Revistas

Revista:
SCIENTIFIC REPORTS

ISSN 2045-2322
Vol. 12
N° 1
Año 2022
Págs.2647

The emergence of coherent vortices has been observed in a wide variety of many-body systems such as animal flocks, bacteria, colloids, vibrated granular materials or human crowds. Here, we experimentally demonstrate that pedestrians roaming within an enclosure also form vortex-like patterns which, intriguingly, only rotate counterclockwise. By implementing simple numerical simulations, we evidence that the development of swirls in many-particle systems can be described as a phase transition in which both the density of agents and their dissipative interactions with the boundaries play a determinant role. Also, for the specific case of pedestrians, we show that the preference of right-handed people (the majority in our experiments) to turn leftwards when facing a wall is the symmetry breaking mechanism needed to trigger the global counterclockwise rotation observed.

Revista:
PHYSICAL REVIEW LETTERS

ISSN 0031-9007
Vol. 127
N° 14
Año 2021
Págs.148002

The existence of a transition from a clogged to an unclogged state has been recently proposed for the flow of macroscopic particles through bottlenecks in systems as diverse as colloidal suspensions, granular matter, or live beings. Here, we experimentally demonstrate that, for vibrated granular media, such a transition genuinely exists, and we characterize it as a function of the outlet size and vibration intensity. We confirm the suitability of the "flowing parameter" as the order parameter, and we find out that the resealed maximum acceleration of the system should be replaced as the control parameter by a dimensionless velocity that can be seen as the square root of the ratio between kinetic and potential energy. In all the investigated scenarios, we observe that, for a critical value of this control parameter S-c, there seems to be a continuous transition to an unclogged state. The data can be resealed with this critical value, which, as expected, decreases with the orifice size D. This leads to a phase diagram in the S-D plane in which clogging appears as a concave surface.

Revista:
JOURNAL OF STATISTICAL MECHANICS-THEORY AND EXPERIMENT

ISSN 1742-5468
Vol. 2021
N° 4
Año 2021
Págs.043401

We report experimental results of the speed-density relation emerging in pedestrian dynamics when individuals keep a prescribed safety distance among them. To this end, we characterize the movement of a group of people roaming inside an enclosure varying different experimental parameters: (i) global density, (ii) prescribed walking speed, and (iii) suggested safety distance. Then, by means of the Voronoi diagram we are able to compute the local density associated to each pedestrian, which is afterward correlated with its corresponding velocity at each time. In this way, we discover a strong dependence of the speed-density relation on the experimental conditions, especially with the (prescribed) free speed. We also observe that when pedestrians walk slowly, the speed-density relation depends on the global macroscopic density of the system, and not only on the local one. Finally, we demonstrate that for the same experiment, each pedestrian follows a distinct behavior, thus giving rise to multiple speed-density curves.

Revista:
SCIENTIFIC REPORTS

ISSN 2045-2322
Vol. 11
N° 1
Año 2021
Págs.1534

With people trying to keep a safe distance from others due to the COVID-19 outbreak, the way in which pedestrians walk has completely changed since the pandemic broke out1,2. In this work, laboratory experiments demonstrate the effect of several variables-such as the pedestrian density, the walking speed and the prescribed safety distance-on the interpersonal distance established when people move within relatively dense crowds. Notably, we observe that the density should not be higher than 0.16 pedestrians per square meter (around 6 m2 per pedestrian) in order to guarantee an interpersonal distance of 1 m. Although the extrapolation of our findings to other more realistic scenarios is not straightforward, they can be used as a first approach to establish density restrictions in urban and architectonic spaces based on scientific evidence.

Revista:
THE EUROPEAN PHYSICAL JOURNAL CONFERENCES

ISSN 2101-6275
Vol. 249
Año 2021
Págs.03009

Flowing grains can clog an orifice by developing arches, an undesirable event in many cases. Several strategies have been put forward to avoid this. One of them is to vibrate the system in order to undo the clogging. Nevertheless, the time taken to break an arch under a constant vibration has a distribution displaying a heavy tail. This can lead to a situation where the average breaking time is not well defined. Moreover, it has been observed in some experiments that these tails tend to flatten for very long times, exacerbating the problem. Here we will review two conceptual frameworks that have been proposed to understand the phenomenon and discuss their physical implications.

