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Granular
Lab - Dynamics of Complex Systems |
 (above) The giant component of the broken link network after a shear of 30 degrees. Particles are represented by nodes [dots] , edges [lines] represent contacts that have broken after the applied shear. |
Network Analysis of Granular Fracture Mark Herrera PhD student, Physics Shear zones and reproducible flow fields are key features of granular flows. We experimentally study flows in a split bottom geometry by tracking the motion of all particles in three dimensions. In particular, we investigate how shear zones emerge from individual particle rearrangements, and how the rearrangements transition from reversible to irreversible with increasing strain. In order to analyze rearrangements at the level of particle motion, we define a broken links network, the set of particle pairs that have separated from each other and are no longer in contact. The emergence of a giant component occurs at the same characteristic strain at which a steady shear zone forms. We propose network theory as a new framework to characterize granular flows at the intermediate scale. See Herrera, et al. PRE 2011 (with M. Girvan, Physics, University of Maryland) Supported by NSF- Division of Materials Research |
 (above) Instantaneous velocities of glass beads due to the impact of a falling cylinder |
Dynamics of Granular
Impacts Kerstin Nordstrom, Postdoc Matt Harrington, Ph.D. student, Physics A meteor hitting the moon and a golf ball hitting a sand trap are the same common process: an object strikes a granular medium. Granular impacts have been studied for many years in the scientific community. However, most studies have focused on macroscopic quantities, such as impact depth vs projectile speed. Many force laws have been characterized for different systems, and recently a universal scaling of these force laws has been developed [1]. Despite this, little is known regarding the microscopic origin of these observations. In other words: we know what the projectile is doing, but what are the grains doing? We seek to better understand how the grains themselves rearrange at the moment of impact. Using our Refractive Index Matched Scanning imaging method [2], a high speed camera, and particle tracking, we can measure the trajectory of each individual grain. Physical processes such as plastic deformation and crystallization are observed and quantified. We are currently focused on characterizing nonaffine motion [3] within the system, and comparing our results with 2D systems (Behringer) and simulation (Kondic). Future work will view the system through the lens of network theory [4] to quantify fracture events within the granular medium. (References: [1] Katsuragi and Durian, Nature Physics 2007, [2] Slotterback et al. PRL 2008, [3] Falk and Langer, PRE 1998, [4] Herrera, et al. PRE 2011) Supported by the Defense Threat Reduction Agency |
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Compaction
Dynamics of Granular Matter using 3D imaging Though sandpiles are packed tightly they can be forced to compact into an even denser configuration. Using a laser sheet scanning technique, we are able to see the motion of all particles inside a pile of sand in three dimensions and to determine how the grains rearrange relative to their neighbors to allow such a compaction. To see inside,
our grains of sand, glass beads, are immersed in a fluid
with a matching refractive index. The
fluid is dyed so that it fluoresces as a sheet of laser
light passes through it, while the beads appear as dark
circles. This gives us a
cross-sectional image of the system.
We scan the sandpile by moving the sheet through
the system and taking pictures as we go.
To compact a container full of glass beads we
employ a method that is much gentler than tapping
- we cyclically expand and contract the container
size.
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 (above) A “Broken Link” Network is constructed from a 3D reconstruction of the a real granular packing. First, a nearest neighbor network is constructed (purple links). Then, at a later time, a new nearest neighbor network is compared to the original. Any links which have disappeared in the present network make up the Broken Link network (red links). |
Origins of Reversibility in Quasistatic Granular
Flows Mitch Mailman, Postdoc Steve Slotterback, Ph.D Student, Physics We have been investigating the properties of quasistatic granular flows under reversed cyclic shear. Unlike, for instance, dilute suspensions, dense granular flows do not exhibit exact structural reversibility when the strain is reversed. However, topological properties of the granular packing associated with the contact (or nearest neighbor) network do exhibit reproducible behavior in the steady state. Our approach is multi-faceted: motivated by experiments done in the Losert lab, we use numerical techniques to simulate quasistatic shear flow. Simulations provide insight into the process of self-organization that appears to be responsible for the onset of reproducible shear flow properties. In addition, we explore properties of minimal models of the evolution of the contact network under strain. (with Michelle Girvan, University of Maryland) Supported by NSF- Division of Materials Research |
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From Granular to Suspension Dynamics
J. Dijksman (Visiting Student, Univ. Leiden) and S. Slotterback (PhD Student) We probe the three dimensional flow structure and rheology of gravitational (non-density matched) suspensions for a range of driving rates. We establish that for sufficiently slow flows, the suspension flows as if it were a dry granular medium, with essentially the same effective friction coefficient as the dry particles. For faster flows, the suspension flows as if it were a Newtonian fluid, with viscosity larger than, but surprisingly close to, that of the interstitial fluid. Both regimes and the transition that separates them are shown to be consistent with recently developed inertial number approaches. Recent Publications: J.A. Dijksman et al, Phys. Rev. E 82, 060301(R) (2010) (with M. van Hecke, E. Wandersman (Univ Leiden), C. Berardi, and W.D. Updegraff (undergraduates, Univ. of Maryland) Supported by NSF- International Exchange Support |
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Dynamics of granular matter under local
perturbation. L. Jacob (Visiting Student, Univ. Rennes) We measure the 3D motion of particle in response to a local penetrating object (with A. Winter, A. Hosoi (MIT)) |
| Last updated by Rachel on Feb 1, 2012. Please contact rmlee @ umd.edu for updates to this page and wlosert @ umd.edu for questions about the Dynamics of Complex Systems lab. | |