Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P15: Granular Matter |
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Sponsoring Units: GSNP GSOFT Chair: Abe Clark, Yale University Room: 274 |
Wednesday, March 15, 2017 2:30PM - 2:42PM |
P15.00001: Outflow and clogging of shape-anisotropic grains in hoppers Ralf Stannarius, Ahmed Ashour, Sandra Wegner, Tamas Börzsönyi Silos have been in use in human history for millennia, but still today, the discharge of grains from silos is a process with potential risks and imponderabilities. Models and quantitative predictions have been developed almost exclusively for spherical grains shapes. We study the discharge and clogging processes of shape-anisotropic grains in hoppers, and describe the peculiarities of these materials both in their dynamical properties and in the observed clogging structures. An attempt is made to adapt the well-known equations for spherical material to describe anisometric particles. [Preview Abstract] |
Wednesday, March 15, 2017 2:42PM - 2:54PM |
P15.00002: The effect of particle shape on granular flow Eli Owens, Salem Wright It has long been observed that the pressure at the bottom of a granular container, for instance a grain silo, saturates as the height of the container increases relative to its width. However, the precise effect grain shape has on the buildup of sidewall pressure is not well understood. Using a model silo, we investigated the influence of grain shape on sidewall pressure during the filling process. Our silo is 125 cm tall and 16 cm in diameter, and it is filled with either corn, peas, or rice via a cone shaped hopper. As the silo fills, we monitor the pressure the grains exert on four sections of the wall. We see that the corn and peas behave very differently from the rice. When using the rice, the pressure frequently reaches a peak value and then decays with time. We attribute this decay to rice's large aspect ratio which causes grains higher in the silo to jam and shield the lower grains from the weight above. However, this decay is not as pronounced when using the peas or corn. Since the peas and corn are more round, they can more easily rearrange than the irregular rice particles. As a result, they are not as effective at screening the pressure. [Preview Abstract] |
Wednesday, March 15, 2017 2:54PM - 3:06PM |
P15.00003: Collapse of Hexapod Packings under Vibration Yuchen Zhao, Jingqiu Ding, Jonathan Bar\'{e}s, Hu Zheng, Karola Dierichs, Achim Menges, Robert Behringer Columns made of convex non-cohesive grains like sand collapse after being released from a confining container. However, structures built from non-convex grains can be stable without external support. Our previous research shows that thinner and taller columns collapse with higher probability. While the column stability weakly depends on packing density, it strongly depends on inter-particle friction. Experiments that cause columns to collapse also reveal a similar trend, as more effort (such as heavy loading or shearing) is required to destabilize columns that are intrinsically more stable. In the current experiments, we investigate the effect of vibration on destroying stable columns of hexapods. Under vertical, sinusoidal vibrations, the collapses of short columns are well approximated by stretched exponentials, a function which has successfully described relaxation of disordered systems including collapse of stable staple packings. However, tall columns collapse faster at the beginning, in addition to the relaxation process coming after. Using high-speed imaging, we analyze column collapse data from different column geometries. Ongoing work is focusing on characterizing the stability of hexapod packings subjected to vibration. [Preview Abstract] |
Wednesday, March 15, 2017 3:06PM - 3:18PM |
P15.00004: Eshelby inclusions in granular matter: from simulations to experiments Jérôme Crassous, Axelle Amon, Sean McNamara We present a numerical implementation of an active inclusion in a granular material submitted to a biaxial test. We discuss the dependence of the response to this perturbation on two parameters: the intragranular friction coefficient on one hand, and the degree of the loading on the other hand. We compare the numerical results to theoretical predictions taking into account the change of volume of the inclusion as well as the anisotropy of the elastic matrix. We compare the results of this numerical study with experimental measurements of strain fluctuations into a granular material near failure. [Preview Abstract] |
Wednesday, March 15, 2017 3:18PM - 3:30PM |
P15.00005: Mechanisms of intruder motion in cyclically sheared granular media Hu Zheng, Jonathan Barés, Dong Wang, Robert Behringer We perform an experimental study showing how an intruder, a Teflon disk that experiences a moderate constant force, F, can advance through a granular material that is subject to quasi-static cyclic shear. The large Teflon disk is embedded in a layer of smaller bidisperse photoelastic disks. The granular medium and disk are contained in a horizontal cell, which is deformed from a square to a parallelogram and back again. The area of the cell remains constant throughout, and the protocol corresponds to cyclical simple shear. We find that the net intruder motion relative to the granular background occurs primarily following strain reversals. [Preview Abstract] |
Wednesday, March 15, 2017 3:30PM - 3:42PM |