Autores:
Parisi, D. R. (Autor de correspondencia); Sartorio, A. G.; Colonnello, J. R.; et al.

Revista:
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA

ISSN 0027-8424
Vol. 118
N° 50
Año 2021
Págs.e2107827118

We characterize the dynamics of runners in the famous Running of the Bulls Festival by computing the individual and global velocities and densities, as well as the crowd pressure. In contrast with all previously studied pedestrian systems, we unveil a unique regime in which speed increases with density that can be understood in terms of a time-dependent desired velocity of the runners. Also, we discover the existence of an inaccessible region in the speed-density state diagram that is explained by falls of runners. With all these ingredients, we propose a generalization of the pedestrian fundamental diagram for a scenario in which people with different desired speeds coexist.

Revista:
SAFETY SCIENCE

ISSN 0925-7535
Vol. 121
Año 2020
Págs.394 - 402

We report experimental measurements obtained during the evacuation of 180 soldiers through a narrow door. Several conditions are analyzed in the evacuation drills, such as the degree of competitiveness (from rush to shove) and the influence of an obstacle placed before the exit. From the data, we compute the flow rate through the door and the velocity and density fields, as well as a map of the local evacuation time. We also present novel results on the pressure that the individuals exert on the wall adjacent to the door. Our study challenges the idea that an obstacle could be beneficial for pedestrian evacuations because of a hypothetical alleviation of pressure at the door. At the same time, we discover a correlation among the largest pressure peaks and the development of clogging.

Revista:
SCIENTIFIC REPORTS

ISSN 2045-2322
Vol. 10
N° 1
Año 2020
Págs.15947

Although some experimental evidence showed that an obstacle placed in front of a door allows making people's evacuations faster, the efficacy of such a solution has been debated for over 15 years. Researchers are split between those who found the obstacle beneficial and those who could not find a significant difference without it. One of the reasons for the several conclusions lies in the variety of the experiments performed so far, both in terms of competitiveness among participants, geometrical configuration and number of participants. In this work, two unique datasets relative to evacuations with/without obstacle and comprising low and high competitiveness are analyzed using state-of-the-art definitions for crowd dynamics. In particular, the so-called congestion level is employed to measure the smoothness of collective motion. Results for extreme conditions show that, on the overall, the obstacle does not reduce density and congestion level and it could rather slightly increase it. From this perspective, the obstacle was found simply shifting the dangerous spots from the area in front of the exit to the regions between the obstacle and the wall. On the other side, it was however confirmed, that the obstacle can stabilize longitudinal crowd waves, thus reducing the risk of trampling, which could be as important (in terms of safety) as improving the evacuation time.

Revista:
PHYSICAL REVIEW E

ISSN 2470-0045
Vol. 99
N° 3
Año 2019
Págs.1 - 6

We present experimental results of the effect of the hopper angle on the clogging of grains discharged from a two-dimensional silo under gravity action. We observe that the probability of clogging can be reduced by three orders of magnitude by increasing the hopper angle. In addition, we find that for very large hopper angles, the avalanche size (s) grows with the outlet size (D) stepwise, in contrast to the case of a flat-bottom silo for which s grows smoothly with D. This surprising effect is originated from the static equilibrium requirement imposed by the hopper geometry to the arch that arrests the flow. The hopper angle sets the bounds of the possible angles of the vectors connecting consecutive beads in the arch. As a consequence, only a small and specific portion of
the arches that jam a flat-bottom silo can survive in hoppers.

Revista:
PHYSICAL REVIEW E

ISSN 2470-0045
Vol. 100
N° 3
Año 2019
Págs.032901

We report an experiment on the unclogging dynamics in a two-dimensional silo submitted to a sustained gentle vibration. We find that arches present a jerking motion where rearrangements in the positions of their beads are interspersed with quiescent periods. This behavior occurs for both arches that break down and those that withstand the external perturbation: Arches evolve until they either collapse or get trapped in a stable configuration. This evolution is described in terms of a scalar variable characterizing the arch shape that can be modeled as a continuous-time random walk. By studying the diffusivity of this variable, we show that the unclogging is a weakly nonergodic process. Remarkably, arches that do not collapse explore different configurations before settling in one of them and break ergodicity much in the same way than arches that break down.

