When an initially homogeneous binary mixture of granular media such as fine and coarse sand is poured near the closed edge of a “quasi-two-dimensional” Hele-Shaw cell consisting of two vertical transparent plates held a narrow distance apart, the mixture spontaneously forms alternating segregated layers. Experimental measurements of this stratification effect are reported in order to determine which model, one which suggests that segregation only occurs when the granular material contained within a metastable heap between the critical and maximum angle of repose avalanches down the free surface, or one for which the segregation results from smaller particles becoming trapped in the top surface and being removed from the moving layer during continuous flow. The result reported here indicate that the Metastable Wedge model provides a natural explanation for the initial mixed zone which precedes the formation of the layers, while the Continuous Flow model explains the observed upward moving kink of segregated material for higher granular flux rates, and that both mechansims are necessary in order to understand the observed pairing of segregated layersfor intermediate flow rates and cell separations.

Glass spheres were used as a model system to investigate granule failure during die compaction. Stresses within an assembly of spheres follow a network of pathways. When the spheres are of uniform composition, the magnitude of the stresses within a pair of contacting granules is a function of the locally transmitted stress and the diameter of the two spheres. Results obtained using glass spheres demonstrated the statistical nature of granule failure during compaction, with some granules failing at very low applied pressures while others (∼40% by volume) persist at even the highest applied loads. Within a distribution of granule sizes, those granules with smaller diameter were seen to have a higher probability of failure at low pressure than were larger granules. These results are consistent with those observed during die compaction of granulated alumina powder.

The ultrasonic pulse transmission method (100-500 kHz) was adapted to measure compressional (P) and shear (S) wave velocities for synthetic soils fabricated from quartz-clay and quartz-peat mixtures. Velocities were determined as samples were loaded by small (up to 0.1 MPa) uniaxial stress to determine how stress at grain contacts affects wave amplitudes, velocities, and frequency content. Samples were fabricated from quartz sand mixed with either a swelling clay or peat (natural cellulose). P velocities in these dry synthetic soil samples were low, ranging from about 230 to 430 m/s for pure sand, about 91 to 420 m/s for sand-peat mixtures, and about 230 to 470 m/s for dry sand-clay mixtures. S velocities were about half of the P velocity in most cases, about 130 to 250 m/s for pure sand, about 75-220 m/s for sand-peat mixtures, and about 88-220 m/s for dry sand-clay mixtures. These experiments demonstrate that P and S velocities are sensitive to the amount and type of admixed second phase at low concentrations. We found that dramatic increases in all velocities occur with small uniaxial loads, indicating strong nonlinearity of the acoustic properties. Composition and grain packing contribute to the mechanical response at grain contacts and the nonlinear response at low stresses.

Size segregation of particulates is of concern in a number of industries that handle materials such as chemicals, pharmaceuticals, fertilizers, and food products. Of particular interest in this paper is segregation resulting from externally applied vibration. In industrial applications this vibration may either be applied intentionally in devices such as vibrating conveyors or “live wall” hoppers, or unintentionally during material handling and transport. This paper investigates size segregation in granular beds subject to discrete “taps” and continuous, sinusoidal vertical vibration. The results from discrete element computer simulations indicate that the rise rate of a single impurity increases monotonically with amplitude for discrete vibrations but for continuous vibrations the rise rate increases, reaches a maximum value, then decreases as the oscillation amplitude increases.

The unusual feature of granular state is the negligible linear range of the interaction force between neighboring particles resulting in zero sound speed in uncompressed material. This makes linear and weakly nonlinear continuum approach based on Korteveg - de Vries equation invalid. This paper describes a results based on more general approach.

A theoretical approach to the description of stress distribution in granular compacts is presented on the basis of a memory function formalism. Experiments which have motivated the approach are mentioned. The formalism is shown to provide an explanation of observed features of stress distribution in compacts, and to lead to existing theories in extreme limits, thereb y providing a unification of the theories. The memory functions are shown to be intimately related to characteristic spatial correlations in the granular system and are discussed on the basis of stochastic considerations.

We study the rheology of a granular material in a confined geometry. The grains are stacked in a vertical cylinder and pushed at different driving velocities. The resistance force encountered by the bottom piston is monitored while the piston is pushing the granular column upwards. Above a critical velocity, the motion is characterized by a steady sliding and by a force level increasing rather slowly with the pushing velocity. For driving velocities under this threshold, the system undergoes a dynamic instability and then, a stick-slip motion occurs. The amplitude of the slipping events, and thus, the elastic energy release, increase strongly when the velocity decreases. The critical velocity depends on the stiffness of the driving system and on the height of the granular column. This transition can be shifted towards higher velocity values by increasing the friction at the walls of the cylinder. It is also very sensitive to the state of compaction of the grains. Moreover, the mean energy release during a stick-slip motion seems to increase as a power-law when the pushing velocity is decreased. We also show that the distribution of energy release is strongly dependent on the level of disorder in the grains (polydispersity, friction, etc.). We argue that this complex phenomenology characterizes confined granular packing in connection with arching and aging phenomena.

