email = candela@physics.umass.edu

Institution = University of Massachusetts Amherst

Title = Using NMR to Study Granular Media

Nuclear magnetic resonance (NMR) can serve as a powerful, noninvasive probe of dense three-dimensional granular media. NMR has been used to study a variety of granular states including static packings, quasistatic tapping flow, continuous rotating drum flow, and rapid granular flows such as the vibrofluidized bed. In our lab at UMass we have used NMR to measure density and granular temperature profiles in vibrofluidized beds, providing a quantitative test of granular hydrodynamic theory. Using this work as an example, I will provide a brief tutorial for nonspecialists on the principles and capabilities of NMR as an experimental probe of granular systems.

Name = David L. Johnson

email = johnson10@slb.com

Institution = Schlumberger-Doll Research

Title = Linear and nonlinearelasticity of granular media

We consider the linear and non-linear elasticity ofunconsolidated granular aggregates, considered as a function ofapplied stress, which may be anisotropic. A major considerationis the path-dependence of the stress-strain relation. Forexample, if the system is compressed and sheared, the order inwhich this is done is important. For special cases in which theinter-granular forces are path independent, we derive expressionsfor the so-called "3rd order" nonlinear elastic constants. Wecompare our results for the speeds of sound as a function ofstress against experimental data on both hydrostaticallycompressed and on uniaxially strained systems of loose glassbeads. We present results on molecular dynamic simulations of thedependence of the bulk and shear modulii as a function ofhydrostatic stress. Here we see that the bulk modulus is wellunderstood but the shear modulus depends upon grain rotations andtranslations in a manner not captured by the simple theory.Initial attempts to understand this in terms of correlated oruncorrelated particle relaxation are only partially successful

Name = Mark Shattuck

email = shattuck@ccny.cuny.edu

Institution = Levich Institute, City College of New York

Title = Granular Kinetic Theory

Granular materials are ubiquitous in nature and of great importance in industries ranging from food preparation to pharmaceuticals to coal processing. They also represent a new forum in which to study collective behavior and pattern formation in a system with weak scale separation, but one in which the microscopic scale is very large ranging from tens of microns to centimeters. The large-scale microstructure allows unprecedented experimental access to the underlying particle motions and provides an interesting test bed for kinetic theories of hard spheres that incorporate inelastic particle interactions.

In our laboratory, we combine experiment, simulation, and theory to examine the validity of kinetic theories of dense inelastic hard spheres, which have been developed over the last 15 years using Grad's 13-moment method, Enskog theory, or revised Enskog theory. All of these theories lead to equations similar to the Navier-Stokes equations for dense gases but with the addition of a temperature loss rate term in the energy equation. We find that transport coefficients predicted by the Grad 13-moment approach agree (typically within 20 percent) with event-driven molecular dynamics simulation of a homogeneously heated gas of inelastic hard spheres. These simulations have been previously validated and quantitatively reproduce the complex patterns found in experiments in thin oscillated granular layers. The same simulations agree to within 1 percent with experiments in dilute gravity-driven flow past a wedge producing granular shockwaves. Further, numerical integrations of the Navier-Stokes equations for dense gases also agree with the simulations to within 1 percent. We are currently investigating dense rotating flows and find near Maxwell-Boltzmann velocity distributions in agreement with the assumptions of kinetic theory

In this presentation an introduction to the kinetic theory of dense granular gases will be presented, with an emphasis on basic concepts.

Name = Greg Voth

email = gvoth@wesleyan.edu

Institution = Wesleyan University

Title = Fluidization and collapse of driven granular materials

While theories have been developed that accurately describe many properties of granular solids and gasses, a comprehensive understanding of how materials transition between these states has remained elusive. This is particularly problematic because many systems exhibit both flow types in different spatial domains, and so an adequate understanding of the transitional regime is crucial to understanding even the steady state of these flows. I will discuss what we know and identify some open questions about granular fluidization by examining experimental and computational studies of two systems: a vertically shaken system and a Couette shear flow. Important issues we will consider include: Can a fluidization transition be defined? What are the relevant parameters that control whether a specific system will collapse or not? And are there useful general (flow-independent) criteria for predicting collapse?

(in order of receipt)

Name = Paul Nakroshis

email = pauln@maine.edu

Institution = University of Southern Maine

talk = yes-talk

Title = Force Fluctuations and Angular Rotations in a driven granular array.

A quasi-two dimensional array of cylinders was pushed along a horizontal aluminum track at constant velocity and the total driving force was sampled at 10 KHz. Operating in a continuous motion regime, we found driving force fluctuations that follow a power law distribution with a sharp upper cutoff, a behavior which persisted even while varying the number of cylinders, the geometry and the driving velocity. We also recorded the angular rotations of the individual cylinders and report on how the total force correlates with the total angular velocity within the array, as well as the vital role dilatency plays in the collapse of the array.

