Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S44: Focus Session: Granular Materials and Continuum Descriptions of Discrete Media I |
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Sponsoring Units: GSNP GSOFT Chair: Bulbul Chakraborty, Brandeis University Room: 214D |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S44.00001: Giant drag reduction due to interstitial air in sand Devaraj van der Meer, Tess Homan When an object impacts onto a bed of very loose, fine sand, the drag it experiences depends on the ambient pressure in a surprising way: Drag is found to increase significantly with decreasing pressure. We use a modified penetrometer experiment to investigate this effect and directly measure the drag on a sphere as a function of both velocity and pressure. We observe a drag reduction of over 90\% and trace this effect back to the presence of air in the pores between the sand grains. Finally, we construct a model based on the modification of grain-grain interactions that is in full quantitative agreement with the experiments. [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S44.00002: Scattering of a legged robot in a heterogeneous granular terrain Feifei Qian, Daniel Goldman Many granular substrates are composed of particulates of varying size, from fine sand to pebbles and boulders. Ambulatory locomotion on such heterogeneous substrates is complicated in part due to fluctuations introduced by heterogeneities. To discover principles of movement in such substrates, we developed an automated system, the ``Systematic Creation of Arbitrary Terrain and Testing of Exploratory Robots'' (SCATTER), to create heterogeneous granular substrates of varying properties such as compaction, inclination, obstacle shape/size/distribution and obstacle mobility within the substrate. We investigate how the presence of a single ``boulder'' affects the locomotion of a 6-legged robot (15cm, 150g). The robot's trajectory is straight before boulder interaction, and is scattered to an angle after the interaction. Surprisingly, the interactions with the boulder can lead to both negative and positive scattering angles--an effective attraction and repulsion between the robot and the boulder. The scattering pattern depends sensitively on the leg-boulder contact position and the boulder mobility within the fine sand. However, the scattering pattern dependence upon contact position on the boulder is insensitive to boulder shape (created using 3D printing), orientation and roughness. [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S44.00003: Stability of an isolated granular band in a rotating tumbler Paul B. Umbanhowar, Darius Wheeler, Julio M. Ottino, Richard M. Lueptow Granular mixtures tend to segregate into axial bands when tumbled in long, horizontal cylinders. To better understand this phenomenon we experimentally and computationally studied the stability of a single band of large spherical particles initially located between two regions of small spherical particles. Unlike previous work with bidisperse particles, where the band spread axially in a diffusive-like fashion, we found that a single band can stabilize to a constant width much smaller than the cylinder length depending on the size ratio of large to small particles, $R$, and the fill fraction of the tumbler, $f.$ Stable bands were observed for $f<0.3$ and $1.3 \leq R \leq 2.3$; for $R$ outside this interval and $f>0.3$, bands were unstable and grew diffusively. For $R < 1.3$ large particles diffuse axially in the flowing layer, while for $R> 2.3$ axial motion of large particles occurs mainly at the intersection of the downstream terminus of the flowing layer and the cylinder wall. Lastly, band stability was independent of initial band width for the range we tested (4-40 mm). We discuss possible band stabilization mechanisms in light of these observations. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 8:48AM |
S44.00004: The cause of coarsening Matthias Schroeter, Tilo Finger, Ralf Stannarius The coarsening process of bands of smaller grains in a horizontally rotated cylindrical drum is a counterintuitive process. Our X-ray tomography results point to an effective surface tension as the driving mechanism. Additionally, we report on a novel microsegregation phenomenon. [Preview Abstract] |
Thursday, March 5, 2015 8:48AM - 9:00AM |
S44.00005: Analysis of inter-event times for avalanches on a conical bead pile with cohesion Susan Lehman, Nathan Johnson, Catherine Tieman, Elliot Wainwright We investigate the critical behavior of a 3D conical bead pile built from uniform 3~mm steel spheres. Beads are added to the pile by dropping them onto the apex one at a time; avalanches are measured through changes in pile mass. We investigate the dynamic response of the pile by recording avalanches from the pile over tens of thousands of bead drops. We have previously shown that the avalanche size distribution follows a power law for beads dropped onto the pile apex from a low drop height. We are now tuning the critical behavior of the system by adding cohesion from a uniform magnetic field and find an increase in both size and number for very large avalanches and decreases in the mid-size avalanches. The resulting bump in the avalanche distribution moves to larger avalanche size as the cohesion in the system is increased. We compare the experimental inter-event time distribution to both the Brownian passage-time and Weibull distributions, and observe a shift from the Weibull to Brownian passage-time as we raise the threshold from measuring time between events of all sizes to time between only the largest system-spanning events. These results are both consistent with those from a mean-field model of slip avalanches in a shear system [Dahmen, Nat Phys 7, 554 (2011)]. [Preview Abstract] |
Thursday, March 5, 2015 9:00AM - 9:12AM |
