CiteULike: Tag qual

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.

RAPID GRANULAR FLOWS

Isaac Goldhirsch — 2008-03-06T05:32:38-00:00

Annual Review of Fluid Mechanics, Vol. 35, No. 1. (2003), pp. 267-293.

Abstract The recent avalanche of research activity in the field of granular matter has yielded much progress. The use of state-of-the-art (and other) computational and experimental methods has led to the discovery of new states and patterns and enabled detailed tests of theories and models. The application of statistical mechanical methods and phenomenology has contributed to the understanding of the microscopic a nd macroscopic properties of granular systems. Some previously open problems seem to be solved. Fluidized granular systems (rapid granular flows), recently referred to as granular gases, are often modeled by hydrodynamic equations of motion, some of which are based on systematic expansions applied to the pertinent Boltzmann equation. The undeniable success of granular hydrodynamics is somewhat surprising in view of the lack of scale separation in these systems and the neglect of certain correlations in most derivations of the hydrodynamic equations. Microstructures have been recognized as key features of granular gases; explanations for their existence have been proposed, and many of their properties elucidated. Granular-gas multistability can often be traced back to microstructure dynamics. In spite of these and other impressive advances, this field still poses serious challenges.

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.

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.

Slow Drag in a Granular Medium

R Albert — 2008-05-15T05:55:40-00:00

Physical Review Letters, Vol. 82, No. 1. (4 January 1999), 205.

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.

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.

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.

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.

Low density fragile states in cohesive powders

Paul Umbanhowar — 2008-02-21T23:08:20-00:00

American Journal of Physics, Vol. 74, No. 8. (2006), pp. 720-721.

We discuss the difference between cohesive and non-cohesive granular media in the context of dry quicksand, recently proposed as a new fragile state of sand. We demonstrate that weak low density configurations with properties like dry quicksand are readily formed in many common household powders. In contrast, such states cannot be formed in non-cohesive granular media such as ordinary sand. ©2006 American Association of Physics Teachers

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.

Projectile impact and penetration in loose granular bed

M Hou — 2008-02-21T16:18:52-00:00

Science and Technology of Advanced Materials, Vol. 6, No. 7. (October 2005), pp. 855-859.

Dynamics of a projectile impacting on a loose granular bed under the acceleration due to gravity g has been studied by fast video photograph. Granular jet formation and projectile penetration are observed in 3D and quasi-2D experiments. It is found that the penetration velocity u can be described by , where [gamma] and [kappa] are the parameters which characterize the viscous damping and hydrostatic drag forces of the bed, respectively, z is the penetration distance of the projectile, and g' is a modified gravity term. The viscous damping term is found important in quasi-2D experiments. For 3D, the damping term is only important at the beginning of the impact, and can be neglected during penetration.

Dynamics of a projectile penetrating in granular systems

M Hou — 2008-02-21T16:18:08-00:00

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

The impact of a sphere with velocity u0 on a fine, loose granular system under the acceleration due to gravity has been studied by fast video photography. The behavior of the granular bed is found to be similar to a fluid during initial impact, followed by a cavity drag during projectile penetration. From the trajectory of the projectile it is found that the drag on the projectile can be well described by adding a bulk frictional force f to the hydrostatic force z where is a constant and z denotes the penetration depth. Both and f are u0 dependent. This form of the drag force suggests that fluidlike viscous dissipations in the bed can be neglected in these three-dimensional (3D) experiments. However, due to the imposed boundary this hydrodynamic term of the drag force is found to be not negligible in quasi-2D granular beds.

Mauve: multiple alignment of conserved genomic sequence with rearrangements.

AC Darling — 2005-11-14T16:09:29-00:00

Genome Res, Vol. 14, No. 7. (July 2004), pp. 1394-1403.