P15.00006: Helical propulsion in granular media Alejandro Ibarra, Baptiste Darbois Texier, Fransisco Melo It is known that the forces experienced by a slender body moving into a granular media present similar trends that the ones undergone in a viscous fluid at low Reynolds numbers. Thus, the solutions employed by the living kingdom in order to move in fluids in such conditions are expected to operate in granular media at a larger scale. The present work investigates the case of the helical propulsion into a granular material. The horizontal velocity of a rotating helix has been studied depending on its geometry, its rotation speed and the properties of the granular material. Our observations are proved to be consistent with a simple modelisation of the problem based on the anisotropical friction force experienced by the helix. The previous theoretical analysis also provides the optimal geometry of the helix in order to maximize the locomotion speed. Finally, our results were used to build an autonomous robot able to progress in non-cohesive materials such as sand. The optimal design of such a robot in term of energy consumption and moving speed will be discussed. [Preview Abstract] |
Wednesday, March 15, 2017 3:42PM - 3:54PM |
P15.00007: The dependence of granular plasticity on particle shape. Kieran Murphy, Heinrich Jaeger Granular materials plastically deform through reworking an intricate network of particle-particle contacts.~ Some particle rearrangements have only a fleeting effect before being forgotten while others set in motion global restructuring.~ How particle shape affects local interactions and how those, in turn, influence the nature of the aggregate's plasticity is far from clear, especially in three dimensions.~ Here we investigate the remarkably wide range of behaviors in the yielding regime, from quiescent flow to violent jerks, depending on particle shape.~ We study this complex dependence via uniaxial compression experiments on aggregates of 3D-printed particles, and complement stress-strain data with simultaneous x-ray videos and volumetric strain measurements.~ We find power law distributions of the slip magnitudes, and discuss their universality.~ Our data show that the multitude of small slips serves to gradually dilate the packing whereas the fewer large ones accompany significant compaction events.~ Our findings provide new insights into general features of granular materials during plastic deformation and highlight how small changes in particle shape can give rise to drastic differences in yielding behavior. [Preview Abstract] |
Wednesday, March 15, 2017 3:54PM - 4:06PM |
P15.00008: Densest Local Structures of Uniaxial Ellipsoids Sebastian C Kapfer, Fabian M Schaller, Robert FB Weigel Connecting the collective behavior of disordered systems with local structure on the particle scale is an important challenge in granular and glassy systems. In many scientific and industrial applications, particles are polydisperse, aspherical, or even of varying shape. Here, we investigate a generalization of the classical kissing problem in order to understand the local building blocks of packings of aspherical grains. We numerically determine the densest local structures of uniaxial ellipsoids by minimizing the Set Voronoi cell volume around a given particle. Depending on the particle aspect ratio, different local structures are observed and classified by symmetry and Voronoi coordination number. In extended disordered packings of frictionless particles, knowledge of the densest structures allows us to rescale the Voronoi volume distributions onto the single-parameter family of k-Gamma distributions. Moreover, we find that approximate icosahedral clusters are found in random packings, while the optimal local structures for more aspherical particles are not formed. [Preview Abstract] |
Wednesday, March 15, 2017 4:06PM - 4:18PM |
P15.00009: Jamming transition in granular systems of regular polygons Cacey Stevens Bester, Yiqiu Zhao, Jonathan Barés, Yuanyuan Xu, Meredith Cox, Robert Behringer The study of the onset of mechanical stability, known as the jamming transition, of granular systems provides key insights into properties of amorphous materials. A fundamental challenge to understanding this transition is to determine the influence of particle properties. Here, we investigate how nontrivial particle shapes affect the jamming transition as controlled by the packing fraction. Our experiments are performed by compression of two-dimensional arrangements of photoelastic particles, allowing us to visualize force information. To explore the role of particle shape, we systematically change the number of sides of polygonal particles used in the experiments and compare the force chain network, contact number and pressure evolution of compressed systems of polygons to the well-studied systems of disks. We also explore the influence of geometric features, such as face-face contacts and ordering within packings, in connection with the jamming transition. [Preview Abstract] |
Wednesday, March 15, 2017 4:18PM - 4:30PM |
P15.00010: Random close packing of rods in confinement Julian Freeman, Eric Weeks We conduct experiments to observe the effects surfaces have on the internal packing structure of particles. In order to observe this, we run an experiment using cylindrical containers of different diameters, and rods of aspect ratios ranging from 4 to 32. We find that the rods packed into smaller cylindrical containers yielded lower volume fractions than in larger containers. Our results are extrapolated to an infinite container size, and the subsequent volume fraction decreases with increasing aspect ratios, in agreement with previous simulations. We find that the surface effect on internal packing decreases with aspect ratio as well. We also perform simulations in order to gather more data on boundary effects on internal packing and volume fraction. The simulations allow us to examine how the orientation of the rods differs with the distance from the container walls, which gives greater insight to random rod packing. [Preview Abstract] |
Wednesday, March 15, 2017 4:30PM - 4:42PM |