Revista:
GRANULAR MATTER

ISSN 1434-5021
Vol. 21
N° 3
Año 2019
Págs.47

In this paper we report experimental and numerical results on the velocity fluctuations of grains inside silos. Although simple models exist for the stationary and continuous approximation of the flow, the variability at the microscopic level (both ensemble averages and the fluctuations of individual particles around the average) reveal non-Gaussian statistics that resist a straightforward treatment. We also show that decreasing the orifice size causes an increase in the relative amplitude of the velocity fluctuations, meaning that the intermittency grows bigger.

Autores:
Aumaître, S.; Behringer, R. P.; Cazaubiel, A.; et al.

Revista:
REVIEW OF SCIENTIFIC INSTRUMENTS

ISSN 0034-6748
Vol. 89
N° 075103
Año 2018
Págs.1 - 10

A new experimental facility has been designed and constructed to study driven granular media in a low-gravity environment. This versatile instrument, fully automatized, with a modular design based on several interchangeable experimental cells, allows us to investigate research topics ranging from dilute to dense regimes of granular media such as granular gas, segregation, convection, sound propagation, jamming, and rheology¿all without the disturbance by gravitational stresses active on Earth. Here, we present the main parameters, protocols, and performance characteristics of the instrument. The current scientific objectives are then briefly described and, as a proof of concept, some first selected results obtained in low gravity during parabolic flight campaigns are presented.

Revista:
NEW JOURNAL OF PHYSICS

ISSN 1367-2630
Vol. 20
Año 2018

The placement of obstacles in front of doors is believed to be an effective strategy to increase the flow of pedestrians, hence improving the evacuation process. Since it was first suggested, this counter-intuitive feature is considered a hallmark of pedestrian flows through bottlenecks. Indeed, despite the little experimental evidence, the placement of an obstacle has been hailed as the panacea for solving evacuation problems. In this work, we challenge this idea and experimentally demonstrate that the pedestrians flow rate is not necessarily altered by the presence of an obstacle. This result-which is at odds with recent demonstrations on its suitability for the cases of granular media, sheep and mice- differs from the outcomes of most of existing numerical models, and warns about the risks of carelessly extrapolating animal behaviour to humans. Our experimental findings also reveal an unnoticed phenomenon in relation with the crowd movement in front of the exit: in competitive evacuations, an obstacle attenuates the development of collective transversal rushes, which are hazardous as they might cause falls.

Revista:
PHYSICAL REVIEW E

ISSN 2470-0045
Vol. 97
N° 4
Año 2018
Págs.042904

We experimentally explore the vibration-induced unclogging of arches halting the flow in a two-dimensional silo. The endurance of arches is determined by carrying out a survival analysis of their breaking times. By analyzing the dynamics of two morphological variables, we demonstrate that arches evolve toward less regular structures and it seems that there may exist a certain degree of irregularity that the arch reaches before collapsing. Moreover, we put forward that ¿ (the standard deviation of all angles between consecutive beads) describes faithfully the morphological evolution of the arch. Focusing on long-lasting arches, we study ¿ calculating its two-time autocorrelation function and its mean-squared displacement. In particular, the apparent logarithmic increase of the correlation and the decrease of the mean-squared displacement of ¿ when the waiting time is increased reveal a slowing down of the dynamics. This behavior is a clear hallmark of aging phenomena and confirms the lack of ergodicity in the unclogging dynamics. Our findings provide new insights on how an arch tends to destabilize and how the probability that it breaks with a long sustained vibration decreases with time.

Revista:
PHYSICAL REVIEW LETTERS

ISSN 0031-9007
Vol. 120
N° 19
Año 2018
Págs.198002

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.

Autores:
Patterson, G. A. ; Fierens, P. I. ; Sangiuliano Jimka, F. ; et al.