As genomes evolve, they undergo large-scale evolutionary processes that present a challenge to sequence comparison not posed by short sequences. Recombination causes frequent genome rearrangements, horizontal transfer introduces new sequences into bacterial chromosomes, and deletions remove segments of the genome. Consequently, each genome is a mosaic of unique lineage-specific segments, regions shared with a subset of other genomes and segments conserved among all the genomes under consideration. Furthermore, the linear order of these segments may be shuffled among genomes. We present methods for identification and alignment of conserved genomic DNA in the presence of rearrangements and horizontal transfer. Our methods have been implemented in a software package called Mauve. Mauve has been applied to align nine enterobacterial genomes and to determine global rearrangement structure in three mammalian genomes. We have evaluated the quality of Mauve alignments and drawn comparison to other methods through extensive simulations of genome evolution.

Whole-genome sequencing and assembly with high-throughput, short-read technologies.

A Sundquist — 2007-08-08T14:23:56-00:00

PLoS ONE, Vol. 2 (2007)

While recently developed short-read sequencing technologies may dramatically reduce the sequencing cost and eventually achieve the $1000 goal for re-sequencing, their limitations prevent the de novo sequencing of eukaryotic genomes with the standard shotgun sequencing protocol. We present SHRAP (SHort Read Assembly Protocol), a sequencing protocol and assembly methodology that utilizes high-throughput short-read technologies. We describe a variation on hierarchical sequencing with two crucial differences: (1) we select a clone library from the genome randomly rather than as a tiling path and (2) we sample clones from the genome at high coverage and reads from the clones at low coverage. We assume that 200 bp read lengths with a 1% error rate and inexpensive random fragment cloning on whole mammalian genomes is feasible. Our assembly methodology is based on first ordering the clones and subsequently performing read assembly in three stages: (1) local assemblies of regions significantly smaller than a clone size, (2) clone-sized assemblies of the results of stage 1, and (3) chromosome-sized assemblies. By aggressively localizing the assembly problem during the first stage, our method succeeds in assembling short, unpaired reads sampled from repetitive genomes. We tested our assembler using simulated reads from D. melanogaster and human chromosomes 1, 11, and 21, and produced assemblies with large sets of contiguous sequence and a misassembly rate comparable to other draft assemblies. Tested on D. melanogaster and the entire human genome, our clone-ordering method produces accurate maps, thereby localizing fragment assembly and enabling the parallelization of the subsequent steps of our pipeline. Thus, we have demonstrated that truly inexpensive de novo sequencing of mammalian genomes will soon be possible with high-throughput, short-read technologies using our methodology.

De novo bacterial genome sequencing: millions of very short reads assembled on a desktop computer.

David Hernandez — 2008-03-17T23:46:00-00:00

Genome Res (10 March 2008)

Novel high-throughput DNA sequencing technologies allow researchers to characterize a bacterial genome during a single experiment and at a moderate cost. However, the increase in sequencing throughput that is allowed by using such platforms is obtained at the expense of individual sequence read length, which must be assembled into longer contigs to be exploitable. This study focuses on the Illumina sequencing platform that produces millions of very short sequences that are 35 bases in length. We propose a de novo assembler software that is dedicated to process such data. Based on a classical overlap graph representation and on the detection of potentially spurious reads, our software generates a set of accurate contigs of several kilobases that cover most of the bacterial genome. The assembly results were validated by comparing datasets that were obtained experimentally for Staphylococcus aureus strain MW2 and Helicobacter acinonychis strain Sheeba with that of their published genomes acquired by conventional sequencing of 1.5 - 3.0 kb fragments. We also provide indications that the broad coverage achieved by high throughput sequencing might allow for the detection of clonal polymorphisms in the set of DNA molecules being sequenced.

A general approach to single-nucleotide polymorphism discovery.

GT Marth — 2007-02-14T15:38:23-00:00

Nat Genet, Vol. 23, No. 4. (December 1999), pp. 452-456.