P15.00011: Jamming of semiflexible polymers Robert S. Hoy We study jamming in model freely rotating polymers as a function of chain length $N$ and bond angle $\theta_0$. The volume fraction at jamming, $\phi_J(\theta_0)$, is minimal for rigid-rod-like chains ($\theta_0 = 0$), and increases monotonically with increasing $\theta_0 \leq \pi/2$. In contrast to flexible polymers, marginally jammed states of freely rotating polymers are highly hypostatic, even when bond and angle constraints are accounted for. Large aspect ratio (small $\theta_0$) chains behave comparably to stiff fibers: resistance to large-scale bending plays a major role in their jamming phenomenology. Low aspect ratio (large $\theta_0$) chains behave more like flexible polymers, but still jam at much lower densities due to the presence of frozen-in 3-body correlations corresponding to the fixed bond angles. Long-chain systems jam at lower $\phi$ and are more hypostatic at jamming than short-chain systems. Implications of these findings for polymer solidification are discussed. [Preview Abstract] |
Wednesday, March 15, 2017 4:42PM - 4:54PM |
P15.00012: Common features in the mechanics of fiber networks, spring networks, and emulsions Karsten Baumgarten, Brian P. Tighe In many soft matter systems with a network structure, the mean connectivity plays a key role in determining the mechanical response. In the typical scenario for central force networks, there is a critical connectivity (the isostatic point) above which the system can withstand shear and bulk deformations. However, there are several different ways to induce the material to rigidify below the critical connectivity. Here we explore three such mechanisms: bending rigidity in the Mikado model for semi-flexible fiber networks, pre-tensioning in random spring networks, and finite-ranged attraction in soft sphere packings. Here we show common features in the rigidity transition of all three seemingly disparate systems. In particular we identify a band of low frequency normal modes for low perturbation strengths whose height and width are related to the distance to isostaticity. These in turn control the elastic moduli, as we explain with simple scaling arguments. [Preview Abstract] |
Wednesday, March 15, 2017 4:54PM - 5:06PM |
P15.00013: The Uniformity of Jammed Soft Disk Packings Anthony Chieco, Carl Goodrich, Ning Xu, Andrea Liu, Douglas Durian Rattler-free jammed packings were conjectured by Torquato \& Stillinger to be hyperuniform, such that volume-fraction fluctuations across a set of $L^d$ measuring windows is $\sigma_\phi^2(L)\sim 1/L^{d+1}$. For simulations of bidisperse soft disks of average area $\langle a\rangle$, we thus propose to quantify the uniformity of the packings by the value of a hyperuniformity disorder length, $h_e$, defined by $\sigma_\phi^2(L)/\phi = 4 \langle a\rangle h_e/L^{3}$ and equal to the distance from the window boundary over which density fluctuations occur. Independent of system size, preparation protocol, and fraction of rattlers, we find $h_e=0.084\sqrt{\langle a\rangle}$, which is only 14\% larger (i.e. only 14\% less uniform) than for a triangular lattice of close-packed disks. However, for windows larger than a certain size $L_e$ we find liquid-like Poissonian fluctuations of $h(L)=(h_e/L_e)L$, as defined by $\sigma_\phi^2(L)/\phi = 4 \langle a\rangle h(L)/L^{3}\sim 1/L^2$. For a rapid quench protocol, the value is $L_e=65\sqrt{\langle a\rangle}$, independent of system size and fraction of rattlers. For slower quenches, $L_e$ increases and is the subject of current study. [Preview Abstract] |
Wednesday, March 15, 2017 5:06PM - 5:18PM |
P15.00014: Experimental evidences of the Gardner phase in a granular glass Olivier Dauchot, Antoine Seguin The constituent particles of a glass are caged by their neighbors and thus cannot relax density fluctuations. This is also true for hard particles under compression. The associated slowing down of the dynamics is related to a complex free energy landscape. It was recently shown theoretically that the hard sphere glass in infinite dimension undergoes a Gardner transition, at which the glass basin breaks into a hierarchy of marginally stable sub-basins. This was very recently confirmed in simulations of 2d and 3d hard sphere (HS) glasses. We present the first direct experimental evidences of the Gardner phase, taking advantage of a well controlled granular experiment, which has already proven to successfully probe the vicinity of the jamming transition in a bi-dimensional granular glass former. More precisely, we perform independent compressions of a carefully prepared granular glass and show that for large enough compression, the final state differs from one compression to another. To do so we compare the average cage size within one state, and the average distance separating the cages of the same particles across successive compression cycles. The latter plateaus to a constant value, when entering the Gardner phase. [Preview Abstract] |
Wednesday, March 15, 2017 5:18PM - 5:30PM |
P15.00015: Stress, Structure, and Force Measurements in Jammed 3D Granular Solids Ryan Hurley, Stephen Hall, Jonathan Wright, Eric Herbold When subjected to external load, a jammed granular material develops an internal network of contact forces at the micro-scale. This force network controls the stress, stability, and transport properties of the material at the macro-scale. In order to understand this transition between scales and evaluate the accuracy of continuum descriptions of granular solids, measurement of variables at both length scales is needed. In this work, we discuss experiments that furnish such measurements. In particular, we combine X-ray diagnostics with numerical analysis to furnish intra-grain strains, continuum strains, contact network fabric, and force networks in a deforming 3D granular solid composed of microscopic ruby grains. We evaluate the statistical and structural nature of the contact and force network and study homogenization length scales that determine when various variables become correlated. By observing changes to contact and force networks during local rearrangements and grain fracture events, we also examine the grain-scale origins of non-locality. We discuss the current capabilities of such experimental approaches and how they will aid in developing continuum descriptions of granular solids in the future. [Preview Abstract] |
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