Name = Jaehyuk Choi

email = jaehyuk@mit.edu

Institution = Dept of Math, MIT

talk = yes-talk

Title = Diffusion in gravity driven dense granular flows

Abstract: We measure the diffusion of particles in a silo drainage using direct particle tracking. The mean squared deviations are observed to grow in proportion to time indicating cage rearrangement. The diffusion rates, however, are very small compared with mean flow. We also measure the rearrangement time scales from the distributions of velocity fluctuations and topological correlation function.

Name = Lou Kondic

email = kondic@oak.njit.edu

Institution = New Jersey Institute of Technology

talk = yes-talk

Title = Extended granular temperature

Kondic and Behringer

We consider the role of elastic energy in the context of granular materials undergoing shear flow. Depending on the ratio of pressure to Young's modulus of the material from which grains are made and the typical velocity of shearing, there is a transition from a regime in which kinetic energy is the dominant form of energy storage to one in which elastic energy is dominant. We then consider a generalization of the granular temperature that includes both types of terms and changes smoothly from one regime to the other. We conclude by relating this generalized temperature to the concepts based on equilibrium statistical mechanics.

Name = Allison Ferguson

email = shamrock@brandeis.edu

Institution = Brandeis University

talk = yes-talk

Title = Signatures of Large-Scale Spatial Structures in Dense Granular Flows

Measurements of impulse distributions P(I) on a simulated 2D gravity driven hopper system show flow-velocity-invariant behavior of the distribution for impulses larger than the average impulse. For small impulses, however, P(I) decreases significantly with flow velocity, a phenomenon which can be attributed exclusively to collisions between grains undergoing frequent collisions. Visualizations of the system also show that these frequently colliding particles tend to form increasingly large linear clusters as the flow velocity decreases. A model is proposed for the form of P(I), given distributions of cluster size and velocity, which accurately predicts the observed form of the distribution. Thus the impulse distribution provides some insight into the formation and properties of these "dynamic" force chains.

Name = John R Williams

email = jrw@mit.edu

Institution = MIT

talk = yes-talk

Title = A two dimensional Discrete Element Model is used to study the deformation of granular materials under slow quasi-static deformations.

The results of uni-axial compression tests under steady confinement show that the particles move by forming distinct circulation cells. The size of these cells grows as the deformation proceeds until a shear band is formed. This behavior is exhibited by a range of particle shapes and for a wide range of inter-particle friction and stiffness. The formation of these structures whose characteristic length is significantly larger than the particle size has implications for continuum analysis of granular materials. At failure the characteristic length may be as large as the sample size. Therefore continuum assumptions involving the definition of an elementary volume of the material may be invalid because of the formation of the circulation structures within the material. A brief review of the latest advances in the Discrete Element Method will also be given.

Name = Martin Z. Bazant

email = bazant@mit.edu

Institution = Department of Mathematics, MIT

talk = yes-talk

Title = Towards a model of cooperative diffusion in dense granular flows

There is currently no mathematical model to describe particle dynamics in a slow, dense granular flow, such as gravity-driven drainage from a silo. Although classical continuum models, based on the concept of diffusing voids, are able to capture the mean velocity profile, particle mixing (relative to the mean flow) is grossly over-estimated if the presumed microscopic picture is taken seriously. We demonstrate these successes and failures by an exact analysis of the void model compared with drainage experiments. The basic problem is that particles move independently in the Void Model, whereas in a real dense flow, they clearly move with strong local correlations related to dynamical random packings.

As an alternative, we propose a new stochastic model in which particles undego strongly correlated random motion in response to diffusing `spots' of influence, which correspond to extended regions of slightly reduced density. Although it is eventually unstable to density fluctuations, the Spot Model is in good agreement with experimental measurements, at least for early times. The model also predicts spatial correlations in local velocities, which we later observed in experiments. This could be interpreted as direct experimental evidence for the existence of spots.

Collaborators: Jaehyuk Choi, Ruben Rosales, Chris Rycroft, Camilo Guaqueta (MIT), Arshad Kudrolli (Clark).

Name = GLAND

email = gland@geologie.ens.fr

Institution = ECOLE NORMALE SUPERIEURE, PARIS

talk = yes-talk

Title = Effects of Grain Size Heterogeneities on Packing Order and Stress Distribution in Compact Granular Assemblies.