Single-nucleotide polymorphisms (SNPs) are the most abundant form of human genetic variation and a resource for mapping complex genetic traits. The large volume of data produced by high-throughput sequencing projects is a rich and largely untapped source of SNPs (refs 2, 3, 4, 5). We present here a unified approach to the discovery of variations in genetic sequence data of arbitrary DNA sources. We propose to use the rapidly emerging genomic sequence as a template on which to layer often unmapped, fragmentary sequence data and to use base quality values to discern true allelic variations from sequencing errors. By taking advantage of the genomic sequence we are able to use simpler yet more accurate methods for sequence organization: fragment clustering, paralogue identification and multiple alignment. We analyse these sequences with a novel, Bayesian inference engine, POLYBAYES, to calculate the probability that a given site is polymorphic. Rigorous treatment of base quality permits completely automated evaluation of the full length of all sequences, without limitations on alignment depth. We demonstrate this approach by accurate SNP predictions in human ESTs aligned to finished and working-draft quality genomic sequences, a data set representative of the typical challenges of sequence-based SNP discovery.

Whole-genome comparison of disease and carriage strains provides insights into virulence evolution in Neisseria meningitidis.

C Schoen — 2008-03-11T13:48:44-00:00

Proc Natl Acad Sci U S A, Vol. 105, No. 9. (4 March 2008), pp. 3473-3478.

Neisseria meningitidis is a leading cause of infectious childhood mortality worldwide. Most research efforts have hitherto focused on disease isolates belonging to only a few hypervirulent clonal lineages. However, up to 10% of the healthy human population is temporarily colonized by genetically diverse strains mostly with little or no pathogenic potential. Currently, little is known about the biology of carriage strains and their evolutionary relationship with disease isolates. The expression of a polysaccharide capsule is the only trait that has been convincingly linked to the pathogenic potential of N. meningitidis. To gain insight into the evolution of virulence traits in this species, whole-genome sequences of three meningococcal carriage isolates were obtained. Gene content comparisons with the available genome sequences from three disease isolates indicate that there is no core pathogenome in N. meningitidis. A comparison of the chromosome structure suggests that a filamentous prophage has mediated large chromosomal rearrangements and the translocation of some candidate virulence genes. Interspecific comparison of the available Neisseria genome sequences and dot blot hybridizations further indicate that the insertion sequence IS1655 is restricted only to N. meningitidis; its low sequence diversity is an indicator of an evolutionarily recent population bottleneck. A genome-based phylogenetic reconstruction provides evidence that N. meningitidis has emerged as an unencapsulated human commensal from a common ancestor with Neisseria gonorrhoeae and Neisseria lactamica and consecutively acquired the genes responsible for capsule synthesis via horizontal gene transfer.

Two-dimensional granular Poiseuille flow on an incline: Multiple dynamical regimes

JC Tsai — 2008-04-26T16:34:11-00:00

Physical Review E, Vol. 65, No. 1. (19 December 2001), 011306.

Clogging Time of a Filter

S Redner — 2008-05-28T11:02:21-00:00

Physical Review Letters, Vol. 84, No. 26. (26 June 2000), 6018.

Monolithic Microfabricated Valves and Pumps by Multilayer Soft Lithography

Marc Unger — 2008-02-23T06:41:19-00:00

Science, Vol. 288, No. 5463. (7 April 2000), pp. 113-116.

10.1126/science.288.5463.113

Suspension extraction through an opening before clogging

Guillermo Goldsztein — 2008-04-29T11:16:55-00:00

Applied Physics Letters, Vol. 85, No. 19. (2004), pp. 4535-4537.

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Granular flow from a silo: Discrete-particle simulations in three dimensions

D Hirshfeld — 2008-06-13T12:50:30-00:00

The European Physical Journal E - Soft Matter, Vol. 4, No. 2. (19 January 2001), pp. 193-199.