Revista:
PHYSICAL REVIEW LETTERS

ISSN 0031-9007
Vol. 119
N° 24
Año 2017
Págs.248301

We report experimental results on the competitive passage of elongated self-propelled vehicles rushing through a constriction. For the chosen experimental conditions, we observe the emergence of intermittencies similar to those reported previously for active matter passing through narrow doors. Noteworthy, we find that, when the number of individuals crowding in front of the bottleneck increases, there is a transition from an unclogged to a clogged state characterized by a lack of convergence of the mean clog duration as the measuring time increases. It is demonstrated that this transition-which was reported previously only for externally vibrated systems such as colloids or granulars-appears also for self-propelled agents. This suggests that the transition should also occur for the flow through constrictions of living agents (e.g., humans and sheep), an issue that has been elusive so far in experiments due to safety risks.

Revista:
SCIENTIFIC REPORTS

ISSN 2045-2322
Vol. 7
N° 1
Año 2017
Págs.10792

When a sizable number of people evacuate a room, if the door is not large enough, an accumulation of pedestrians in front of the exit may take place. This is the cause of emerging collective phenomena where the density is believed to be the key variable determining the pedestrian dynamics. Here, we show that when sustained contact among the individuals exists, density is not enough to describe the evacuation, and propose that at least another variable -such as the kinetic stress- is required. We recorded evacuation drills with different degrees of competitiveness where the individuals are allowed to moderately push each other in their way out. We obtain the density, velocity and kinetic stress fields over time, showing that competitiveness strongly affects them and evidencing patterns which have been never observed in previous (low pressure) evacuation experiments. For the highest competitiveness scenario, we detect the development of sudden collective motions. These movements are related to a notable increase of the kinetic stress and a reduction of the velocity towards the door, but do not depend on the density.

Revista:
PHYSICAL REVIEW E

ISSN 2470-0045
Vol. 94
N° 3
Año 2016
Págs.032302

In a recent work [Phys. Rev. E 91, 022808 (2015)] it was reported that placing an obstacle in front of a gate has a beneficial effect in the flow of sheep through it. Here, we extend such results by implementing three different obstacle positions. We have observed that the flow is improved in two cases, while it worsens in the other one; the last instance happens when the obstacle is too close to the door. In this situation, the outcomes suggest that clogging develops between the doorjamb and the obstacle, contrary to the cases when the obstacle is farther, in which case clogging always occurs at the very door. The effectiveness of the obstacle (a strategy put forward to alleviate clogging in emergency exits) is therefore quite sensitive to its location. In addition, the study of the temporal evolution of the flow rate as the test develops makes evident a steady behavior during the entire duration of the entrance. This result is at odds with recent findings in human evacuation tests where the flow rate varies over time, therefore challenging the fairness of straightforward comparisons between pedestrian behavior and animal experimental observations.

Revista:
JOURNAL OF STATISTICAL MECHANICS-THEORY AND EXPERIMENT

ISSN 1742-5468
N° 4
Año 2016
Págs.043402

We report a thorough analysis of the intermittent flow of pedestrians through a narrow door. The observations include five different sets of evacuation drills with which we have investigated the effect of door size and competitiveness on the flow dynamics. Although the outcomes are in general compatible with the existence of the faster-is-slower effect, the temporal evolution of the instantaneous flow rate provides evidence of new features. These stress the crucial role of the number of people performing the tests, which has an influence on the obtained results. Once the transients at the beginning and end of the evacuation are removed, we have found that the time lapses between the passage of two consecutive pedestrians display heavy-tailed distributions in all the scenarios studied. Meanwhile, the distribution of burst sizes decays exponentially; this can be linked to a constant probability of finding a long-lasting clog during the evacuation process. Based on these results, a discussion is presented on the caution that should be exercised when measuring or describing the intermittent flow of pedestrians through narrow doors.

Revista:
GRANULAR MATTER

ISSN 1434-5021
Vol. 17
N° 5
Año 2015
Págs.545 - 551

We report experimental results for pipe flow of granular materials discharged through vertical narrow tubes by means of a conveyor belt placed at the bottom. When the diameter of the tube is not much larger than the particle size, the system clogs due to the development of hanging arches that are able to support the weight of the grains above them. We find that the time it takes to develop a stable clog decays exponentially, which is compatible with a clogging probability that remains constant during the discharge. From this, and making an analogy with the discharge of silos, we introduce the avalanche size, measured in terms of the number of discharged tubes before the system clogs. The mean avalanche size is found to increase as the tube diameter is enlarged, the velocity of the conveyor belt grows, and the tube tilt deviates from the vertical. © 2015, Springer-Verlag Berlin Heidelberg.