We use Molecular Dynamics numerical simulations in order to study the connection between microscopic parameters and macroscopic properties of compacted granular assemblies. Confined packings of spherical grains of equal size with identical mechanical and surface properties spontaneously exhibit structures with local Hexagonal Close-Packed (HCP) order. We observed that introducing grain size heterogeneities (for instance by using binary mixtures) inhibits the local organisation of HCP crystals. We studied numerically structural transitions in binary mixtures of grains with different size ratios and different compositions (fractions of each specie). We use both local and global order parameters to characterize the geometry and directional properties of these structures, which result from collective rearrangement of grains. We also analyse mechanical properties of the packings (elastic moduli), using macroscopic response measurements. We discuss the problem of homogenisation of the structures of packings (towards Random Close Packing) obtained by adding of small amounts of grains of different size, and its implications on the mechanical responses.

Name = Scott Johnson

email = smjohns@mit.edu

Institution = MIT

talk = yes-talk

Title = THE DISCRETE ELEMENT MODELING APPROACH TO ADDRESSING GRANULAR MATERIAL PROBLEMS

Discrete element modeling (DEM) has become a popular method to analyze a variety of granular discontinua where continuum methods are inappropriate. There are several current applications of DEM to engineering problems, including modeling of the near well-bore region in drilling operations, hopper flow, pharmaceutical powder mixing, and particle behavior in fluidized beds. The usefulness of DEM in addressing these problems has led to several advances over the past 30 years, a brief overview of which is provided.

Many of the problems currently of interest to researchers, however, require a 3-D formulation, and addressing this has been the focus of past and current research efforts at the MIT Intelligent Engineering Systems Laboratory. A recent project, which developed a new method of modeling ellipsoidal shapes with C1 parametric continuity called the Equivalent Spheres Method, is discussed. A brief discussion of its applicability and computational requirements is also presented.

(in order of reciept)

Name = Benjamin Dupuy

email = bdupuy@mit.edu

Institution = MIT

talk = yes-poster

Title = Fingering instabilities in particle-laden flow down an inclined plane

The instability of a two-dimensional moving contact line is studied for a thin liquid film of a high-concentrated suspension flowing down an inclined plane, leading to the formation of rivulets. We perform experiments in various ranges of volume fraction, particle size and particle density and obtain measurements of the characteristic wave length, tip velocity and position of the onset. A laser imaging technique was used to obtain data for rivulet shapes and contact angles. Comparisons are made with the existing data and theoretical models.

Name = Fabricio Q. Potiguar

email = potiguar@levdec.engr.ccny.cuny.edu

Institution = City College of CUNY

talk = yes-poster

Title= Structural study of Granular assmblies

We measured some functions, which are standard in measuring the structure of liquids, for sheared granular systems. We are mainly interested in the incoherent scattering function which, as in the glass literature, can give us a good way to mearuse the granular system's temperature.

Name = Mason Klein

email = mklein@fas.harvard.edu

Institution = Harvard-Smithsonian CFA

talk = yes-poster

Title = The role of interstitial gas in the segregation of vertically vibrated granular media

We report experimental studies of vibrofluidized mixtures of like-sized bronze and glass spheres. We show how changes in the vibration conditions (amplitude and frequency) and the interstitial gas affect the formation of spatial patterns, including segregation of the two species and hysteretic behavior.

Name = Ruopeng Wang

email = wangrp@mit.edu

Institution = Harvard Smithsonian Center for Astrophysics

talk = yes-poster

Title= Study of homogeneous gas fluidization with both phenomenal observation and hyperpolarized 129Xe NMR

We present our recent work on the gas-fluidized glass beads with diameter in the range 45 to 70 micrometers. This type of particles is classified as Geldart A, and was observed to demonstrate that, in homogeneous fluidization of the particles, hysteretic behavior in terms of bed height and pressure drop as gas flow rate was cycled up and down. Our measurements of bed height as a function of gas flow rate exhibited this phenomenon, and were also able to show that obvious homogeneous fluidization only occurred when the flow rate was decreasing in the backward cycle. When the flow rate increased in the forward cycle, the height of the particle bed stayed still until the minimum bubbling flow rate was reached, and then increased significantly and suddenly to accommodate bubbles. We also measured 129Xe NMR spectra and gas velocity distribution at different gas flow rate, which shows correlation to the results available from phenomenal observation.

Name = Alexander Lobkovsky

email = leapfrog@mit.edu

Institution = MIT

talk = yes-poster

Description = Physics of erosion driven by subsurface flow

We attempt to understand the fundamental a spects of erosion in a lab-scale experiment. In the experiment, water is injected into a pile of glass beads. When it emerges on the surface of the pile, it incises channels which grow fed by the seeping water. A laser tomography setup allows us to extract time dependent topographic map of the eroding slope. We will present our current understanding of the initiation and growth of the channels.

Name = Emily Gardel/Nalini Easwar

email = egardel@email.smith.edu

Institution = Smith College

talk = yes-poster

Description = Force and Velocity Fluctuations in 2D Granular Flow