Collisional regime during the discharge of a two-dimensional silo.

The present work reports an investigation into the collisional dynamics of particles in the vicinity of the outlet of a two-dimensional silo using molecular dynamics simulations. Most studies on this granular system focus in the bulk of the medium. In this region, contacts are permanent or long-lived, so continuous approximations are able to yield results for velocity distributions or mass flow. Close to the exit, however, the density of the medium decreases and contacts are instantaneous. Thus, the collisional nature of the dynamics becomes significant, warranting a dedicated investigation as carried out in this work. More interesting, the vicinity of the outlet is the region where the arches that block the flow for small apertures are formed. It is found that the transition from the clogging regime (at small apertures) to the continuous flow regime is smooth in collisional variables. Furthermore, the dynamics of particles as reflected by the distributions of the velocities is as well unaffected. This result implies that there is no critical outlet size that separates both regimes, as had been proposed in the literature. Instead, the results achieved support the alternative picture in which a clog is possible for any outlet size.

Clogging of granular materials in silos: effect of gravity and outlet size.

By means of extensive numerical simulations we disclose the role of the driving force in the clogging of inert particles passing through a constriction. We uncover the effect of gravity and outlet size on the flow rate and kinetic energy within the system, and use these quantities to deepen our understanding of the blocking process. First, we confirm the existence of a finite avalanche size when the driving force tends to zero. The magnitude of this limit avalanche size grows with the outlet size, as expected due to geometrical reasons. In addition, there is an augment of the avalanche size when the driving force is increased, an effect that is enhanced by the outlet size. This phenomenology is explained by assuming that in order to get a stable clog developed, two conditions must be fulfilled: (1) an arch spanning the outlet size should be formed; (2) the arch should resist until the complete dissipation of the kinetic energy within the system. From these assumptions, we are able to obtain the probability that an arch gets destabilized, which is shown to primarily depend on the square root of the kinetic energy. A minor additional dependence of the outlet size is also observed which is explained in the light of recent results of the arch resistance in vibrated silos.

Topological analysis of tapped granular media using persistent homology.

We use the first Betti number of a complex to analyze the morphological structure of granular samples in mechanical equilibrium. We investigate two-dimensional granular packings after a tapping process by means of both simulations and experiments. States with equal packing fraction obtained with different tapping intensities are distinguished after the introduction of a filtration parameter which determines the particles (nodes in the network) that are joined by an edge. This is accomplished by just using the position of the particles obtained experimentally and no other information about the possible contacts, or magnitude of forces.

Role of driving force on the clogging of inert particles in a bottleneck.

We present numerical results of the effect that the driving force has on the clogging probability of inert particles passing through a bottleneck. When the driving force is increased by four orders of magnitude, the mean avalanche size remains almost unaltered (increases 1.6 times) while the flow rate and the avalanche duration display strong dependence on this magnitude. This indicates that in order to characterize the ability of a system to clog, the right variable to consider is the number of particles that pass through the outlet. The weak dependence of this magnitude on the driving force is explained in terms of the average kinetic energy of the flowing grains that has to be dissipated in order to get an arch stabilized.

Size and density avalanche scaling near jamming.

The current microscopic picture of plasticity in amorphous materials assumes local failure events to produce displacement fields complying with linear elasticity. Indeed, the flow properties of nonaffine systems, such as foams, emulsions and granular materials close to jamming, that produce a fluctuating displacement field when failing, are still controversial. Here we show, via a thorough numerical investigation of jammed materials, that nonaffinity induces a critical scaling of the flow properties dictated by the distance to the jamming point. We rationalize this critical behavior by introducing a new universal jamming exponent and hyperscaling relationships, and we use these results to describe the volume fraction dependence of the friction coefficient.

Contact network topology in tapped granular media.

We analyze the contact network of simulated two-dimensional granular packings in different states of mechanical equilibrium obtained by tapping. We show that topological descriptors of the contact network allow one to distinguish steady states of the same mean density obtained with different tap intensities. These equal-density states were recently proven to be distinguishable through the mean force moment tensor. In contrast, geometrical descriptors, such as radial distribution functions, bond order parameters, and Voronoi cell distributions, can hardly discriminate among these states. We find that small-order loops of contacts-the polygons of the network-are especially sensitive probes for the contact structure.

Silo clogging reduction by the presence of an obstacle.

We present experimental results on the effect that inserting an obstacle just above the outlet of a silo has on the clogging process. We find that, if the obstacle position is properly selected, the probability that the granular flow is arrested can be reduced by a factor of 100. This dramatic effect occurs without any remarkable modification of the flow rate or the packing fraction above the outlet, which are discarded as the cause of the change in the clogging probability. Hence, inspired by previous results of pedestrian crowd dynamics, we propose that the physical mechanism behind the clogging reduction is a pressure decrease in the region of arch formation.

Topology of the force network in the jamming transition of an isotropically compressed granular packing.

The jamming transition of an isotropically compressed granular packing is studied by means of molecular dynamics simulations. The system is shown to undergo a critical transition which is analyzed by looking at the topological structure of the force network. At the critical packing fraction there is a sudden growth of the number of polygons in the network. Above the critical packing fraction the number of triangles keeps growing while the number of the rest of polygons is weakly reduced. Then, we prove that in the jammed regime, there is a linear relationship between the number of triangles and the coordination number. Furthermore, the presence of these minimal structures is revealed to be connected with the evolution of some important topological properties, suggesting its importance to understand the physical properties of the packing and the onset of rigidity during the compression.

Identification of arches in two-dimensional granular packings.

We identify arches in a bed of granular disks generated by a molecular dynamic-type simulation. We use the history of the deposition of the particles to identify the supporting contacts of each particle. Then, arches are defined as sets of mutually stable disks. Different packings generated through tapping are analyzed. The possibility of identifying arches from the static structure of a deposited bed, without any information on the history of the deposition, is discussed.

Strategies for management of Sicyos polyacanthus Cogn. (Cucurbitaceae) in sugarcane crops of Tucumán, Argentina.

Sicyos polyacanthus is one of the most important weed in sugarcane crops of Tucumán (Argentina). The objective of this work was to establish strategies that would decrease the weed incidence in the crop to a minimum level. The study was carried out during 1998--2003 at five localities of sugarcane production of Tucumán (Argentina). The plots were 20 m long (192 m2) with 6 furrows and each plot was replicated five times. Treatments were: (i) Mechanical-chemical cultivation without fire; (ii) Mechanical-chemical culltivation with fire; (iii) Mechanical cultivation with handle pulled and with fire; (iv) Mechanical cultivation with handle pulled without fire; (v) Mechanical and chemical variants with fertilization and without fertilization; (vi) Mechanical and chemical variants with watering and without watering; (vii) Fallow and rotation, at the sugarcane crop renovation; (viii) Mechanical and chemical variants for plant cane and ratoon cane; and (ix) Mulching of harvest rests. The results suggest that besides the use of preemergent herbicides, fire marks the entry point of control, influencing fluxes and seed viability. It appears that fallow, mulching, and rotation of crops is fundamental for eliminating seeds that live short time in the soil and increase the mortality rate of species.

Vaccinia necrosum: Report on a fatal case

Vaccinia necrosum, a rare complication of smallpox vaccination, appeared in a young adult male, developed rapidly, and proved fatal in less than six weeks.