CiteULike: kdesmond's library [200 articles]

Hysteresis and packing in gas-fluidized beds

R Ojha — 2008-05-10T19:33:45-00:00

Physical Review E, Vol. 62, No. 3. (2000), 4442.

Compaction dynamics of a granular medium under vertical tapping

P Philippe — 2008-05-10T19:04:42-00:00

EPL (Europhysics Letters), Vol. 60, No. 5. (2002), pp. 677-683.

We report new experimental results on granular compaction under consecutive vertical taps. The evolution of the mean volume fraction and of the mean potential energy of a granular packing presents a slow densification until a final steady state, and is reminiscent of usual relaxation in glasses via a stretched exponential law. The intensity of the taps seems to rule the characteristic time of the relaxation according to an Arrhenius's type relation. Finally, the analysis of the vertical volume fraction profile reveals an almost homogeneous densification in the packing.

Density fluctuations in vibrated granular materials

Edmund Nowak — 2007-06-03T19:15:17-00:00

Physical Review E, Vol. 57, No. 2. (February 1998), 1971.

We report systematic measurements of the density of a vibrated granular material as a function of time. Monodisperse spherical beads were confined to a cylindrical container and shaken vertically. Under vibrations; the density of the pile slowly reaches a final steady-state value about which the density fluctuates. We have investigated the frequency dependence and amplitude of these fluctuations as a function of vibration intensity Γ. The spectrum of density fluctuations around the steady state value provides a probe of the internal relaxation dynamics of the system and a link to recent thermodynamic theories for the settling of granular material. In particular; we propose a method to evaluate the compactivity of a powder; first put forth by Edwards and co-workers; that is the analog to temperature for a quasistatic powder. We also propose a stochastic model based on free volume considerations that captures the essential mechanism underlying the slow relaxation. We compare our experimental results with simulations of a one-dimensional model for random adsorption and desorption.

Torque Generated by the Flagellar Motor of Escherichia coli while Driven Backward

Richard Berry — 2008-05-04T21:32:13-00:00

Biophys. J., Vol. 76, No. 1. (1 January 1999), pp. 580-587.

The technique of electrorotation was used to apply torque to cells of the bacterium Escherichia coli tethered to glass coverslips by single flagella. Cells were made to rotate backward, that is, in the direction opposite to the rotation driven by the flagellar motor itself. The torque generated by the motor under these conditions was estimated using an analysis that explicitly considers the angular dependence of both the viscous drag coefficient of the cell and the torque produced by electrorotation. Motor torque varied approximately linearly with speed up to over 100 Hz in either direction, placing constraints on mechanisms for torque generation in which rates of proton transfer for backward rotation are limiting. These results, interpreted in the context of a simple three-state kinetic model, suggest that the rate-limiting step in the torque-generating cycle is a powerstroke in which motor rotation and dissipation of the energy available from proton transit occur synchronously.

Stress propagation: Getting to the bottom of a granular medium

Matthew Stone — 2008-05-03T00:07:18-00:00

Nature, Vol. 427, No. 6974. (5 February 2004), pp. 503-504.

Dynamics of meteor impacts

Karen Daniels — 2008-05-03T00:06:18-00:00

Chaos: An Interdisciplinary Journal of Nonlinear Science, Vol. 14, No. 4. (2004), pp. S4-S4.

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Anomalous density dependence of static friction in sand

Viktor Horváth — 2008-05-03T00:04:28-00:00

Physical Review E, Vol. 54, No. 2. (1996), 2005.

Pressure screening and fluctuations at the bottom of a granular column

L Vanel — 2008-05-03T00:03:46-00:00

The European Physical Journal B - Condensed Matter and Complex Systems, Vol. 11, No. 3. (19 October 1999), pp. 525-533.

Abstract: We report sets of precise and reproducible measurements on the static pressure at the bottom of a granular column. We make a quantitative analysis of the pressure saturation when the column height is increased. We evidence a great sensitivity of the measurements with the global packing fraction and the eventual presence of shear bands at the boundaries. We also show the limit of the classical Janssen model and discuss these experimental results under the scope of recently proposed theoretical frameworks.