S44.00006: Ultra-fast parallel magnetic resonance imaging of granular systems Alexander Penn, Klaas P. Pruessmann, Christoph M\"uller Several non-intrusive techniques have been applied to probe the dynamics of two-phase granular systems, with the most prominent examples being X-ray tomography, positron emission particle tracking (PEPT), electrical capacitance tomography and magnetic resonance imaging (MRI). MRI comes with the particular advantage that by implementing suitable pulse sequences not only spin densities (i.e. voidage), but also velocity, acceleration, diffusion and chemical reactions can be measured. However, so far the investigation of two-phase granular systems has been performed on relatively small-bore systems (max. diameter 60 mm). Such systems are, however, heavily influenced by wall effects. Furthermore, largely only single-coil detection has been employed, limiting severely the temporal resolution of the data acquisition. Here, we report the acquisition of ultra-fast MRI measurements in large volume vessels using medical MRI scanners. Specifically, parallel MRI, i.e. the simultaneous use of multiple receiver coils, has been exploited to speed up the data acquisition. In combination with advanced pulse sequences, we were able to probe the rapid dynamics (voidage and velocity measurements) of gas-solid systems. [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S44.00007: 3D imaging of particle-scale rotational motion in granular flows Matt Harrington, Michael Lin, Wolfgang Losert In current granular research, many strides have been made in the characterization of three-dimensional motion and structure through the use of novel imaging techniques. In the context of measuring individual motion of spherical grains, these techniques tend to be limited to translational motion. While this is often sufficient, it neglects the rotational motion that can arise from torques that grains exert on each other, and that potentially propagate across mesoscopic structures. This has left a gap that prevents researchers from fully characterizing the behavior of real granular flows. In particular, the role of individual rotational motion has not been fully explored in the context of bulk processes such as shear-banding, segregation, and irreversibility. In our current work, the Refractive Index Matched Scanning technique is expanded to extract the orientation of near-spherical grains in a quasistatic shear flow. Particle tracking is then applied to directly measure the rotational motion of individual grains. In an initial study, the presence of rolling modes in the shear band of a circular shear cell has been confirmed. From here, we are extending the method further to determine the role of collective rotations within and across neighborhoods. [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S44.00008: Simulation of granular flows through their many phases Sachith Dunatunga The material point method (MPM) is combined with a constitutive model which allows material to traverse through its many common phases during the flow process. When dense, the material is treated as a pressure sensitive elasto-viscoplastic solid obeying a yield criterion and a plastic flow rule given by the $\mu(I)$ inertial rheology of granular materials. When the free volume exceeds a critical level, the material is deemed to separate and is treated as disconnected, stress-free media. By using the MPM framework, extremely large strains and nonlinear deformations such as those common to granular flows can be represented. The method has been shown to replicate results such as Beveloo scaling in silo discharge, as well as the Bagnold profile on an inclined plane. [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S44.00009: Modeling granular inclined plane flow phenomena with Nonlocal Granular Fluidity Ken Kamrin, David Henann The continuum theory of Nonlocal Granular Fluidity (NGF) has previously been shown to predict steady granular flow fields in many different geometries, including those such as split-bottom cells, which have been historically resistant to continuum modeling. Central to NGF is a direct inclusion of a particle length-scale, which renders the rheology nonlocal, capturing the cooperatively of granular motion. In this talk we demonstrate that the same model also captures the behaviors observed in granular inclined plane flows. We show that the model predicts a quantitatively accurate ``stopping curve'' which indicates the conditions that determine when a flowing layer comes to a stop, which depends explicitly on the thickness of the layer. We also explore other known phenomena in this geometry, such as the dependence of the flow profile on layer thickness, the collapse of the Froude number as a function of thickness vs the stopping height, and the possibility of modeling both starting and stopping curves within the same model. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S44.00010: Flow Profiles and Fluctuations Measured for Granular Flow in a Vertical Channel Donald Candela, Kevin Facto The average velocity profiles and the velocity fluctuations were measured for flows of a dense granular medium (corn poppy seeds) through a long vertical channel, using NMR. The flow profiles seem to be in good agreement with non-local constitutive laws that have been proposed. In particular, there is a shear band near the channel wall with width that is independent of the flow rate. However, the measured velocity fluctuations do not agree with expectations from a simple interpretation of the underpinnings of the non-local rheology. For example, there are large fluctuations in the velocity of the central portion of the flow, away from the walls. This apparent discrepancy may be due to the absence of constant-pressure boundary conditions in granular flow through a fixed-size channel. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S44.00011: High speed impacts on a granular material Yue Zhang, Abrahm Clark, Lou Kondic, Bob Behringer When an object strikes a granular material, its momentum and energy are transferred to the grains and dissipated. When the ratio of the intruder speed, $v_0$, to a typical granular sound speed, $c$, is small, this energy transfer is intermittent along force-chain-like structures, leading to an inertial drag term proportional to the square of the intruder speed. However, many natural and industrial examples of granular impact occur much closer to $M'\equiv v_0/c \sim 1$, a regime which is difficult to reach in a lab setting using many common granular materials. To address this, we perform experiments (and matching simulations) with granular materials comprised of photoelastic disks of varying stiffness (and thus, varying $c$), in order to probe regimes closer to $M'\sim 1$. As $M'$ increases, the inertial drag law fails and the material begins to behave more elastically, with a shock-like front propagating away at impact. This causes the penetration depth to be greatly reduced, and in extreme cases, the intruder can rebound temporarily. We understand this transition to damped, elastic-like behavior by comparing the grain-grain collision time to the time for the intruder to move one grain diameter. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S44.00012: Avalanches, and evolution of stress and fabric for a cyclically sheared granular material Dengming Wang, Jonathan Bares, Dong Wang, Bob Behringer Granular materials yield for large enough shear stress, leading to avalanches. We seek to understand the relation between macroscopic avalanches and the the microscopic granular structure. We present an experimental study of a 2D granular material subjected to cyclic pure shear, which we visualized by a photo-elastic technique. We start from a stress-free sample of frictional particles in the shear-jamming regime ($\phi_S \le \phi \le \phi_J$). We apply multiple cycles of pure shear: shear in one direction, followed by a reversal to the original boundary configuration. The strain is made in small quasi-static steps: after each small step, we obtain polarized and unpolarized images yielding particle-scale forces and locations. Statistical measures of the avalanches are in reasonable agreement with recent mean-field avalanche models by Dahmen et al. (Nature Physics {7}, 554 (2011)) The system structure evolves slowly to reduce the stress at the extrema of strain, similar to the relaxation observed by Ren et al. (Phys. Rev. Lett. 110, 018302 (2013)) in a simple shear experiment. To understand how this relaxation occurs, we track the stress and fabric tensors and measures of the strain field over many cycles of shear. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S44.00013: Statistics from granular stick-slip experiment Aghil Abed Zadeh, Jonathan Bares, Robert Behringer We carry out experiments to characterize stick-slip for granular materials. In our experiment, a constant speed stage pulls a slider which rests on a vertical bed of circular photoelastic particles in a 2D system. The stage is connected to the slider by a spring. We measure the force on the spring as well as the slider's acceleration by a force sensor attached to the spring and accelerometers on the slider. The distributions of energy release and time duration of avalanches during slip obey power laws. We apply a novel event recognition approach using wavelets to extract the avalanche properties. We compare statistics from the wavelet approach with those obtained by typical methods, to show how noise can change the distribution of events. We analyze the power spectrum of various quantities to understand the effect of the loading speed and of the spring stiffness on the statistical behavior of the system. Finally, from a more local point of view and by using a high speed camera and the photoelastic properties of our particles, we characterize the internal granular structure during avalanches. [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S44.00014: Force network in a three-dimensional sheared material Nicolas Brodu, Jonathan Bares, Joshua Dijksman, Robert Behringer Force chains in 2D granular material have been widely studied over the past decade. However the force network evolution when a 3D granular medium is sheared remains poorly unterstood due to the complexity of experimental observations. We present an experimental set-up to measure interparticle forces in the case of the quasi-static deformation of a 3D sphere packing subjected to shear and compression. We perform these experiments on slightly polydisperse and los-friction soft hydrogel spheres. We resolve the microscopic force network in a this three dimensional packing through imaging the entire packing at each loading steps. By resolving particle deformations via custom image analysis software, we extract all particle contacts and contact forces with a very good accuracy. We address the rising up of the Reynolds pressure from the microscopic force network and a statistical ensemble analogous to equilibrium counterpart for 3D frictionless particles. [Preview Abstract] |
Thursday, March 5, 2015 10:48AM - 11:00AM |
S44.00015: Granular dynamics under shear with deformable boundaries Drew Geller, Scott Backhaus, Robert Ecke Granular materials under shear develop complex patterns of stress as the result of granular positional rearrangements under an applied load. We consider the simple planar shear of a quasi two-dimensional granular material consisting of bi-dispersed nylon cylinders confined between deformable boundaries. The aspect ratio of the gap width to total system length is 50, and the ratio of particle diameter to gap width is about 10. This system, designed to model a long earthquake fault with long range elastic coupling through the plates, is an interesting model system for understanding effective granular friction because it essentially self tunes to the jamming condition owing to the hardness of the grains relative to that of the boundary material, a ratio of more than 1000 in elastic moduli. We measure the differential strain displacements of the plates, the inhomogeneous stress distribution in the plates, the positions and angular orientations of the individual grains, and the shear force, all as functions of the applied normal stress. There is significant stick-slip motion in this system that we quantify through our quantitative measurements of both the boundary and the grain motion, resulting in a good characterization of this sheared 2D hard sphere system. [Preview Abstract] |
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