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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.Item Exact predictions from Edwards ensemble vs realistic simulations of tapped narrow two dimensional granular columns(Journal of Statistical Mechanics: Theory and Experiment, 2013) Irastorza, Ramiro; Carlevaro, Manuel; Pugnaloni, LuisWe simulate, via a discrete element method, the tapping of a narrow column of disks under gravity. For frictionless disks, this system has a simple analytical expression for the density of states in the Edwards volume ensemble. We compare the predictions of the ensemble at constant compactivity against the results for the steady states obtained in the simulations. We show that the steady states cannot be properly described since the microstates sampled are not in correspondence with the predicted distributions, suggesting that the postulates of flat measure and ergodicity are, either or both, invalid for this simple realization of a static granular system. However, we show that certain qualitative features of the volume fluctuations which are difficult to predict from simple arguments are captured by the theory.Item First order phase transition during displacement of amphiphilic biomacromolecules from interfaces by surfactant molecules(Journal of Physics: Condensed Matter, 2014) Ettelaie, Rammile; Dickinson, Eric; Pugnaloni, LuisThe adsorption of surfactants onto a hydrophobic interface, already laden with a fixed number of polymers, is studied using the self consistent field (SCF) calculation method of Scheutjens and Fleer. For polymers having unfavourable interactions with the surfactant molecules, the adsorption isotherms show an abrupt jump at a certain value of surfactant bulk concentration. Alternatively, the same behaviour is exhibited when the number of polymers on the interface is decreased. We show that this sudden jump is associated with a first-order phase transition, by calculating the free energy values for the stable and the metastable states at both sides of the transition point. We also observe that the transition can occur for two approaching surfaces, from a high surfactant coverage phase to a low surfactant coverage one, at sufficiently close separation distances. The consequence of this finding for the steric colloidal interactions, induced by the overlap of two polymer + surfactant films, is explored. In particular, a significantly different interaction, in terms of its magnitude and range, is predicted for these two phases. We also consider the relevance of the current study to problems involving the competitive displacement of proteins by surfactants in food colloid systems.Item Arching during the segregation of two dimensional tapped granular systems Mixtures versus intruders(Springer, 2014) Uñac, Rodolfo; Benito, Jesica; Vidales, Ana María; Pugnaloni, LuisWe present numerical simulations of binary mixtures of granular disks subjected to tapping. We consider the size segregation process in terms of the arches formed by small and big particles. Although arching has been proposed as one of the chief mechanisms that determines size segregation in non-convecting systems, there is no direct data on arching to support the existing proposals. The pseudo-dynamic approach chosen for this work allows for a straightforward identification of arches in the bulk of the column. We find that, indeed, arch formation and breakage are crucial to the segregation process. Our results show that the presence of large particles induce the formation of more arches than found in mono-sized samples. However, tapping leads to the progressive breakage of big arches where large particles are involved as the segregation process takes place. Interestingly, isolated intruders may or may not rise under tapping depending not only on the size ratio (as it is well known) but also on the degree of ordering of the environment.Item Effect of particle shape and fragmentation on the response of particle dampers(SAGE Publications, 2014) Sánchez, Martín; Carlevaro, Manuel; Pugnaloni, LuisA particle damper (PD) is a device that can attenuate mechanical vibrations thanks to the dissipative collisions between grains contained in a cavity attached to the vibrating structure. It has been recently suggested that, under working conditions in which the damping is optimal, the PD has a universal response in the sense that the specific dissipative properties of the grains cease to be important for the design of the device. We present evidence from simulations of PDs containing grains of different sizes, shapes and restitution coefficients, that the universal response is also valid when fragmentation of the grains occurs (generally due to intensive operation of the PD). In contrast, the welding of grains (caused by operation under high temperatures) can take the PD out of the universal response and deteriorate the attenuation. Interestingly, we observed that even at working conditions off the optimal damping, the shape of the grains remains unimportant for the response of the PD.Item On and off dynamics of a creeping frictional system(Springer, 2014) Blanc, Baptiste; Geminard, Jean Christophe; Pugnaloni, LuisWe report on the dynamics of a model frictional system submitted to minute external perturbations. The system consists of a chain of sliders connected through