Compaction force in a confined granular column

D Arroyo-Cetto — 2008-05-03T00:02:33-00:00

Physical Review E, Vol. 68, No. 5. (7 November 2003), 051301.

Force Fluctuations in Bead Packs

Liu — 2008-01-23T15:01:10-00:00

Science, Vol. 269, No. 5223. (28 July 1995), pp. 513-515.

Experimental observations and numerical simulations of the large force inhomogeneities present in stationary bead packs are presented. Forces much larger than the mean occurred but were exponentially rare. An exactly soluble model reproduced many aspects of the experiments and simulations. In this model, the fluctuations in the force distribution arise because of variations in the contact angles and the constraints imposed by the force balance on each bead in the pile. 10.1126/science.269.5223.513

Flows of Dense Granular Media

Yoel Forterre — 2008-03-06T05:28:36-00:00

Annual Review of Fluid Mechanics, Vol. 40, No. 1. (2008), pp. 1-24.

We review flows of dense cohesionless granular materials, with a special focus on the question of constitutive equations. We first discuss the existence of a dense flow regime characterized by enduring contacts. We then emphasize that dimensional analysis strongly constrains the relation between stresses and shear rates, and show that results from experiments and simulations in different configurations support a description in terms of a frictional visco-plastic constitutive law. We then discuss the successes and limitations of this empirical rheology in light of recent alternative theoretical approaches. Finally, we briefly present depth-averaged methods developed for free surface granular flows.

Scaling vertical drag forces in granular media

G Hill — 2008-05-02T23:58:00-00:00

EPL (Europhysics Letters), Vol. 72, No. 1. (2005), pp. 137-143.

The average drag forces on intruders slowly plunging into and withdrawing from shallow beds of monodisperse smooth glass beads and of sifted rough sand scale with the immersion depth and lateral dimensions of the intruder. The withdrawal forces are comparable for both types of media; however, the plunging forces for sand are substantially greater than for smooth glass beads. Furthermore, for glass beads, the rescaled plunging and withdrawal forces have two different power law dependences on the immersion depth, with exponents greater than unity.

Rheophysics of dense granular materials: Discrete simulation of plane shear flows

Frédéric da Cruz — 2008-05-02T23:57:00-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 72, No. 2. (2005)

We study the plane shear flow of a dense assembly of dissipative disks using discrete simulation and prescribing the pressure and the shear rate. Those shear states are steady and uniform, and become intermittent in the quasistatic regime. In the limit of rigid grains, the shear state is determined by a single dimensionless number, called the inertial number I, which describes the ratio of inertial to pressure forces. Small values of I correspond to the quasistatic critical state of soil mechanics, while large values of I correspond to the fully collisional regime of kinetic theory. When I increases in the intermediate dense flow regime, we measure an approximately linear decrease of the solid fraction from the maximum packing value, and an approximately linear increase of the effective friction coefficient from the static internal friction value. From those dilatancy and friction laws, we deduce the constitutive law for dense granular flows, with a plastic Coulomb term and a viscous Bagnold term. The mechanical characteristics of the grains (restitution, friction, and elasticity) have a small influence in the dense flow regime. Finally, we show that the evolution of the relative velocity fluctuations and of the contact force anisotropy as a function of I provides a simple explanation of the friction law.

Probing creep motion in granular materials with light scattering

Linda Djaoui — 2008-05-02T23:12:19-00:00

Granular Matter, Vol. 7, No. 4. (11 November 2005), pp. 185-190.

We describe a dynamic light scattering experiment designed in order to study creep motion in granular materials. This method is based on the recording of the speckle pattern with a charge coupled device (CCD) camera. The autocorrelation function of the scattered electric field is calculated and related to the displacement field of the beads. As an application, the measurement of the thermal expansion of a granular material subjected to temperature variations is presented.

Flow of dense granular material: towards simple constitutive laws

O Pouliquen — 2006-08-01T00:34:07-00:00

J. Stat. Mech., Vol. 2006, No. 07. (July 2006), P07020.

Dense flows of dry granular material

Olivier Pouliquen — 2008-05-02T23:02:34-00:00

Comptes Rendus Physique, Vol. 3, No. 2. (2002), pp. 163-175.

