deals with large assemblies of microscopic particles is not adequate.Generally speaking granular materials cannot be described as equilibrium systems neither from the configurational point of view nor from the dynamical point of view. It is known in fact that these systems remain easily trapped in some metastable configurations which can last for long time intervals unless they are shaken or perturbed [2]. On the other hand while in equilibrium statistical mechanics the kinetic energy per particle is proportional to the temperature and the velocities are gaussianly distributed, in the systems we consider the tails of the distribution deviate from the Maxwell law [8] . This phenomenon is accompanied by a pronounced clustering of the particles [3,4] or inelastic collapse [6].Several approaches have been proposed for the study of the so-called "granular gases" [7,9]. One crucial difference between ordinary gases and granular media is represented by the intrinsic inelasticity of the interactions among the grains, which makes any theory based on energy conservation , e.g. for ideal gases, not suitable.In the present work we study a one dimensional mechanical model, in the spirit of the one recently introduced by Kadanoff and coworkers [9], but containing some important differences regarding the energyexchange process. We consider N identical particles on a circle of length L [10] obeying to the following equations:where, 1 ≤ i ≤ N , T F is the temperature of a microscopic medium that we discuss below, τ is the relaxation time, in absence of collisions, and f i (t) is a standard white noise with zero average and variance < f i (t)f j (t ′ ) >= δ ij δ(t − t ′ ). In addition the particles are subject to inelastic collisions according to the rulewhere r is the restit...

The rectification of unbiased fluctuations, also known as the ratchet effect, is normally obtained under statistical nonequilibrium conditions. Here we propose a new ratchet mechanism where a thermal bath solicits the random rotation of an asymmetric wheel, which is also subject to Coulomb friction due to solid-on-solid contacts. Numerical simulations and analytical calculations demonstrate a net drift induced by friction. If the thermal bath is replaced by a granular gas, the well-known granular ratchet effect also intervenes, becoming dominant at high collision rates. For our chosen wheel shape the granular effect acts in the opposite direction with respect to the friction-induced torque, resulting in the inversion of the ratchet direction as the collision rate increases. We have realized a new granular ratchet experiment where both these ratchet effects are observed, as well as the predicted inversion at their crossover. Our discovery paves the way to the realization of micro and submicrometer Brownian motors in an equilibrium fluid, based purely upon nanofriction.

We present results from a series of experiments on a granular medium sheared in a Couette geometry and show that their statistical properties can be computed in a quantitative way from the assumption that the resultant from the set of forces acting in the system performs a Brownian motion. The same assumption has been utilized, with success, to describe other phenomena, such as the Barkhausen effect in ferromagnets, and so the scheme suggests itself as a more general description of a wider class of driven instabilities.

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The elastic properties of ZnO films deposited by rf magnetron sputtering on Al2O3 substrates have been analyzed by means of an acoustic investigation technique. The phase velocities of a spectrum of acoustic modes propagating along the layered structure have been measured and the results exploited for determining the complete set of elastic constants of the film. The effective constants of the film are lower than those of the bulk material by amounts which depend on the elastic constant considered and range from −1.2% for c33 to −24.8% for c11. The values obtained were used for determining the dispersion curves of acoustic modes propagating along ZnO layers deposited on fused quartz and silicon and showed good agreement with experimental results.

We report the study of an experimental granular Brownian motor, inspired by the one published in Eshuis et al. [Phys. Rev. Lett. 104, 248001 (2010)], but different in some ingredients. As in that previous work, the motor is constituted by a rotating blade, the surfaces of which break the rotation-inversion symmetry through alternated patches of different inelasticity, immersed in a gas of granular particles. The main difference of our experimental setup is in the orientation of the main axis, which is parallel to the (vertical) direction of shaking of the granular fluid, guaranteeing an isotropic distribution for the velocities of colliding grains, characterized by a variance v(0)(2). We also keep the granular system diluted, in order to compare with Boltzmann-equation-based kinetic theory. In agreement with theory, we observe the crucial role of Coulomb friction which induces two main regimes: (i) rare collisions, with an average angular velocity <ω>~v(0)(3), and (ii) frequent collisions (FC), with <ω>~v(0). We also study the fluctuations of the angle spanned in a large-time interval Δθ, which in the FC regime is proportional to the work done upon the motor. We observe that the fluctuation relation is satisfied with a slope which weakly depends on the relative collision frequency.

We report on experimentally observed shear stress fluctuations in both granular solid and fluid states, showing that they are non-Gaussian at low shear rates, reflecting the predominance of correlated structures (force chains) in the solidlike phase, which also exhibit finite rigidity to shear. Peaks in the rigidity and the stress distribution's skewness indicate that a change to the force-bearing mechanism occurs at the transition to fluid behavior, which, it is shown, can be predicted from the behavior of the stress at lower shear rates. In the fluid state stress is Gaussian distributed, suggesting that the central limit theorem holds. The fiber bundle model with random load sharing effectively reproduces the stress distribution at the yield point and also exhibits the exponential stress distribution anticipated from extant work on stress propagation in granular materials.