Revista:
PHYSICAL REVIEW E

ISSN 2470-0045
Vol. 92
N° 6
Año 2015
Págs.062817

The ¿faster-is-slower¿ (FIS) effect was first predicted by computer simulations of the egress of pedestrians through a narrow exit [D. Helbing, I. J. Farkas, and T. Vicsek, Nature (London) 407, 487 (2000)]. FIS refers to the finding that, under certain conditions, an excess of the individuals' vigor in the attempt to exit causes a decrease in the flow rate. In general, this effect is identified by the appearance of a minimum when plotting the total evacuation time of a crowd as a function of the pedestrian desired velocity. Here, we experimentally show that the FIS effect indeed occurs in three different systems of discrete particles flowing through a constriction: (a) humans evacuating a room, (b) a herd of sheep entering a barn, and (c) grains flowing out a 2D hopper over a vibrated incline. This finding suggests that FIS is a universal phenomenon for active matter passing through a narrowing.

Revista:
PHYSICAL REVIEW E

ISSN 2470-0045
Vol. 91
Año 2015
Págs.022808

We present an experimental study of a flock passing through a narrow door. Video monitoring of daily routines in a farm has enabled us to collect a sizable amount of data. By measuring the time lapse between the passage of consecutive animals, some features of the flow regime can be assessed. A quantitative definition of clogging is demonstrated based on the passage time statistics. These display broad tails, which can be fitted by power laws with a relatively large exponent. On the other hand, the distribution of burst sizes robustly evidences exponential behavior. Finally, borrowing concepts from granular physics and statistical mechanics, we evaluate the effect of increasing the door size and the performance of an obstacle placed in front of it. The success of these techniques opens new possibilities regarding their eventual extension to the management of human crowds.

Revista:
PHYSICAL REVIEW LETTERS

ISSN 0031-9007
Vol. 114
N° 17
Año 2015
Págs.178002

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.

Revista:
PHYSICAL REVIEW E

ISSN 2470-0045
Vol. 91
N° 6
Año 2015
Págs.062203

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.

Revista:
SCIENTIFIC REPORTS

ISSN 2045-2322
Vol. 4
Año 2014
Págs.7324

When a large set of discrete bodies passes through a bottleneck, the flow may become intermittent due to the development of clogs that obstruct the constriction. Clogging is observed, for instance, in colloidal suspensions, granular materials and crowd swarming, where consequences may be dramatic. Despite its ubiquity, a general framework embracing research in such a wide variety of scenarios is still lacking. We show that in systems of very different nature and scale -including sheep herds, pedestrian crowds, assemblies of grains, and colloids- the probability distribution of time lapses between the passages of consecutive bodies exhibits a power-law tail with an exponent that depends on the system condition. Consequently, we identify the transition to clogging in terms of the divergence of the average time lapse. Such a unified description allows us to put forward a qualitative clogging state diagram whose most conspicuous feature is the presence of a length scale qualitatively related to the presence of a finite size orifice. This approach helps to understand paradoxical phenomena, such as the faster-is-slower effect predicted for pedestrians evacuating a room and might become a starting point for researchers working in a wide variety of situations where clogging represents a hindrance.

Revista:
TRANSPORTATION RESEARCH PROCEDIA

ISSN 2352-1465
Vol. 2
Año 2014
Págs.760 - 767

The Faster-Is-Slower effect (Helbing et al (2000)) is an important instance of self-organized phenomenon in pedestrian dynamics. Despite this, an experimental demonstration is still lacking. We present controlled tests where a group of students are asked to exit a room through a door. Instead of just measuring the evacuation times, we have analyzed the probability distribution of the time lapses between consecutive individuals. We show how it displays a power-law tail. This method displays clearly the Faster Is Slower effect, and also allows to assess the impact of several tactics that can be put in place to alleviate the problem.