The behavior of dense assemblies of dry grains submitted to continuous shear deformation is still not well understood. Recently it has been the subject of several experiments and discrete particle simulations. For both confined and free surface geometries, we present the general features of such flows as well as grain-level information. We then describe the main rheological models and their predictions. To cite this article: O. Pouliquen, F. Chevoir, C. R. Physique 3 (2002) 163-175.

On dense granular flows

GDR Midi — 2008-05-02T21:47:15-00:00

The European Physical Journal E - Soft Matter, Vol. 14, No. 4. (2004), pp. 341-365.

The behaviour of dense assemblies of dry grains submitted to continuous shear deformation has been the subject of many experiments and discrete particle simulations. This paper is a collective work carried out among the French research group Groupement de Recherche Milieux Divisés (GDR MiDi). It proceeds from the collection of results on steady uniform granular flows obtained by different groups in six different geometries both in experiments and numerical works. The goal is to achieve a coherent presentation of the relevant quantities to be measured i.e. flowing thresholds, kinematic profiles, effective friction, etc. First, a quantitative comparison between data coming from different experiments in the same geometry identifies the robust features in each case. Second, a transverse analysis of the data across the different configurations, allows us to identify the relevant dimensionless parameters, the different flow regimes and to propose simple interpretations. The present work, more than a simple juxtaposition of results, demonstrates the richness of granular flows and underlines the open problem of defining a single rheology.

The steady-state counterclockwise/clockwise ratio of bacterial flagellar motors is regulated by protonmotive force.

S Khan — 2007-04-18T13:34:20-00:00

J Mol Biol, Vol. 138, No. 3. (15 April 1980), pp. 563-597.

Proton chemical potential, proton electrical potential and bacterial motility.

S Khan — 2008-05-02T18:28:34-00:00

Journal of molecular biology, Vol. 138, No. 3. (15 April 1980), pp. 599-614.

The proton flux through the bacterial flagellar motor.

M Meister — 2008-05-02T18:25:04-00:00

Cell, Vol. 49, No. 5. (5 June 1987), pp. 643-650.

Bacterial flagella are driven by a rotary motor that utilizes the free energy stored in the electrochemical proton gradient across the cytoplasmic membrane to do mechanical work. The flux of protons coupled to motor rotation was measured in Streptococcus and found to be directly proportional to motor speed. This supports the hypothesis that the movement of protons through the motor is tightly coupled to the rotation of its flagellar filament. Under this assumption the efficiency of energy conversion is close to unity at the low speeds encountered in tethered cells but only a few percent at the high speeds encountered in swimming cells. This difference appears to be due to dissipation by processes internal to the motor. The efficiency at high speeds exhibits a steep temperature dependence and a sizable deuterium solvent isotope effect.

Solvent-Isotope and pH Effects on Flagellar Rotation in Escherichia coli

Xiaobing Chen — 2008-05-02T18:23:23-00:00

Biophys. J., Vol. 78, No. 5. (1 May 2000), pp. 2280-2284.

We studied changes in speed of the flagellar rotary motor of Escherichia coli when tethered cells or cells carrying small latex spheres on flagellar stubs were shifted from H2O to D2O or subjected to changes in external pH. In the high-torque, low-speed regime, solvent isotope effects were found to be small; in the low-torque, high-speed regime, they were large. The boundaries between these regimes were close to those found earlier in measurements of the torque-speed relationship of the flagellar rotary motor (Berg and Turner, 1993, Biophys. J. 65:2201-2216; Chen and Berg, 2000, Biophys. J., 78:1036-1041). This observation provides direct evidence that the decline in torque at high speed is due primarily to limits in rates of proton transfer. However, variations of speed (and torque) with shifts of external pH (from 4.7 to 8.8) were small for both regimes. Therefore, rates of proton transfer are not very dependent on external pH.

Successive incorporation of force-generating units in the bacterial rotary motor

Steven Block — 2008-05-02T18:22:41-00:00

Nature, Vol. 309, No. 5967. (31 May 1984), pp. 470-472.

Restoration of torque in defective flagellar motors

DF Blair — 2008-05-02T18:21:55-00:00

Science, Vol. 242, No. 4886. (23 December 1988), pp. 1678-1681.