Abstract: Molecular (or granular) dynamics methods are used to study the gravity-driven flow of granular material through a horizontal aperture in three dimensions. The grains are spherical and modeled using a short-range repulsive interaction, together with normal and tangential frictional damping forces. The material is contained in a rough-walled cylindrical container with a circular hole in its base, and to permit flow measurements under steady-state conditions a continuous feed approach is employed in which exiting grains are replaced at the upper surface of the material. The dependence of flow velocity and discharge rate on aperture diameter is found to agree with experiment; other quantities such as the kinetic energy and pressure distributions are also examined.

ENGINEERING FLOWS IN SMALL DEVICESMicrofluidics Toward a Lab-on-a-Chip

HA Stone — 2008-05-28T10:48:48-00:00

Annual Review of Fluid Mechanics, Vol. 36, No. 1. (2004), pp. 381-411.

Abstract Microfluidic devices for manipulating fluids are widespread and finding uses in many scientific and industrial contexts. Their design often requires unusual geometries and the interplay of multiple physical effects such as pressure gradients, electrokinetics, and capillarity. These circumstances lead to interesting variants of well-studied fluid dynamical problems and some new fluid responses. We provide an overview of flows in microdevices with focus on electrokinetics, mixing and dispersion, and multiphase flows. We highlight topics important for the description of the fluid dynamics: driving forces, geometry, and the chemical characteristics of surfaces.

Microfluidics: Fluid physics at the nanoliter scale

Todd Squires — 2006-01-16T15:14:37-00:00

Reviews of Modern Physics, Vol. 77, No. 3. (2005)

Microfabricated integrated circuits revolutionized computation by vastly reducing the space, labor, and time required for calculations. Microfluidic systems hold similar promise for the large-scale automation of chemistry and biology, suggesting the possibility of numerous experiments performed rapidly and in parallel, while consuming little reagent. While it is too early to tell whether such a vision will be realized, significant progress has been achieved, and various applications of significant scientific and practical interest have been developed. Here a review of the physics of small volumes (nanoliters) of fluids is presented, as parametrized by a series of dimensionless numbers expressing the relative importance of various physical phenomena. Specifically, this review explores the Reynolds number Re, addressing inertial effects; the Péclet number Pe, which concerns convective and diffusive transport; the capillary number Ca expressing the importance of interfacial tension; the Deborah, Weissenberg, and elasticity numbers De, Wi, and El, describing elastic effects due to deformable microstructural elements like polymers; the Grashof and Rayleigh numbers Gr and Ra, describing density-driven flows; and the Knudsen number, describing the importance of noncontinuum molecular effects. Furthermore, the long-range nature of viscous flows and the small device dimensions inherent in microfluidics mean that the influence of boundaries is typically significant. A variety of strategies have been developed to manipulate fluids by exploiting boundary effects; among these are electrokinetic effects, acoustic streaming, and fluid-structure interactions. The goal is to describe the physics behind the rich variety of fluid phenomena occurring on the nanoliter scale using simple scaling arguments, with the hopes of developing an intuitive sense for this occasionally counterintuitive world.

Molecular dynamics studies of granular flow through an aperture

D Hirshfeld — 2008-06-13T12:48:18-00:00

Physical Review E, Vol. 56, No. 4. (1 October 1997), 4404.

Molecular simulations of lubrication and solvation forces

Sivakumar Challa — 2007-12-31T22:24:24-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 73, No. 1. (2006)

We report on molecular-dynamics simulations of the drag force experienced by a smooth sphere as it approaches a smooth planar surface to test the predictions of classical hydrodynamic theory. We use a simple repulsive Lennard-Jones-like model to represent the fluid interactions, and calculate the total force on the sphere as a function of its radius, velocity, and distance from the surface. We find that the presence of static solvation forces complicates the testing of hydrodynamic theory which predicts a divergent repulsive lubrication force as the gap vanishes. The solvation force contribution is most prominent at small gaps and small velocities. For a smooth wall its presence can lead to a total force that is oscillating between positive and negative, quite different from the hydrodynamic prediction. To enable an improved test of the lubrication predictions, we propose a different approach that measures the total force for approaching as well as receding spheres. We suggest a simple general analysis that decouples the dynamic and static force contributions on the sphere. The new decoupling method is applicable to simulations and laboratory experiments alike. We illustrate its power by applying it to the molecular-dynamics data.