In Figure 2 of the aforementioned paper by Alam, Clelland, and Hrenya, the stresses obtained using molecular-dynamic simulations of binary-sized systems were compared to stresses obtained using the kinetic-theory model for binary mixtures developed by Willits and Arnarson (Phys. Fluids, 11, p. 3116, 1999). An error was found in calculations for the Willits and Arnarson theory. Figure 1 (below) shows the corrected theoretical predictions, along with the original simulation data. These corrections result in an improved match between simulations and theory. In particular, the theoretical predictions compare well with the simulation data across the entire range of diameter ratios investigated.

We present numerical simulations to demonstrate that it may be possible to eject ferrofluid grains from a ferrofluid using non-linear acoustic impulses. The study considers a container with some dilute ferrofluid that is placed in a strong, vertical, homogeneous magnetic field. The field induces the formation of magnetic dipoles into vertical chains that approximately span the region between the base and the surface of the container. We use particle dynamical simulations to show that an impulse generated at the base of any chain, will typically travel as a weakly dispersive bundle of energy. When the impulse magnitudes are appropriate (typically ∼60 m/s or more) the ferrofluid grain nearest to the surface of the liquid may be ejected by the impulse. Since all ferrofluid grains possess a coating of the liquid host, the ejected grain can be used as an ink-drop, with typical diameter of 15 or so nanometers. The velocities of the ejecting grains can be controlled and hence the method, if experimentally feasible, may have wide ranging applications. One of these applications is likely to be in designing special-purpose nozzle-free inkjet printers of unprecedented resolution.

We present the results of different experiments which aim is to investigate the response of a static granular assemblies to various local perturbations. We design a sensitive experimental method to probe the response of a localized stress at the top of a granular piling. The piling is an horizontal 3D granular layer confined in a box and the spatial distribution of loads at the bottom is monitored. This analysis defines some crucial experimental test to inform a currently debated issue on the mechanical status of static granular assemblies.

We present a forced torsion pendulum designed to measure the dynamic moduli of granular media. In the method, the oscillating probe of the pendulum is immersed into various materials, such as sand, glass beads, snow. The apparatus operates at low-frequency and provides information about the quasi-static mechanical properties of the medium. In particular, a peak in the losses is ascribed to friction and cohesion between the grains, and a measure of the macroscopic failure limit can be obtained. As an example the effect of moisture-induced ageing in small glass beads, and the effect of sintering of ice grains in snow, are shown.

We study fluctuations in the drag force resisting the motion of an object being pulled through a dense spherical granular medium. These fluctuations are stick-slip in nature due to the jamming and reorganization of the grains. The fluctuations in the force are periodic at small depths, but they become “stepped” at large depths. We interpret this transition as a consequence of the long-range nature of the force chains.

Nuclear magnetic resonance (NMR) is a non-intrusive method that can characterize not only the particulate density but also velocity and velocity fluctuation parameters. A survey of all the known NMR measurements of granular flow will be followed by a brief description of NMR as it applies to granular flow. Two new experiments, both involving flows in partially filled rotating horizontal cylinders, will be described. First, the effect of a stationary blade to suppress the azimuthal velocity of particles being brought up and deposited into the flowing layer on flow-velocity profiles will be studied. Suppressing the azimuthal velocity reduces the deviation of the velocity profile from a quadratic dependence on the height above the rigid layer. Second, a new NMR scheme will be presented that yields spatial distributions of collisional correlation times for macroscopic particles undergoing granular flow. It is based on Pulsed-Gradient Spin-Echo strategy that is commonly used to measure molecular diffusion in liquids. The scheme will be demonstrated with an example from shear flow in a partially-filled horizontal cylinder. Spatially resolved collisional correlation times and velocity fluctuation intensities are derived from the measurements and have values of ∼1 ms and ∼10-3 m2/s2 respectively, at the center of the free surface for 2 mm particles in a 70 mm diameter cylinder rotating at 2.36 rad/s.