Paralyzed motors of motA and motB point and deletion mutants of Escherichia coli were repaired by synthesis of wild-type protein. As found earlier with a point mutant of motB, torque was restored in a series of equally spaced steps. The size of the steps was the same for both MotA and MotB. Motors with one torque generator spent more time spinning counterclockwise than did motors with two or more generators. In deletion mutants, stepwise decreases in torque, rare in point mutants, were common. Several cells stopped accelerating after eight steps, suggesting that the maximum complement of torque generators is eight. Each generator appears to contain both MotA and MotB. 10.1126/science.2849208

A Protonmotive Force Drives Bacterial Flagella

Michael Manson — 2008-05-02T18:21:07-00:00

Proceedings of the National Academy of Sciences, Vol. 74, No. 7. (15 July 1977), pp. 3060-3064.

Streptococcus strain V4051 is motile in the presence of glucose. The cells move steadily along smooth paths (run), jump about briefly with little net displacement (twiddle), and then run in new directions. They stop swimming when deprived of glucose. These cells become motile when an electrical potential or a pH gradient is imposed across the membrane. Starved cells suspended in a potassium-free medium respond to the addition of valinomycin by a brief period of vigorous twiddling. They also twiddle, although less vigorously, when the external pH is lowered. Valinomycin-induced twiddling occurs in the absence of external alkali or alkaline earth cations and without significant net synthesis of ATP. When a chemoattractant is added to cells swimming in the presence of glucose, twiddles are transiently suppressed, and the cells run for a time. Similarly, when starved cells are suspended in a potassium-free medium containing both valinomycin and an attractant, many cells initially run rather than twiddle. We conclude that the flagella are driven by a protonmotive force. 10.1073/pnas.74.7.3060

Minimal requirements for rotation of bacterial flagella.

S Ravid — 2008-05-02T18:20:17-00:00

J. Bacteriol., Vol. 158, No. 3. (1 June 1984), pp. 1208-1210.

An in vitro system of cell envelopes from Salmonella typhimurium with functional flagella was used to determine the minimal requirements for flagellar rotation. Rotation in the absence of cytoplasmic constituents could be driven either by respiration or by an artificially imposed chemical gradient of protons. No specific ionic requirements other than protons (or hydroxyls) were found for the motor function.

Chemomechanical Coupling without ATP: The Source of Energy for Motility and Chemotaxis in Bacteria

Steven Larsen — 2008-05-02T18:18:49-00:00

Proceedings of the National Academy of Sciences, Vol. 71, No. 4. (15 April 1974), pp. 1239-1243.

The source of energy for bacterial motility is the intermediate in oxidative phosphorylation, not ATP directly. For chemotaxis, however, there is an additional requirement, presumably ATP. These conclusions are based on the following findings. (i) Unlike their parents, mutants of Escherichia coli and Salmonella typhimurium that are blocked in the conversion of ATP to the intermediate of oxidative phosphorylation failed to swim anaerobically, even when they produced ATP. When respiration was restored to the mutants, motility was simultaneously restored. (ii) Carbonylcyanide m-chlorophenyl-hydrazone, which uncouples oxidative phosphorylation, completely inhibited motility even though ATP remained present. (iii) Arsenate did not inhibit motility in the presence of an oxidizable substrate, though it did reduce ATP levels to less than 0.3%. (iv) Arsenate completely inhibited chemotaxis under conditions where motility was normal. 10.1073/pnas.71.4.1239

Constraints on Models for the Flagellar Rotary Motor

Howard Berg — 2008-05-02T18:17:48-00:00

Philosophical Transactions: Biological Sciences, Vol. 355, No. 1396. (2000), pp. 491-501.

Most bacteria that swim are propelled by flagellar filaments, each driven at its base by a rotary motor embedded in the cell wall and cytoplasmic membrane. A motor is about 45 nm in diameter and made up of about 20 different kinds of parts. It is assembled from the inside out. It is powered by a proton (or in some species, a sodium-ion) flux. It steps at least 400 times per revolution. At low speeds and high torques, about 1000 protons are required per revolution, speed is proportional to protonmotive force, and torque varies little with temperature or hydrogen isotope. At high speeds and low torques, torque increases with temperature and is sensitive to hydrogen isotope. At room temperature, torque varies remarkably little with speed from about -100 Hz (the present limit of measurement) to about 200 Hz, and then it declines rapidly, reaching zero at about 300 Hz. These are facts that motor models should explain. None of the existing models for the flagellar rotary motor completely do so.

