# Browsing by Author "Behringer, RP"

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Item Open Access Dense granular flow - A collaborative study(Powder Technology, 2015-11-01) Mort, P; Michaels, JN; Behringer, RP; Campbell, CS; Kondic, L; Kheiripour Langroudi, M; Shattuck, M; Tang, J; Tardos, GI; Wassgren, C© 2015 Elsevier B.V..The International Fine Powder Research Institute (IFPRI) has funded an extensive program in dry powder and granular flows, including a focused study on dense flows of interest to a range of industrial handling and process unit operations, especially dense flows at relatively high shear rates. The dense flow program included experimental studies of granular rheology in 3D axial Couette and 2D hopper geometries, wherein the effect of force chains and jamming interactions were investigated as relevant to flow, stress and packing dynamics. The program cumulated in a collaborative study funded by the NSF, wherein a group of academic collaborators was invited to model experimental systems used in IFPRI-sponsored projects. This paper provides a summary of the IFPRI program, details of the collaborative modeling study, and perspective on what is needed to progress the work further.Item Open Access Granular Impact Dynamics: Acoustics and FluctuationsClark, Abram H; Behringer, RPIn the corresponding fluid dynamics video, created for the APS DFD 2012 Gallery of Fluid Motion, we show high-speed videos of 2D granular impact experiments, where an intruder strikes a collection of bidisperse photoelastic disks from above. We discuss the force beneath the intruder, which is strongly fluctuating in space and time. These fluctuations correspond to acoustic pulses which propagate into the medium. Analysis shows that this process, in our experiments, is dominated by collisions with grain clusters. The energy from these collisions is carried into the granular medium along networks of grains, where is it dissipated.Item Open Access Jamming for a 2D granular material(Soft Matter, 2010-07-07) Zhang, J; Majmudar, TS; Sperl, M; Behringer, RPThis paper focuses on the nature of jamming, as seen in two-dimensional frictional granular systems consisting of photoelastic particles. The photoelastic technique is unique at this time, in its capability to provide detailed particle-scale information on forces and kinematic quantities such as particle displacements and rotations. These experiments first explore isotropic stress states near point J through measurements of the mean contact number per particle, Z, and the pressure, P as functions of the packing fraction, . In this case, the experiments show some but not all aspects of jamming, as expected on the basis of simulations and models that typically assume conservative, hence frictionless, forces between particles. Specifically, there is a rapid growth in Z, at a reasonable which we identify with as c. It is possible to fit Z and P, to power law expressions in - c above c, and to obtain exponents that are in agreement with simulations and models. However, the experiments differ from theory on several points, as typified by the rounding that is observed in Z and P near c. The application of shear to these same 2D granular systems leads to phenomena that are qualitatively different from the standard picture of jamming. In particular, there is a range of packing fractions below c, where the application of shear strain at constant leads to jammed stress-anisotropic states, i.e. they have a non-zero shear stress, τ. The application of shear strain to an initially isotropically compressed (hence jammed) state, does not lead to an unjammed state per se. Rather, shear strain at constant first leads to an increase of both τ and P. Additional strain leads to a succession of jammed states interspersed with relatively localized failures of the force network leading to other stress-anisotropic states that are jammed at typically somewhat lower stress. The locus of jammed states requires a state space that involves not only and τ, but also P. P, τ, and Z are all hysteretic functions of shear strain for fixed . However, we find that both P and τ are roughly linear functions of Z for strains large enough to jam the system. This implies that these shear-jammed states satisfy a Coulomb like-relation, τ = μP. © 2010 The Royal Society of Chemistry.Item Open Access Orientation, flow, and clogging in a two-dimensional hopper: Ellipses vs. disks(EPL, 2016-05-01) Tang, J; Behringer, RP© EPLA, 2016.Two-dimensional (2D) hopper flow of disks has been extensively studied. Here, we investigate hopper flow of ellipses with aspect ratio α = 2, and we contrast that behavior to the flow of disks. We use a quasi-2D hopper containing photoelastic particles to obtain stress/force information. We simultaneously measure the particle motion and stress. We determine several properties, including discharge rates, clogging probabilities, and the number of particles in clogging arches. For both particle types, the size of the opening, D, relative to the size of particles, ℓ, is an important dimensionless