Revista:
PHYSICAL REVIEW E

ISSN 1539-3755
Vol. 89
N° 4
Año 2014
Págs.042205

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.

Revista:
REVISTA DE EDIFICACION

ISSN 0213-8948
N° 41 - 42
Año 2013
Págs.102 - 107

Los medios granulados ¿materia inerte compuesta por sólidos divididos¿ al pasar por una abertura que sea sólo un poco mayor
al tamaño de las partículas se pueden atascar. Se ha estudiado como disminuye la probabilidad de que este sistema se atasque
gracias a la colocación de un obstáculo delante de la salida. El resultado obtenido demuestra que se disminuye la probabilidad
de atasco dependiendo dónde esté colocado el obstáculo, siendo una posición óptima donde la distancia entre el obstáculo y
la salida es similar al tamaño de la abertura. Estos resultados pueden ser interesantes para el diseño óptimo de una salida de
evacuación.

Revista:
GRANULAR MATTER

ISSN 1434-5021
Vol. 15
N° 6
Año 2013
Págs. 841 - 848

In this work, we examine a quasi-2D silo that clogs due to the spontaneous formation of stable arches. We validate a numerical scheme comparing the morphology of clogging arches with previous experimental findings. Additionally, we inspect the forces that act on particles, both on those in the bulk of the silo as well as those belonging to the arches formed above the outlet. In the silo, we have found that normal forces are higher close to the wall, in contrast to the central part of the silo, where normal forces are notably lower. Besides, it is revealed that normal forces on particles belonging to the clogging arches are significantly larger than in their surroundings. In a particle of the arch, the magnitude of the force strongly depends on the angle subtended from its center to the contact points with its two neighbors in the arch. Indeed, for angles exceeding , the larger the angle, the lower the normal force and the higher the tangential one. On the contrary, for smaller angles the behavior is reversed, so the normal forces increase with the angle. Finally, we present a comparison of the normal and tangential force distributions for the particles within the arch and in the bulk. All this shows the special nature of the forces developed in clogging arches, which suggests that direct extrapolations of bulk properties should not be taken for granted.

Revista:
PLOS ONE

ISSN 1932-6203
Vol. 8
N° 8
Año 2013
Págs.e67838

While ¿vibrational noise¿ induced by rotating components of machinery is a common problem constantly faced by engineers, the controlled conversion of translational into rotational motion or vice-versa is a desirable goal in many scenarios ranging from internal combustion engines to ultrasonic motors. In this work, we describe the underlying physics after isolating a single degree of freedom, focusing on devices that convert a vibration along the vertical axis into a rotation around this axis. A typical Vibrot (as we label these devices) consists of a rigid body with three or more cantilevered elastic legs attached to its bottom at an angle. We show that these legs are capable of transforming vibration into rotation by a ¿ratchet effect¿, which is caused by the anisotropic stick-slip-flight motion of the leg tips against the ground. Drawing an analogy with the Froude number used to classify the locomotion dynamics of legged animals, we discuss the walking regime of these robots. We are able to control the rotation frequency of the Vibrot by manipulating the shaking amplitude, frequency or waveform. Furthermore, we have been able to excite Vibrots with acoustic waves, which allows speculating about the possibility of reducing the size of the devices so they can perform tasks into the human body, excited by ultrasound waves from the outside.

Revista:
PHYSICAL REVIEW LETTERS

ISSN 0031-9007
Vol. 109
N° 6
Año 2012

We present experimental and numerical results regarding the stability of arches against external vibrations. Two-dimensional strings of mutually stabilizing grains are geometrically analyzed and subsequently submitted to a periodic forcing at fixed frequency and increasing amplitude. The main factor that determines the granular arch resistance against vibrations is the maximum angle among those formed between any particle of the arch and its two neighbors: the higher the maximum angle is, the easier it is to break the arch. On the basis of an analysis of the forces, a simple explanation is given for this dependence. From this, interesting information can be extracted about the expected magnitudes of normal forces and friction coefficients of the particles composing the arches.