Torque-speed relationship of the flagellar rotary motor of Escherichia coli.

X Chen — 2005-12-31T02:16:45-00:00

Biophys J, Vol. 78, No. 2. (February 2000), pp. 1036-1041.

The output of a rotary motor is characterized by its torque and speed. We measured the torque-speed relationship of the flagellar rotary motor of Escherichia coli by a new method. Small latex spheres were attached to flagellar stubs on cells fixed to the surface of a glass slide. The angular speeds of the spheres were monitored in a weak optical trap by back-focal-plane interferometry in solutions containing different concentrations of the viscous agent Ficoll. Plots of relative torque (viscosity x speed) versus speed were obtained over a wide dynamic range (up to speeds of approximately 300 Hz) at three different temperatures, 22.7, 17.7, and 15.8 degrees C. Results obtained earlier by electrorotation (, Biophys. J. 65:2201-2216) were confirmed. The motor operates in two dynamic regimes. At 23 degrees C, the torque is approximately constant up to a knee speed of nearly 200 Hz, and then it falls rapidly with speed to a zero-torque speed of approximately 350 Hz. In the low-speed regime, torque is insensitive to changes in temperature. In the high-speed regime, it decreases markedly at lower temperature. These results are consistent with models in which torque is generated by a powerstroke mechanism (, Biophys. J. 76:580-587).

Torque and switching in the bacterial flagellar motor. An electrostatic model.

RM Berry — 2008-05-02T18:15:51-00:00

Biophys. J., Vol. 64, No. 4. (1 April 1993), pp. 961-973.

A model is presented for the rotary motor that drives bacterial flagella, using the electrochemical gradient of protons across the cytoplasmic membrane. The model unifies several concepts present in previous models. Torque is generated by proton-conducting particles around the perimeter of the rotor at the base of the flagellum. Protons in channels formed by these particles interact electrostatically with tilted lines of charges on the rotor, providing "loose coupling" between proton flux and rotation of the flagellum. Computer simulations of the model correctly predict the experimentally observed dynamic properties of the motor. Unlike previous models, the motor presented here may rotate either way for a given direction of the protonmotive force. The direction of rotation only depends on the level of occupancy of the proton channels. This suggests a novel and simple mechanism for the switching between clockwise and counterclockwise rotation that is the basis of bacterial chemotaxis.

Protein turbines. I: The bacterial flagellar motor.

TC Elston — 2008-05-02T18:14:57-00:00

Biophys. J., Vol. 73, No. 2. (1 August 1997), pp. 703-721.

The bacterial flagellar motor is driven by a flux of ions between the cytoplasm and the periplasmic lumen. Here we show how an electrostatic mechanism can convert this ion flux into a rotary torque. We demonstrate that, with reasonable parameters, the model can reproduce many of the experimental measurements.

The rotary motor of bacterial flagella.

HC Berg — 2005-12-31T02:14:38-00:00

Annu Rev Biochem, Vol. 72 (2003), pp. 19-54.

Flagellated bacteria, such as Escherichia coli, swim by rotating thin helical filaments, each driven at its base by a reversible rotary motor, powered by an ion flux. A motor is about 45 nm in diameter and is assembled from about 20 different kinds of parts. It develops maximum torque at stall but can spin several hundred Hz. Its direction of rotation is controlled by a sensory system that enables cells to accumulate in regions deemed more favorable. We know a great deal about motor structure, genetics, assembly, and function, but we do not really understand how it works. We need more crystal structures. All of this is reviewed, but the emphasis is on function.

Torque generated by the bacterial flagellar motor close to stall.

RM Berry — 2006-01-12T00:39:55-00:00

Biophys J, Vol. 71, No. 6. (December 1996), pp. 3501-3510.