elastic springs that rest on an incline. By introducing cyclic expansions and contractions of the rest length of the springs, we induce the reptation of the chain. Decreasing the amplitude of the perturbation below a critical value, we observe an intermittent creep regime characterized by alternated periods of reptation (owing state) and rest (quiescent state). A further decrease of the perturbation leads to the disappearance of the reptation. The width of the transition region between the continuous creep and the full stop (i.e., the range of excitation amplitudes where the intermittent creep is observed) is shown to depend on the di_erence between the static (µs) and the dynamic (µd) friction coefficients. For µs = µd the intermittent creep is not observed. Studying the statistical features of the intermittent creep regime for any given perturbation amplitude, we find that the time the system resides in each state (owing or quiescent) suggests that: (i) reptation events are uncorrelated, and (ii) rest events are history dependent. We show that this latter history dependency is consistent with the aging of the stress state inside the chain of sliders during the quiescent periods.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 Dynamic transition in conveyor belt driven granular flow(Elsevier, 2015) Cordero, José; Pugnaloni, LuisWe consider the flow of disks of diameter d driven by a conveyor belt of dynamic friction coefficient μ through an aperture on a flat barrier. The flow rate presents two distinct regimes. At low belt velocities v the flow rate is proportional to v and to the aperture width A. However, beyond a critical velocity, the flow rate becomes independent of v and proportional to (A − kd)3/2 in correspondence with a two-dimensional Beverloo scaling. In this high-velocity regime we also show that the flow rate is proportional to μ1/2. We discuss that these contrasting behaviors arise from the competition between two characteristic time scales: the typical time a disk takes to stop on the belt after detaching from the granular pack and the time it takes to reach the aperture.Item Flow rate of polygonal grains through a bottleneck Interplay between shape and size(Papers in Physics, 2015) Goldberg, Ezequiel; Carlevaro, Manuel; Pugnaloni, LuisWe report two-dimensional simulations of circular and polygonal grains passing through an aperture at the bottom of a silo. The mass flow rate for regular polygons is lower than for disks\red{,} as observed by other authors. We show that both the exit velocity of the grains and the packing fraction are lower for polygons, which leads to the reduced flow rate. We point out the importance of the criteria used to define when two objects of different shape are considered to be of the same size. Depending on this criteria, the mass flow rate may vary significantly for some polygons. Moreover, the particle flow rate is non-trivially related to a combination of mass flow rate, particle shape and particle size. For some polygons, the particle flow rate may be lower or higher than that of the corresponding disks depending on the size comparison criteria.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 Wang Landau algorithm for entropic sampling of arch based microstates in the volume ensemble of static granular packings(Papers in Physics, 2015) Slobinsky, Demian; Pugnaloni, LuisWe implement the Wang-Landau algorithm to sample with equal probabilities the static configurations of a model granular system. The "non-interacting rigid arch model" used is based on the description of static configurations by means of splitting the assembly of grains into sets of stable arches. This technique allows us to build the entropy as a function of the volume of the packing for large systems. We make a special note of the details that have to be considered when defining the microstates and proposing the moves for the correct sampling in these unusual models. We compare our results with previous exact calculations of the model made at moderate system sizes. The technique opens a new opportunity to calculate the entropy of more complex granular models.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 Arch based configurations in the volume ensemble of static granular systems(Journal of Statistical Mechanics: Theory and Experiment, 2015) Slobinsky, D.; Pugnaloni, LuisWe propose an alternative approach to count the microscopic static configurations of granular packs under gravity by considering arches. This strategy obviates the problem of filtering out configurations that are not mechanically stable, opening the way for a range of granular models to be studied via ensemble theory. Following this arch-based approach, we have obtained the exact density of states for a 2D, non-interacting rigid arch model of granular assemblies. The calculated arch size distribution and volume fluctuations show qualitative agreement with realistic simulations of tapped granular beds. We have also validated our calculations by comparing them with the analytic solution for the limiting case of a quasi-1D column of frictionless disks.Item Structure of force networks in tapped particulate systems of disks and pentagons I Clusters and loops(American Physical Society, 2016) Pugnaloni, Luis; Carlevaro, Manuel; Kramar, M; Mischaikow, K; Kondic, LThe force network of a