Avalanching studies were performed on 2D beds of a unique granular material - uniform length, 2.06 mm diameter flexible metal bead chain. The dynamical behavior of the slowly rotating vertical chain beds was separately characterized using chain lengths of 1, 2, 5, 9 and 13 beads, respectively. Digital images were acquired at 30-second intervals and analyzed to provide comparisons of mean profiles, slope angle variations, avalanche mass distributions and surface step discontinuities. This study is designed to probe how elongation, flexing and rolling affect avalanching through interlocking and polymer-like behavior.

Event-driven molecular dynamics simulations are performed for the simple shear flow of smooth inelastic disks, with a focus on the effect of polydispersity on stresses. Simulations are conducted for both binary mixtures and polydisperse media with a Gaussian size distribution. For the binary mixture, the total solids volume fraction (φ = φ1 + φ2), the solids fraction ratio (φ1 / φ2) the particle diameter ratio (d1 / d2) and the coefficient of restitution (ep) are varied, and the simulation results are compared with an existing kinetic-theory model. The calculated stresses compare reasonably well with the model predictions for d1 / d2 < 2 in the nearly elastic limit, but the agreement deteriorates significantly at larger size ratios. Furthermore, the assumption of equipartition of granular energy between the two particle sizes appears to have a very limited range of validity in terms of both particle-size ratio and inelasticity. For a granular mixture characterized by a Gaussian size distribution, simulations are carried out in which the standard deviation in particle diameter, the coefficient of restitution and the total solids fraction are varied. The calculated stresses are in good agreement with predictions obtained from an existing kinetic-theory model for an “equivalent” monodisperse mixture.

The study of sound propagation in granular beds at frequencies exceeding a MHz has been a subject of study for many years. Much remains to be learnt about sound propagation at lower frequencies. We shall present our studies on the problem of impulse propagation and briefly comment on low frequency acoustic propagation in model granular beds using particle dynamical simulations. The following results will be briefly discussed. (i) Impulses propagating as solitary waves in 1-D granular chains with Hertz contacts in the absence of precompression. (ii) The effects of uniform precompression and gravitational loading on wave propagation. (iii) Impulse propagation in 3-D granular beds. The research presented shall highlight the intrinsically nonlinear nature of wave propagation in granular beds.

Spray-dried ceramic powders (e.g., Al2O3) are composed of a plurality of granules, each of which, includes ceramic particles and organic binders. It is assumed that the binders become concentrated in the surface layer of the granule in accordance with its type or its volume mixed into a ceramic portion of the granule. However, evidence to prove the assumption was limited because conventional microscopes were not able to clearly visualize the segregation. This paper presents a technique for imaging detailed structure of the spray-dried ceramic powders with the ultrasonic-atomic force microscope (U-AFM). The distribution of binder vis-a-vis Al2O3 particles is highly resolved with good contrast. The distribution was confirmed by nano -indentation. Thus, the U-AFM is shown to be a useful diagnostic tool for the development of approaches to spray-dried process evaluation.

We present an experimental study about the stability and dynamics of a granular packing and the influence of the packing size on the avalanche mass and on its characteristic angles: maximum angle of stability and angle of repose.

For a system with a large enough number of granular layers only Nc superficial layers contribute to the avalanche mass. It can be observed that Nc strongly depends on the packing size. Experiments are done on monosize glass beads with packings of different sizes.

Granular material, fed continuously into the top of a slowly rotating, slightly inclined cylinder, forms a moving bed. Much of the bed rotates with the cylinder in solid body motion. When particles reach the surface of the bed, they move rapidly down it, and are absorbed once more into the solid body motion. Such cylinders are used in calcining, pharmaceutical manufacture, and drying. A steady state transport model, applicable when the bed depth varies slowly along the cylinder, has existed for around 50 years. The bed surface is considered locally flat, and particles in it fall along the line of steepest descent, inclined to the horizontal at the angle of repose. There is reasonable agreement with experiment.

We propose a quasi-steady state dynamical model, in which the steady state model is coupled with a volume balance across an axial element. The model takes the form of a nonlinear diffusion equation which was solved numerically. The parameters of the dynamic model are the dimensions of the cylinder and outlet dam, the inclination of the axis of the cylinder, its rotational speed, the angle of repose of the granular material and its feed volumetric flow rate: the dynamic model has no free parameters. Experiments were conducted using sand, mean particle size 490 μm, in a perspex tube of length 1 m, radius 0.0515 m, lined with sandpaper, with a feed end dam of height 0.029 m, and with no exit dam, or an exit dam of height 0.0105 m. With the system initially in steady state, step changes in feed flow rate, rotational speed or axis inclination were imposed, and the resulting discharge flow rate and bed depth axial profile measured as functions of time. Good agreement is found between model and experiment.