Velocity profile of granular flows inside silos and hoppers

Jaehyuk Choi — 2008-03-24T22:32:04-00:00

Journal of Physics: Condensed Matter, Vol. 17, No. 24. (2005), pp. S2533-S2548.

We measure the flow of granular materials inside a quasi-two-dimensional silo as it drains and compare the data with some existing models. The particles inside the silo are imaged and tracked with unprecedented resolution in both space and time to obtain their velocity and diffusion properties. The data obtained by varying the orifice width and the hopper angle allow us to thoroughly test models of gravity driven flows inside these geometries. All of our measured velocity profiles are smooth and free of the shock-like discontinuities ('rupture zones') predicted by critical state soil mechanics. On the other hand, we find that the simple kinematic model accurately captures the mean velocity profile near the orifice, although it fails to describe the rapid transition to plug flow far away from the orifice. The measured diffusion length b, the only free parameter in the model, is not constant as usually assumed, but increases with both the height above the orifice and the angle of the hopper. We discuss improvements to the model to account for the differences. From our data, we also directly measure the diffusion of the particles and find it to be significantly less than predicted by the void model, which provides the classical microscopic derivation of the kinematic model in terms of diffusing voids in the packing. However, the experimental data are consistent with the recently proposed spot model, based on a simple mechanism for cooperative diffusion. Finally, we discuss the flow rate as a function of the orifice width and hopper angles. We find that the flow rate scales with the orifice size to the power of 1.5, consistent with dimensional analysis. Interestingly, the flow rate increases when the funnel angle is increased.

Lubrication effects on the flow of wet granular materials

Qing Xu — 2008-01-23T19:06:56-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 3. (2007)

We investigate the dynamics of a partially saturated grain-liquid mixture with a rotating drum apparatus. The drum is partially filled with the mixture and then rotated about its horizontal axis. We focus on the continuous avalanching regime and measure the impact of the volume fraction and viscosity of the liquid on the dynamic surface angle. The inclination angle of the surface is observed to increase sharply to a peak and then decrease as a function of liquid volume fraction. The height of the peak is observed to increase with rotation rate. For higher liquid volume fractions, the inclination angle of the surface can decrease with viscosity before increasing. The viscosity where the minimum occurs decreases with the rotation rate of the drum. Limited measurements of the flow depth were made, and these were observed to show only fractional changes with volume fraction and rotation speeds. We show that the qualitative features of our observations can be understood by analyzing the effect of lubrication forces on the time scale over which particles come in contact.

Analogies between granular jamming and the liquid-glass transition

Leonardo Silbert — 2008-04-26T14:40:32-00:00

Physical Review E, Vol. 65, No. 5. (21 May 2002), 051307.

Structure and distribution of arches in shaken hard sphere deposits

Pugnaloni — 2008-04-26T14:38:17-00:00

Physica A: Statistical and Theoretical Physics, Vol. 337, No. 3-4. (15 June 2004), pp. 428-442.

We investigate the structure and distribution of arches formed by spherical, hard particles shaken in an external field after they come to rest. Arches (or bridges) are formed during a computer-simulated, non-sequential deposition of the spheres after each shaking cycle. We identify these arches by means of a connectivity criterion and study their structural characteristics and spatial distribution. We find that neither the size distribution nor the shape of the arches is strongly affected by the packing fraction of the deposit. Conversely, the spatial distribution and orientation of the bridges do depend on the volume fraction occupied by the spheres.

Jamming phase diagram for attractive particles

V Trappe — 2006-09-13T15:26:10-00:00

Nature, Vol. 411, No. 6839. (14 June 2001), pp. 772-775.

Hydrodynamic interactions between two spheres at contact

ML Ekiel-Jeżewska — 2008-05-17T03:38:11-00:00

Physical Review E, Vol. 59, No. 3. (1 March 1999), 3182.