Revista:
PHYSICAL REVIEW E

ISSN 1539-3755
Vol. 86
N° 3
Año 2012
Págs.031306

In a recent paper [Zuriguel et al., Phys. Rev. Lett. 107, 278001 (2011)] it has been shown that the presence of an obstacle above the outlet can significatively reduce the clogging probability of granular matter pouring from a silo. The amount of this reduction strongly depends on the obstacle position. In this work, we present new measurements to analyze different outlet sizes, extending foregoing results and revealing that the effect of the obstacle is enhanced as the outlet size is increased. In addition, the effect of the obstacle position on the flow rate properties and in the geometrical features of arches is studied. These results reinforce previous evidence of the pressure reduction induced by the obstacle. In addition, it is shown how the mean avalanche size and the average flow rate are not necessarily linked. On the other hand, a close relationship is suggested between the mean avalanche size and the flow rate fluctuations.

Revista:
PHYSICAL REVIEW E

ISSN 1539-3755
Vol. 84
N° 3
Año 2011
Págs.1 - 8

We present experimental data corresponding to a two-dimensional dense granular flow, namely, the gravity-driven discharge of grains from a small opening in a silo. We study the local velocity field at the scale of single grains at different places with the help of particle-tracking techniques. From these data, the velocity profiles can be obtained and the validity of some long-standing approaches can be assessed. Moreover, the fluctuations of the velocities are taken into consideration to characterize the features of the advective motion (due to the gravity force) and the diffusive motion, which shows nontrivial behavior.

Revista:
PHYSICAL REVIEW LETTERS

ISSN 0031-9007
Vol. 107
N° 27
Año 2011
Págs.278001

Revista:
PHYSICAL REVIEW E

ISSN 1539-3755
Vol. 82
N° 3
Año 2010
Págs.31306

We present experimental results on the shape of arches that block the outlet of a two-dimensional silo. For a range of outlet sizes, we measure some properties of the arches such as the number of particles involved, the span, the aspect ratio, and the angles between mutually stabilizing particles. These measurements shed light on the role of frictional tangential forces in arching. In addition, we find that arches tend to adopt an aspect ratio (the quotient between height and half the span) close to 1, suggesting an isotropic load. The comparison of the experimental results with data from numerical models of the arches formed in the bulk of a granular column reveals the similarities of both, as well as some limitations in the few existing models.

Capítulos de libros

Libro:
Encyclopedia of complexity and systems science

Editorial: Springer Science+Business Media BV
Año 2020
Págs.1 - 32

Libro:
EPJ Web of Conferences 140, 01002 (2017). Powders & Grains 2017

Editorial: EDP Sciences
Año 2017
Págs.01002

When a group of discrete particles pass through a narrowing, the flow may become arrested due to the
development of structures that span over the size of the aperture. Then, it is said that the system is clogged. Here,
we will discuss about the existence of a phase diagram for the clogged state that has been recently proposed,
arguing on its usefulness to describe different systems of discrete bodies ranging from granular materials, to
colloidal suspensions and live beings. This diagram is built based on the value of a flowing parameter which
characterizes the intermittent flow observed in all these discrete systems provided that there is an external or
internal energy supply. Such requirement, which is necessary to destabilize the clogging arches, is absent in a
standard static silo, which is therefore examined as a particular case. This view will help to understand some a
priori inconsistencies concerning the role of driving force in the clogging process that have been found in the
last years.

Libro:
EPJ Web of Conferences 140, 04004 (2017). Powders & Grains 2017

Editorial: EDP Sciences
Año 2017
Págs.04004

In a recent work [C. Lozano et al. Phys. Rev. Lett 114, 178002 (2015)] segregation in an horizontally shaken granular layer was studied by analysing the particle-particle interactions in the simplest case possible of a two particles cluster. There, it was found that all clusters are transient (they eventually split if one waits long enough) and the probability distribution function of the separation times displays a power law tail, indicating that the splitting probability is not constant over time. Here, we extend this study to clusters of 3, 5, 10 and 20 particles where we also observe the power law decay of the distribution of cluster splitting time. In addition, we observe a weak increase of the average cluster splitting time with the cluster size, suggesting that interaction forces are non-additive. Finally, we show interesting statistics on the way in which clusters break suggesting that escaping of individual particles in the cluster borders is more likely than cluster breakage in subclusters of similar size.