In earlier work in which electrorotation was used to apply external torque to tethered cells of the bacterium Escherichia coli, it was found that the torque required to force flagellar motors backward was considerably larger than the torque required to stop them. That is, there appeared to be substantial barrier to backward rotation. Here, we show that in most, possibly all, cases this barrier is an artifact due to angular variation of the torque applied by electrorotation, of the motor torque, or both; the motor torque appears to be independent to speed or to vary linearly with speed up to speeds of tens of Hertz, in either direction. However, motors often break catastrophically when driven backward, so backward rotation is not equivalent to forward rotation. Also, cells can rotate backward while stalled, either in randomly timed jumps of 180 degrees or very slowly and smoothly. When cells rotate slowly and smoothly backward, the motor takes several seconds to recover after electrorotation is stopped, suggesting that some form of reversible damage has occurred. These findings do not affect the interpretation of electrorotation experiments in which motors are driven rapidly forward.

Torque-speed relationship of the bacterial flagellar motor.

J Xing — 2007-05-03T22:24:38-00:00

Proc Natl Acad Sci U S A, Vol. 103, No. 5. (31 January 2006), pp. 1260-1265.

Many swimming bacteria are propelled by flagellar filaments driven by a rotary motor. Each of these tiny motors can generate an impressive torque. The motor torque vs. speed relationship is considered one of the most important measurable characteristics of the motor and therefore is a major criterion for judging models proposed for the working mechanism. Here we give an explicit explanation for this torque-speed curve. The same physics also can explain certain puzzling properties of other motors.

A constitutive law for dense granular flows

Pierre Jop — 2006-06-07T20:26:31-00:00

Nature, Vol. 441, No. 7094. (June 2006), pp. 727-730.

Overshoot effect in the Janssen granular column: A crucial test for granular mechanics

G Ovarlez — 2008-04-29T13:43:46-00:00

Physical Review E, Vol. 67, No. 6. (30 June 2003), 060302.

Phase transition in a static granular system

Matthias Schröter — 2008-04-21T22:31:55-00:00

EPL (Europhysics Letters), Vol. 78, No. 4. (2007), 44004.

We find that a column of glass beads exhibits a well-defined transition between two phases that differ in their resistance to shear. Pulses of fluidization are used to prepare static sedimented states with well-defined particle volume fractions ph in the range 0.57-0.63. The resistance to shear is determined by slowly inserting a rod into the column of beads. Force measurements and bed height measurements both indicate that the transition occurs at ph = 0.60 for a range of speeds of the rod.

Creep, stick-slip, and dry-friction dynamics: Experiments and a heuristic model

F Heslot — 2008-04-14T02:32:45-00:00

Physical Review E, Vol. 49, No. 6. (1 June 1994), 4973.

Scaling and dynamics of sphere and disk impact into granular media

Daniel Goldman — 2008-04-14T00:46:54-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 2. (2008)

Direct measurements of the acceleration of spheres and disks impacting granular media reveal simple power law scalings along with complex dynamics which bear the signatures of both fluid and solid behavior. The penetration depth scales linearly with impact velocity while the collision duration is constant for sufficiently large impact velocity. Both quantities exhibit power law dependence on sphere diameter and density, and gravitational acceleration. The acceleration during impact is characterized by two jumps: a rapid, velocity-dependent increase upon initial contact and a similarly sharp depth-dependent decrease as the impacting object comes to rest. Examination of the measured forces on the sphere in the vicinity of these features leads to an experimentally based granular force model for collision. We discuss our findings in the context of recently proposed phenomenological models that capture qualitative dynamical features of impact but fail both quantitatively and in their inability to capture significant acceleration fluctuations that occur during penetration and which depend on the impacted material.

Local jamming via penetration of a granular medium

MB Stone — 2008-03-19T18:41:36-00:00

Physical Review E, Vol. 70, No. 4. (29 October 2004), 041301.

We present a series of measurements examining the penetration force required to push a flat plate vertically through a dense granular medium; focusing in particular on the effects of the bottom boundary of the vessel containing the medium. Our data demonstrate that the penetration force near the bottom is strongly affected by the surface properties of the bottom boundary; even many grain diameters above the bottom. Furthermore; the data indicate an intrinsic length scale for the interaction of the penetrating plate with the vessel bottom via the medium. This length scale; which corresponds to the extent of local jamming induced by the penetrating plate; has a square root dependence both upon the plate radius and the ambient granular stress near the bottom boundary; but it is independent of penetration velocity and grain diameter.