Long-Range Correlations in Sedimentation

PN Segrè — 2008-04-11T04:13:52-00:00

Physical Review Letters, Vol. 79, No. 13. (1997), 2574.

Analytical Approach to Continuous and Intermittent Bottleneck Flows

Dirk Helbing — 2008-04-26T14:30:56-00:00

Physical Review Letters, Vol. 97, No. 16. (2006)

We propose a many-particle-inspired theory for granular outflows from a hopper and for the escape dynamics through a bottleneck based on a continuity equation in polar coordinates. If the inflow is below the maximum outflow, we find an asymptotic stationary solution. If the inflow is above this value, we observe queue formation, which can be described by a shock wave equation. We also address the experimental observation of intermittent outflows, taking into account the lack of space in the merging zone by a minimum function and coordination problems by a stochastic variable. This results in avalanches of different sizes even if friction, force networks, inelastic collapse, or delay-induced stop-and-go waves are not assumed. Our intermittent flows result from a random alternation between particle propagation and gap propagation. Erratic flows in congested merging zones of vehicle traffic may be explained in a similar way.

Deposition of Colloidal Asphaltene in Capillary Flow: Experiments and Mesoscopic Simulation

Edo Boek — 2008-04-28T11:22:35-00:00

Energy Fuels, Vol. 22, No. 2. (19 March 2008), pp. 805-813.

Abstract: The aggregation and deposition of colloidal asphaltene in reservoir rock is a significant problem in the oil industry. To obtain a fundamental understanding of this phenomenon, we have studied the deposition and aggregation of colloidal asphaltene in capillary flow by experiment and simulation. For the simulation, we have used the stochastic rotation dynamics (SRD) method, in which the solvent hydrodynamic emerges from the collisions between the solvent particles, while the Brownian motion emerges naturally from the interactions between the colloidal asphaltene particles and the solvent. The asphaltene colloids interact through a screened Coulomb potential. We vary the well depth µcc and the flow rate v to obtain Peflow >> 1 (hydrodynamic interactions dominate) and Re << 1 (Stokes flow). In the simulations, we impose a pressure drop over the capillary length and measure the corresponding solvent flow rate. We observe that the transient solvent flow rate decreases when the asphaltene particles become more sticky. For a well depth µcc = 2kBT, a monolayer deposits on the capillary wall. With an increasing well depth, the capillary becomes totally blocked. The clogging is transient for µcc = 5kBT, but appears to be permanent for µcc = 1020kBT. We compare our simulation results with flow experiments in glass capillaries, where we use extracted asphaltenes in toluene, reprecipitated with n-heptane. In the experiments, the dynamics of asphaltene precipitation and deposition were monitored in a slot capillary using optical microscopy under flow conditions similar to those used in the simulation. Maintaining a constant flow rate of 5 µL min1, we found that the pressure drop across the capillary first increased slowly, followed by a sharp increase, corresponding to a complete local blockage of the capillary. Doubling the flow rate to 10 µL min1, we observe that the initial deposition occurs faster but the deposits are subsequently entrained by the flow. We calculate the change in the dimensionless permeability as a function of time for both experiment and simulation. By matching the experimental and simulation results, we obtain information about (1) the interaction potential well depth for the particular asphaltenes used in the experiments and (2) the flow conditions associated with the asphaltene deposition process.

Continuous Particle Separation Through Deterministic Lateral Displacement

Lotien Huang — 2007-11-30T17:48:27-00:00

Science, Vol. 304, No. 5673. (14 May 2004), pp. 987-990.

