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Item Clogging transition of many particle systems flowing through bottlenecks(Scientific Reports, 2014) Zuriguel, Iker; Parisi, Daniel; Cruz Hidalgo, Raul; Lozano, Celia; Janda, Alvaro; Gago, Paula; Peralta, Juan; Ferrer, Luis; Pugnaloni, Luis; Clement, Eric; Maza, Diego; Pagonabarraga, Ignacio; Garcimartín, AngelWhen 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.Item Experimental proof of faster is slower in systems of frictional particles flowing through constrictions(American Physical Society, 2015) Pastor, Jose; Garcimartín, Angel; Gago, Paula; Peralta, Juan; Martín Gomez, Cesar; Ferrer, Luis; Maza, Diego; Parisi, Daniel; Pugnaloni, Luis; Zuriguel, IkerThe “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.Item Relevance of system size to the steady state properties of tapped granular systems(American Physical Society, 2015) Gago, Paula; Maza, Diego; Pugnaloni, LuisWe investigate the steady-state packing fraction ϕ and force moment tensor Σ of quasi-two-dimensional granular columns subjected to tapping. Systems of different height h and width L are considered. We find that ϕ and Σ, which describe the macroscopic state of the system, are insensitive to L for L>50d (with d the grain diameter). However, results for granular columns of different heights cannot be conciliated. This suggests that comparison between results of different laboratories on this type of experiments can be done only for systems of same height. We show that a parameter ɛ=1+(Aω)2/(2gh), with A and ω the amplitude and frequency of the tap and g the acceleration of gravity, can be defined to characterize the tap intensity. This parameter is based on the effective flight of the granular bed, which takes into account the h dependency. When ϕ is plotted as a function of ɛ, the data collapses for systems of different h. However, this parameter alone is unable to determine the steady state to be reached since different Σ can be observed for a given ɛ if different column heights are considered.Item Ergodic nonergodic transition in tapped granular systems; The role of persistent contacts(Papers in Physics, 2016) Gago, Paula; Maza, Diego; Pugnaloni, LuisStatic granular packs have been studied in the last three decades in the frame of a modi ed equilibrium statistical mechanics that assumes ergodicity as a basic postulate. The canonical example on which this framework is tested consists in the series of static con gurations visited by a granular column subjected to taps. By analyzing the response of a realistic model of grains, we demonstrate that volume and stress variables visit di erent regions of the phase space at low tap intensities in di erent realizations of the experiment. We show that the tap intensity beyond which sampling by tapping becomes ergodic coincides with the forcing necessary to break all particle{particle contacts during each tap. These results imply that the well-known \reversible" branch of tapped granular columns is only valid at relatively high tap intensities.Item Tapped granular packings described as complex networks(Taylor and Francis, 2013) Arévalo, Roberto; Pugnaloni, Luis; Maza, Diego; Zuriguel, IkerWe characterize the structure of simulated two-dimensional granular packings using con- cepts from complex networks theory. The packings are generated by a simulated tapping protocol, which allows us to obtain states in mechanical equilibrium in a wide range of densi- ties. We show that our characterization method is able to discriminate non-equivalent states that have the same density. We do this by examining differences in the topological structure of the contact network of the packings. In particular, we find that the polygons of the network are specially sensitive probes for the contact structure. Additionally, we compare the network properties obtained in two different scenarios: the tapped and a compressed system.Item Contact network topology in tapped granular media(American Physical Society, 2013) Arévalo, Roberto; Pugnaloni, Luis; Zuriguel, Iker; Maza, DiegoWe 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.