Effect of material properties on the packing of fine particles

RY Yang — 2008-03-15T22:46:34-00:00

Journal of Applied Physics, Vol. 94, No. 5. (2003), pp. 3025-3034.

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On the relationship between porosity and interparticle forces

AB Yu — 2008-03-15T22:36:42-00:00

Powder Technology, Vol. 130, No. 1-3. (19 February 2003), pp. 70-76.

This paper presents an attempt to quantify the relationship between porosity and interparticle forces for mono-sized spheres. Two systems are considered: the packing of wet coarse spheres where the dominant interparticle force is the capillary force, and the packing of dry fine spheres where the dominant force is the van der Waals force. The interrelationships between porosity, capillary force and liquid content are first discussed based on the well-established theories and experimental observations. The resultant relationship between porosity and capillary force is then applied to the packing of fine particles to quantify the van der Waals force in a packing. A generalised relationship between porosity and interparticle forces results as an extension of this analysis. The usefulness of this relationship is finally demonstrated in depicting the fundamentals governing the relationship between porosity and particle size.

Computer simulation of the packing of fine particles

RY Yang — 2008-03-15T22:30:16-00:00

Physical Review E, Vol. 62, No. 3. (2000), 3900.

This paper presents a simulation study of the packing of uniform fine-spherical particles where the van der Waals force is dominant. It is shown that porosity increases with the decreases of particle size from about 100 to 1 μm and the simulated relationship can match the literature data well. The packing structure of fine particles is qualitatively depicted by illustrative pictures and quantified in terms of radial distribution function; angular distribution; and coordination number. The results indicate that in line with the increase in porosity; the first component of the split second peak and then the other peaks beyond the second one in the radial distribution function gradually vanish; the first peak becomes narrower; with a sharp decrease to the first minimum. As particle size decreases; the peaks at 120° and then 60° in the angular distribution will gradually vanish; the coordination number distribution shifts to the left and becomes narrower. The mean coordination number can decrease to a value as low as two for 1 μm particles; giving a very loose and chainlike structure. The interparticle forces acting on individual particles in a stable packing are analyzed and shown to be related to the packing properties.

Plug flow and the breakdown of Bagnold scaling in cohesive granular flows

Robert Brewster — 2008-02-24T06:46:02-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 72, No. 6. (2005)

Cohesive granular media flowing down an inclined plane are studied by discrete element simulations. Previous work on cohesionless granular media demonstrated that within the steady flow regime where gravitational energy is balanced by dissipation arising from intergrain forces, the velocity profile in the flow direction scales with depth in a manner consistent with the predictions of Bagnold. Here we demonstrate that this Bagnold scaling does not hold for the analogous steady flows in cohesive granular media. We develop a generalization of the Bagnold constitutive relation to account for our observation and speculate as to the underlying physical mechanisms responsible for the different constitutive laws for cohesive and noncohesive granular media.

Rheology and Contact Lifetimes in Dense Granular Flows

Leonardo Silbert — 2008-02-24T06:45:24-00:00

Physical Review Letters, Vol. 99, No. 6. (2007)

We study the rheology and distribution of interparticle contact lifetimes for gravity-driven, dense granular flows of noncohesive particles down an inclined plane using large-scale, three dimensional, granular dynamics simulations. Rather than observing a large number of long-lived contacts as might be expected for dense flows, brief binary collisions predominate. In the hard-particle limit, the rheology conforms to Bagnold scaling, where the shear stress is quadratic in the strain rate. As the particles are made softer, however, we find significant deviations from Bagnold rheology; the material flows more like a viscous fluid. We attribute this change in the collective rheology of the material to subtle changes in the contact lifetime distribution involving the increasing lifetime and number of the long-lived contacts in the softer particle systems.

Random loose packing of cohesive granular materials

JM Valverde — 2008-02-24T06:40:36-00:00

EPL (Europhysics Letters), Vol. 75, No. 6. (2006), pp. 985-991.