measure. The orientation of the ellipses plays an important role in flow rheology and clogging. The alignment of contacting ellipses enhances the probability of forming stable arches. This study offers insights into applications involving the flow of granular materials consisting of ellipsoidal shapes, and possibly other non-spherical shapes.Item Open Access Packings of 3D stars: stability and structure(Granular Matter, 2016-05-01) Zhao, Y; Liu, K; Zheng, M; Barés, J; Dierichs, K; Menges, A; Behringer, RP© 2016, Springer-Verlag Berlin Heidelberg.We describe a series of experiments involving the creation of cylindrical packings of star-shaped particles, and an exploration of the stability of these packings. The stars cover a broad range of arm sizes and frictional properties. We carried out three different kinds of experiments, all of which involve columns that are prepared by raining star particles one-by-one into hollow cylinders. As an additional part of the protocol, we sometimes vibrated the column before removing the confining cylinder. We rate stability in terms of r, the ratio of the mass of particles that fall off a pile when it collapsed, to the total particle mass. The first experiment involved the intrinsic stability of the column when the confining cylinder was removed. The second kind of experiment involved adding a uniform load to the top of the column, and then determining the collapse properties. A third experiment involved testing stability to tipping of the piles. We find a stability diagram relating the pile height, h, versus pile diameter, (Formula presented.) , where the stable and unstable regimes are separated by a boundary that is roughly a power-law in h versus (Formula presented.) with an exponent that is less than unity. Increasing vibration and friction, particularly the latter, both tend to stabilize piles, while increasing particle size can destabilize the system under certain conditions.Item Open Access Self-organized magnetic particles to tune the mechanical behavior of a granular system(EPL, 2016-09-01) Cox, M; Wang, D; Barés, J; Behringer, RP© 2016, EPLA.Above a certain density a granular material jams. This property can be controlled by either tuning a global property, such as the packing fraction or by applying shear strain, or at the micro-scale by tuning grain shape, inter-particle friction or externally controlled organization. Here, we introduce a novel way to change a local granular property by adding a weak anisotropic magnetic interaction between particles. We measure the evolution of the pressure, P, and coordination number, Z, for a packing of 2D photo-elastic disks, subject to uniaxial compression. A fraction R m of the particles have embedded cuboidal magnets. The strength of the magnetic interactions between particles is too weak to have a strong direct effect on P or Z when the system is jammed. However, the magnetic interactions play an important role in the evolution of latent force networks when systems containing a large enough fraction of the particles with magnets are driven through unjammed to jammed states. In this case, a statistically stable network of magnetic chains self-organizes before jamming and overlaps with force chains once jamming occurs, strengthening the granular medium. This property opens a novel way to control mechanical properties of granular materials.Item Open Access Shear-induced rigidity of frictional particles: Analysis of emergent order in stress space.(Phys Rev E, 2016-04) Sarkar, Sumantra; Bi, Dapeng; Zhang, Jie; Ren, Jie; Behringer, RP; Chakraborty, BulbulSolids are distinguished from fluids by their ability to resist shear. In equilibrium systems, the resistance to shear is associated with the emergence of broken translational symmetry as exhibited by a nonuniform density pattern that is persistent, which in turn results from minimizing the free energy. In this work, we focus on a class of systems where this paradigm is challenged. We show that shear-driven jamming in dry granular materials is a collective process controlled by the constraints of mechanical equilibrium. We argue that these constraints can lead to a persistent pattern in a dual space that encodes the statistics of contact forces and the topology of the contact network. The shear-jamming transition is marked by the appearance of this persistent pattern. We investigate the structure and behavior of patterns both in real space and the dual space as the system evolves through the rigidity transition for a range of packing fractions and in two different shear protocols. We show that, in the protocol that creates homogeneous jammed states without shear bands, measures of shear jamming do not depend on strain and packing fraction independently but obey a scaling form with a packing-fraction-dependent characteristic strain that goes to zero at the isotropic jamming point ϕ_{J}. We demonstrate that it is possible to define a protocol-independent order parameter in this dual space, which provides a quantitative measure of the rigidity of shear-jammed states.Item Open Access Spanning the scales of granular materials through microscopic force imaging.