We report on a microfluidic particle-separation device that makes use of the asymmetric bifurcation of laminar flow around obstacles. A particle chooses its path deterministically on the basis of its size. All particles of a given size follow equivalent migration paths, leading to high resolution. The microspheres of 0.8, 0.9, and 1.0 micrometers that were used to characterize the device were sorted in 40seconds with a resolution of [~]10nanometers, which was better than the time and resolution of conventional flow techniques. Bacterial artificial chromosomes could be separated in 10 minutes with a resolution of [~]12%. 10.1126/science.1094567

Development of particle migration in pressure-driven flow of a Brownian suspension

Denis Semwogerere — 2008-05-14T11:41:14-00:00

Journal of Fluid Mechanics, Vol. 581, No. -1. (2007), pp. 437-451.

An experimental investigation into the influence of Brownian motion on shear-induced particle migration of monodisperse suspensions of micrometre-sized colloidal particles is presented. The suspension is pumped through a 50 μm × 500 μm rectangular cross-section glass channel. The experiments are characterized chiefly by the sample volume fraction (φ = 0.1 − 0.4), and the flow rate expressed as the Péclet number (Pe = 10 − 400). For each experiment we measure the entrance length, which is the distance from the inlet of the channel required for the concentration profile to develop to its non-uniform steady state. The entrance length increases strongly with increasing Pe for Pe ≪ 100, in marked contrast to non-Brownian flows for which the entrance length is flow-rate independent. For larger Pe, the entrance length reaches a constant value which depends on the other experimental parameters. Additionally, the entrance length decreases with increasing φ; this effect is strongest for low φ. Modelling of the migration based on spatial variation of the normal stresses due to the particles captures the primary features observed in the axial evolution over a range of Pe and φ.

Transition in particle capture in deep bed filtration

Claude Ghidaglia — 2008-06-20T00:38:13-00:00

Physical Review E, Vol. 53, No. 4. (1 April 1996), R3028.

Flow and jamming of granular mixtures through obstacles

F Chevoir — 2008-04-26T14:29:09-00:00

EPL (Europhysics Letters), Vol. 79, No. 1. (2007), 14001.

Due to the formation of arches, granular materials may jam when flowing through obstacles, as in the case of hoppers. As a way to quantify this process, we study experimentally the flow of binary granular mixtures through sieves, as a function of two parameters: the proportion of large grains and the ratio of large grains to sieve hole size. We distinguish three regimes: steady flows, jamming, and progressive clogging. In the case of steady flows, we measure the dependencies of the flow rate on the two parameters and observe a generalization of the law known for mono-disperse grains flowing through a single aperture. Moreover we measure how the critical size of the holes leading to jamming depends on the proportion of large grains. In the case of progressive clogging, we measure the slowing down of the flow rate and identify two mechanisms associated to the trapping of the large grains in the holes of the sieves and then to the formation of a filtration cake.

The flow rate of granular materials through an orifice

C Mankoc — 2008-06-18T18:02:02-00:00

Granular Matter, Vol. 9, No. 6. (6 November 2007), pp. 407-414.

Abstract The flow rate of grains through large orifices is known to be dependent on its diameter to a 5/2 power law. This relationship has been checked for big outlet sizes, whereas an empirical fitting parameter is needed to reproduce the behavior for small openings. In this work, we provide experimental data and numerical simulations covering a wide span of outlet sizes, both in three- and two-dimensions. This allows us to show that the laws that are usually employed are satisfactory only if a small range of openings is considered. We propose a new law for the mass flow rate of grains that correctly reproduces the data for all the orifice sizes, including the behaviors for very large and very small outlet sizes.

Dilatant Flow of Concentrated Suspensions of Rough Particles

Didier Lootens — 2007-12-31T22:26:03-00:00

Physical Review Letters, Vol. 95, No. 26. (2005)

The flow anisotropy of a concentrated colloidal suspension at the jamming transition is studied. It is shown that the use of rough spherical particles reduces the hydrodynamic lubrication forces between adjacent colloids and makes possible the study of the stress tensor anisotropy. At low shear rates, the suspension exerts an attractive force between two opposite surfaces, whereas at higher shear rates it becomes dilatant. Direct confocal microscopy observation of the particles organization reveal that crystallites form at high shear rate.