Libro:
EPJ Web of Conferences 140, 03016 (2017). Powders & Grains 2017

Editorial: EDP Sciences
Año 2017
Págs.03016

Granular flow through an orifice can be suddenly halted by the formation of arches in the vicinity of the outlet, which are stable under the action of gravity. They may be broken when an external driving (for instance, vibration) is applied. With the aim of shedding light on the dynamics of arch destruction, we built an experiment consisting of a vertical two-dimensional silo filled with monodisperse beads, to which a constant vibration is applied. It was previously found that an important parameter to predict the robustness of the arch is the angle between consecutive beads. We focus on long-enduring arches and study the angles among the beads along time. We have found that in many cases the dynamics of the largest angle determines the breaking of the arch; it does not only determine where the ¿weakest link¿ is, but also the process that leads to the final destabilization. This is interesting because it can provide information about whether the flow will resume in a well-defined time or not, which is especially useful for industrial processes that have to constantly deal with the possible emergence of clogs.

Libro:
Powder and grains : Proceedings of the 7th International Conference on Micromechanics of Granular Media

Lugar de Edición: Melville
Editorial: AIP Publishing
Año 2013
Págs.686 - 689

A distinctive feature of discrete solids is their ability to form arches. These mechanically stable structures alter
the isotropy of granular packings and can arrest the motion of grains when, for example, they flow through a bottleneck.
Breaking arches can be achieved by means of an external vibration, which effectively eliminates clogging. Indeed, these
phenomena and procedures are quite common in industrial applications. Nevertheless, there are not rigorous, well founded
criteria to determine the most efficient way to break arches and restore the flow of grains. This happens in part because it is not
known which are the relevant characteristics that boost the arch strength. In the experiment presented here, we have carried
out a statistical analysis of the arches that block the exit orifice at the bottom of a two dimensional silo, and described their
geometrical properties. We then submit the silo to an external vibration. We find that the larger the outlet size, the weaker the
arches that clog it. This dependence is just the outcome of a more complicated process that involves geometrical defects in
the arch. The defects are quantitatively defined in terms of contact angles and we show that this is a key factor regarding the
endurance of arches.

Libro:
Powders and grains 2013 : Proceedings of the 7th International Conference on Micromechanics of Granular Media

Lugar de Edición: Melville
Editorial: AIP Publishing
Año 2013
Págs.710 - 713

Transport of material through pipes or channels in mines or gravel quarries seems to be a simple and economic
form of conveying blasted ore between different levels. Despite the apparent advantages of moving the material by means of
the gravity force, there exists an important problem that makes the applicability of this method more difficult: the election of
the pipe diameter to prevent clogging of the stones. It was R. Kvapil in the sixties who extended the ideas of granular flows in
silos to underground mining. Nevertheless, after his pioneering works there are only a few manuscripts focused on this topic,
and many questions remain unsolved. In this work, we present experimental results about the flow of particles (gravel) driven
by gravity through tilted tubes. The amount of material discharged between clogs shows that the probability of clogging can
be estimated by the same procedures introduced for silos. Finally, by changing the ratio between the tube diameter and the
typical particle size, we discuss about the existence or not of a critical size beyond which clogging is not possible.

Libro:
Powders and grains 2013 : Proceedings of the 7th International Conference on Micromechanics of Granular Media

Lugar de Edición: Melville
Editorial: AIP Publishing
Año 2013
Págs.698 - 701

We present an experimental study of the effect that an obstacle above the outlet of a silo has on the clogging
probability. Both, the size of the orice and the obstacle position are varied for a chosen obstacle size and shape. If the
position of the obstacle is properly selected the clogging probability can be importantly reduced. Indeed, as the outlet size
is increased ¿ and we approach the critical size above which there is not clogging ¿ the obstacle effect is enhanced. For the
largest outlet size studied, the clogging probability is reduced by a factor of more than one hundred. We will show, using
numerical simulations, that the physical parameter behind the reduction of the silo clogging seems to be the decrease of the
vertical pressure at the outlet proximities.