(Nat Commun, 2015-03-05) Brodu, N; Dijksman, JA; Behringer, RPIf you walk on sand, it supports your weight. How do the disordered forces between particles in sand organize, to keep you from sinking? This simple question is surprisingly difficult to answer experimentally: measuring forces in three dimensions, between deeply buried grains, is challenging. Here we describe experiments in which we have succeeded in measuring forces inside a granular packing subject to controlled deformations. We connect the measured micro-scale forces to the macro-scale packing force response with an averaging, mean field calculation. This calculation explains how the combination of packing structure and contact deformations produce the observed nontrivial mechanical response of the packing, revealing a surprising microscopic particle deformation enhancement mechanism.Item Open Access Stress correlations in granular materials: An entropic formulation(Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2009-12-28) Lois, G; Zhang, J; Majmudar, TS; Henkes, S; Chakraborty, B; O'Hern, CS; Behringer, RPWe study the response of dry granular materials to external stress using experiment, simulation, and theory. We derive a Ginzburg-Landau functional that enforces mechanical stability and positivity of contact forces. In this framework, the elastic moduli depend only on the applied stress. A combination of this feature and the positivity constraint leads to stress correlations whose shape and magnitude are extremely sensitive to the nature of the applied stress. The predictions from the theory describe the stress correlations for both simulations and experiments semiquantitatively. © 2009 The American Physical Society.Item Open Access Structural templates of disordered granular media(International Journal of Solids and Structures, 2015-01-01) Walker, DM; Tordesillas, A; Zhang, J; Behringer, RP; Andò, E; Viggiani, G; Druckrey, A; Alshibli, K© 2014 Elsevier Ltd. All rights reserved.Granular materials, in common with many complex systems, exhibit a range of self-organization processes that control their mechanical performance. Many of these processes directly manifest in the evolution of the contact network as the material responds to applied stresses and strains. Yet the connections between the topology, structure and dynamics of this evolving contact network remain poorly understood. Here we demonstrate that dense granular systems under a variety of loading conditions exhibit preferred structural ordering reminiscent of a superfamily classification. In particular, two distinct superfamilies are discovered: the first is typically exhibited by materials in the pre-failure regime, while the second manifests in the unstable or failure regime. We demonstrate the robustness of these findings with respect to a range of packing fractions in experimental sand and photoelastic disk assemblies subject to compression and shear, as well as in a series of discrete element simulations of compression tests. We show that the superfamily classification of small connected subgraphs in a granular material can be used to map boundaries in a so-called jamming phase diagram and, consequently, offers a key opportunity to bridge the mechanics and physics perspectives on the constitutive behavior of granular systems.Item Open Access The Jamming point street-lamp in the world of granular media(Soft Matter) Coulais, C; Behringer, RP; Dauchot, OThe Jamming of soft spheres at zero temperature, the J-point, has been extensively studied both numerically and theoretically and can now be considered as a safe location in the space of models, where a street lamp has been lit up. However, a recent work by Ikeda et al, 2013 reveals that, in the Temperature/Packing fraction parameter space, experiments on colloids are actually rather far away from the scaling regime illuminated by this lamp. Is it that the J-point has little to say about real system? What about granular media? Such a-thermal, frictional, systems are a-priori even further away from the idealized case of thermal soft spheres. In the past ten years, we have systematically investigated horizontally shaken grains in the vicinity of the Jamming transition. We discuss the above issue in the light of very recent experimental results. First, we demonstrate that the contact network exhibits a remarkable dynamics, with strong heterogeneities, which are maximum at a packing fraction phi star, distinct and smaller than the packing fraction phi dagger, where the average number of contact per particle starts to increase. The two cross-overs converge at point J in the zero mechanical excitation limit. Second, a careful analysis of the dynamics on time scales ranging from a minute fraction of the vibration cycle to several thousands of cycles allows us to map the behaviors of this shaken granular system onto those observed for thermal soft spheres and demonstrate that some light of the J-point street-lamp indeed reaches the granular universe.