Granular segregation driven by particle interactions

We report the results of an experimental study of particle-particle interactions in a horizontally shaken granular layer that undergoes a second order phase transition from a binary gas to a segregation liquid as the packing fraction C is increased. By focusing on the behavior of individual particles, the effect of C is studied on (1) the process of cluster formation, (2) cluster dynamics, and (3) cluster destruction. The outcomes indicate that the segregation is driven by two mechanisms: attraction between particles with the same properties and random motion with a characteristic length that is inversely proportional to C. All clusters investigated are found to be transient and the probability distribution functions of the separation times display a power law tail, indicating that the splitting probability decreases with time

Origin of waves in surface-tension-driven convection

Waves appear in a liquid layer with a free surface if a sufficiently high horizontal temperature gradient is imposed. These waves have been compared to the hydrothermal waves predicted by a linear stability analysis of a parallel flow. However, depending on the experimental configurations, significant differences with theory are found. We show that there exists another kind of wave that cannot be explained by previous analysis. Our aim is to investigate which is the mechanism leading to this instability. Differential interferometry is used to obtain quantitative information on the temperature field. Experimental evidence is presented suggesting that these waves are the result of a boundary layer instability: the roll near the hot wall begins to oscillate, and the perturbations are dragged and amplified downflow. This mechanism could explain discrepancies between theory and some experimental observations

Failure time and microcrack nucleation

The failure time of samples of heterogeneous materials (wood, ¯berglass) is stud- ied as a function of the applied stress. It is shown that in these materials the failure time is predicted with a good accuracy by a model of microcrack nucleation proposed by Pomeau. It is also shown that the crack growth process presents critical features when the failure time is approached

An experimental test of the critical behaviour of fracture precursors

The statistical properties and the localization of fracture precursors on heteregeneous materials is studied by recording their acoustic emission as a function of the applied load. It's found that the microcrack cluster together as the load increases and the instantaneous acoustic energy has an invariant power law distribution. The integrated acoustic energy presents a critical divergency close to the failure load for the sample. These result support the idea that fracture can be viewed as a critical phenomenon. Finally a measure of the concentration of microcraks, which allows us to predict the failure load, is introduced. These properties are studied also when a periodic load is applied to the sample. It's found that the Kaiser e ect is not strictly satis ed in heteregeneous materials

A non-standard statistical approach to the silo discharge

We present molecular dynamics simulations of the beginning of a silo discharge by gravity. The evolution of the velocity profile and the probability density functions for the displacements of the grains are obtained. These PDFs reveal non-gaussian statistics and superdiffusive behavior similar to that observed in some experiments. We propose an analytical expression for the PDFs and an explanation for its dynamical origin in connection with the ideas of the “spot” model and non-extensive thermodynamics

Stability of clogging arches in a silo submitted to vertical vibrations

We present experimental results on the endurance of arches that block the outlet of a two-dimensional silo when subjected to vertical vibration. In a recent paper [C. Lozano et al., Phys. Rev. Lett. 109, 068001 (2012)], it was shown that the arch resistance against vibrations is determined by the maximum angle among those formed between each particle in the bridge and its two neighbors: the larger the maximum angle is, the weaker the bridge. It has also been reported that the breaking time distribution shows a power-law tail with an exponent that depends on the outlet size, the vibration intensity, and the load [I. Zuriguel et al., Sci. Rep. 4, 7324 (2014)]. Here we connect these previous works, demonstrating the importance of the maximum angle in the arch on the exponent of the breaking time distribution. Besides, we find that the acceleration needed to break an arch does not depend on the ramp rate of the applied acceleration, but it does depend on the outlet size above which the arch is formed. We also show that high frequencies of vibration reveal a change in the behavior of the arches that endure very long times. These arches have been identified as a subset with special geometrical features. Therefore, arches that cannot be broken by means of a given external excitation might exist

Anomalous diffusion in silo drainage

The silo discharge process is studied by molecular dynamics simulations. The development of the velocity profile and the probability density function for the displacements in the horizontal and vertical axis are obtained. The PDFs obtained at the beginning of the discharge reveal non-Gaussian statistics and superdiffusive behaviors. When the stationary flow is developed, the PDFs at shorter temporal scales are non-Gaussian too. For big orifices a well-defined transition between ballistic and diffusive regime is observed. In the case of a small outlet orifice, no well-defined transition is observed. We use a nonlinear diffusion equation introduced in the framework of non-extensive thermodynamics in order to describe the movements of the grains. The solution of this equation gives a well-defined relationship ( = 2/(3 − q)) between the anomalous diffusion exponent and the entropic parameter q introduced by the non-extensive formalism to fit the PDF of the fluctuations

Trap model for clogging and unclogging in granular hopper flows

Granular flows through narrow outlets may be interrupted by the formation of arches or vaults that clog the exit. These clogs may be destroyed by vibrations. A feature which remains elusive is the broad distribution pð¿Þ of clog lifetimes ¿ measured under constant vibrations. Here, we propose a simple model for arch breaking, in which the vibrations are formally equivalent to thermal fluctuations in a Langevin equation; the rupture of an arch corresponds to the escape from an energy trap. We infer the distribution of trap depths from experiments made in two-dimensional hoppers. Using this distribution, we show that the model captures the empirically observed heavy tails in pð¿Þ. These heavy tails flatten at large ¿, consistently with experimental observations under weak vibrations. But, here, we find that this flattening is systematic, which casts doubt on the ability of gentle vibrations to restore a finite outflow forever. The trap model also replicates recent results on the effect of increasing gravity on the statistics of clog formation in a static silo. Therefore, the proposed framework points to a common physical underpinning to the processes of clogging and unclogging, despite their different statistics