The volume fraction ph of a disordered packing of noncohesive hard spheres may vary between the random loose packing at the limit of zero gravitational stress (phRLP [?] 0.56) and the random close packing (phRCP [?] 0.64). When interparticle attractive forces are present it is known however that ph can be as low as [?] 0.01 for nanoparticles. Experimental and numerical works show that ph decreases as the ratio of interparticle attractive force to particle weight (Bog) is increased. We focus on the packing fraction of cohesive particles settled at the limit of zero gravity (jamming transition). From a model of particle agglomeration in suspensions we find the simple equation phJ [?] phJ*Bog(D [?] 3)/(D + 2), where phJ* is the volume fraction of particle agglomerates at jamming, which is close to phRLP, and D their fractal dimension. Our experimental results on fine powders agree with this equation and show that agglomeration of particles in suspension is the relevant physical mechanism to understand the packing of cohesive particles.

On the Brink of Jamming: Granular Convection in Densely Filled Containers

Frank Rietz — 2008-02-23T22:06:54-00:00

Physical Review Letters, Vol. 100, No. 7. (2008)

Granulates are ubiquitous in nature and technology, but, despite their great importance, their dynamics are by far less well understood than those of liquids. We demonstrate in an almost compactly filled flat (Hele-Shaw) cell, where slow horizontal rotation simulates a variable gravitational force, that unexpected dynamic structures may arise under geometrical restrictions. The cell motion drives regular flow in the compact interior, and convection rolls combine with segregation. The container fill level is crucial for the dynamic regime. A transition from chute flow at lower fill levels to convection in densely packed containers is found. These observations suggest the existence of comparable phenomena in situations where so far no systematic search for dynamic patterns has been performed.

Flow, Ordering, and Jamming of Sheared Granular Suspensions

Denis Grebenkov — 2008-02-23T22:06:19-00:00

Physical Review Letters, Vol. 100, No. 7. (2008)

We study the rheological properties of a granular suspension subject to constant shear stress by constant volume molecular dynamics simulations. We derive the system “flow diagram” in the volume fraction or stress plane (, F): at low the flow is disordered, with the viscosity obeying a Bagnold-like scaling only at small F and diverging as the jamming point is approached; if the shear stress is strong enough, at higher an ordered flow regime is found, the order-disorder transition being marked by a sharp drop of the viscosity. A broad jamming region is also observed where, in analogy with the glassy region of thermal systems, slow dynamics followed by kinetic arrest occurs when the ordering transition is prevented.

On the Hausner Ratio and its relationship to some properties of metal powders

RO Grey — 2008-02-23T00:47:34-00:00

Powder Technology, Vol. 2, No. 6. (September 1969), pp. 323-326.

The ratio between the tap density and apparent density of a powder mass is termed the Hausner Ratio. Values of this ratio, flow times and angles of repose are compared for three types of dry powder and for two types of oiled copper powder. The results indicate that the Hausner Ratio measures the friction condition in a moving powder mass. It is concluded that the Hausner Ratio is a characteristic of a powder in addition to angle of repose and flow time. Knowledge of the Hausner Ratio is of use in gaining a basic understanding of the mechanism in sieving and of the initial stage of compaction of powders.

Granular physics: Creating a dry variety of quicksand

Detlef Lohse — 2008-02-21T23:10:01-00:00

Nature, Vol. 432, No. 7018. (December 2004), pp. 689-690.

Impact on Soft Sand: Void Collapse and Jet Formation

Detlef Lohse — 2008-02-21T23:08:38-00:00

Physical Review Letters, Vol. 93, No. 19. (3 November 2004), 198003.

Very fine sand is prepared in a well-defined and fully decompactified state by letting gas bubble through it. After turning off the gas stream; a steel ball is dropped on the sand. On impact of the ball; sand is blown away in all directions (“splash”) and an impact crater forms. When this cavity collapses; a granular jet emerges and is driven straight into the air. A second jet goes downwards into the air bubble entrained during the process; thus pushing surface material deep into the ground. The air bubble rises slowly towards the surface; causing a granular eruption. In addition to the experiments and the discrete particle simulations we present a simple continuum theory to account for the void collapse leading to the formation of the upward and downward jets.