granular assembly, defined by the contact network and the corresponding contact forces, carries valuable information about the state of the packing. Simple analysis of these networks based on the distribution of force strengths is rather insensitive to the changes in preparation protocols or to the types of particles. In this and the companion paper [Kondic et al., Phys. Rev. E 93, 062903 (2016)], we consider two-dimensional simulations of tapped systems built from frictional disks and pentagons, and study the structure of the force networks of granular packings by considering network’s topology as force thresholds are varied. We show that the number of clusters and loops observed in the force networks as a function of the force threshold are markedly different for disks and pentagons if the tangential contact forces are considered, whereas they are surprisingly similar for the network defined by the normal forces. In particular, the results indicate that, overall, the force network is more heterogeneous for disks than for pentagons. Such differences in network properties are expected to lead to different macroscale response of the considered systems, despite the fact that averaged measures (such as force probability density function) do not show any obvious differences. Additionally, we show that the states obtained by tapping with different intensities that display similar packing fraction are difficult to distinguish based on simple topological invariants.Item Structure of force networks in tapped particulate systems of disks and pentagons II Persistence analysis(American Physical Society, 2016) Kondic, L.; Kramar, M.; Pugnaloni, Luis; Carlevaro, Manuel; Mischaikow, K.In the companion paper [Pugnaloni et al., Phys. Rev. E 93, 062902 (2016)], we use classical measures based on force probability density functions (PDFs), as well as Betti numbers (quantifying the number of components, related to force chains, and loops), to describe the force networks in tapped systems of disks and pentagons. In the present work, we focus on the use of persistence analysis, which allows us to describe these networks in much more detail. This approach allows us not only to describe but also to quantify the differences between the force networks in different realizations of a system, in different parts of the considered domain, or in different systems. We show that persistence analysis clearly distinguishes the systems that are very difficult or impossible to differentiate using other means. One important finding is that the differences in force networks between disks and pentagons are most apparent when loops are considered: the quantities describing properties of the loops may differ significantly even if other measures (properties of components, Betti numbers, force PDFs, or the stress tensor) do not distinguish clearly or at all the investigated systems.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 Effect of grain shape on the jamming of two dimensional silos(EPJ Web of Conferences, 2017) Goldberg, Ezequiel; Carlevaro, Manuel; Pugnaloni, LuisWe present discrete element method simulations of the discharge of silos in two dimensions. We study the effect of the grain shape on the clogging of small apertures, considering regular polygons and disks of equal mass. In particular, we analyze the avalanche size distribution and the jamming probability for disks, triangles, squares, pentagons, hexagons and heptagons as a function of the aperture size. We show that the jamming probability presents a non-linear response as a function of the number of vertexes of the polygons.Item A differential equation for the flow rate during silo discharge Beyond the Beverloo rule(EPJ Web of Conferences, 2017) Madrid, Marcos A.; Darias, José Ramón; Pugnaloni, LuisWe present a differential equation for the flow rate of granular materials during the discharge of a silo. This is based in the energy balance of the variable mass system in contrast with the traditional derivations based on heuristic postulates such as the free fall arch. We show that this new equation is consistent with the well known Beverloo rule, providing an independent estimate for the universal Beverloo prefactor. We also find an analytic expression for the pressure under discharging conditions.Item Apparent mass during silo discharge Nonlinear effects related to filling protocols(Elsevier, 2017) Peralta, Juan Pablo; Aguirre, María; Geminard, Jean Christophe; Pugnaloni, LuisWe study the evolution of the force exerted by a granular column on the bottom surface of a silo during its discharge. Previous to the discharge, we prepare the system using different filling procedures: distributed, i.e. a homogeneous rain of grains across the cross-section of the silo; concentric, a granular jet along the silo axis; and a combination of both, i.e. filling half of the silo using one procedure and the second half using the other. We observe that each filling protocol leads to distinctive evolutions of the apparent mass (i.e., the effective weight sensed at the base) during the discharge. Interestingly, the use of combined filling protocols may lead to a reduced apparent mass, smaller than any other achieved with a simple filling. We propose a model based on the Janssen rationale that quantitatively accounts for the latter puzzling experimental observation.