Jamming during the discharge of grains from a silo described as a percolating transition

Iker Zuriguel — 2008-05-28T15:32:21-00:00

Physical Review E, Vol. 68, No. 3. (2003), 030301.

Microfluidic chip for blood cell separation and collection based on crossflow filtration

Xing Chen — 2008-04-26T14:10:58-00:00

Sensors and Actuators B: Chemical, Vol. 130, No. 1. (14 March 2008), pp. 216-221.

Whole blood is a mixture of various cells, such as red blood cell (RBC), white blood cell (WBC) and so on. Separation and collection of WBC and RBC, starting from a sample of whole blood, are the required steps for the subsequent clinica and basic research assays. We created two kinds of microfluidic chips based on the crossflow filtration principle which can be more effective than conventional types in the area of avoiding clogging or jamming. Pillar-type and weir-type filtration microchips were designed and fabricated by microelectromechanical system (MEMS) technology, in which parallel micropillar-array and parallel microweirs were used to separate cells via their different sizes. After separation, WBC and RBC were collected, respectively. Cell concentration and the length of separation channels were investigated and optimized. Under the optimal condition, more than 95% RBC can be removed from the initial whole blood, while 27.4% WBC can be obtained. Isolation efficiency of WBC by using the crossflow filtration microchip is approximately twice as high as that of the dead-end filtration microchip. Furthermore, plasma, WBC and RBC can be simultaneously separated and collected at different outlet ports with multilevel filtration barriers.

From Micro- to Nanofabrication with Soft Materials

Stephen Quake — 2007-09-27T08:45:33-00:00

Science, Vol. 290, No. 5496. (24 November 2000), pp. 1536-1540.

10.1126/science.290.5496.1536

Intermittent granular flow and clogging with internal avalanches

SS Manna — 2008-06-06T14:08:44-00:00

The European Physical Journal E - Soft Matter, Vol. 1, No. 4. (19 April 2000), pp. 341-344.

Abstract: The dynamics of intermittent granular flow through an orifice at the bottom of a granular bin and the associated clogging due to formation of arches blocking the outlet, is studied numerically in two dimensions. When the hole size is less than the grain diameter, only a single grain is removed from the system so that the system self-organizes to a steady state and the distribution of the grain displacements decays as power laws. On the other hand, when hole sizes are within few times of the grain diameter, the outflow distributions are also observed to follow a power law.

Effect of interstitial fluid on a granular flowing layer

Nitin Jain — 2008-06-20T00:13:04-00:00

Journal of Fluid Mechanics, Vol. 508, No. -1. (2004), pp. 23-44.

A dominant aspect of granular flows is flow in thin surface layers. While an understanding of the dynamics of dry granular surface flow has begun to emerge, the case of flow when air is completely replaced by a liquid is largely unexplored. Experiments were performed using particle tracking velocimetry (PTV) in a quasi-two-dimensional rotating tumbler to measure the velocity field within the flowing layer of monodisperse spherical particles fully submerged in liquids, a granular slurry, for a range of Froude numbers, bead sizes, fluid densities and fluid viscosities. The thickness of the flowing layer and the angle of repose with a liquid interstitial fluid are generally larger than for the dry system under similar conditions, although the shear rate is generally smaller. The experimental measurements of shear rate match the theoretical predictions (dependent on the particle size, dynamic angle of repose, and static angle of repose) independent of the interstitial fluid. Furthermore, the velocity profiles for larger beads collapse independent of the interstitial fluid, while for smaller beads these profiles collapse on two distinct curves when using a scaling based on mass balance. However, a normalization based on the velocity of beads at the surface causes a collapse to a nearly linear velocity profile except where the velocity approaches zero logarithmically near the fixed bed, regardless of interstitial fluid. Likewise, the scaled number density profiles collapse, regardless of the interstitial fluid. The similarity in the flows of dry granular materials and granular materials submerged in liquids indicates that the physics of the flow is not strongly altered by the interstitial fluid.

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.