CiteULike: ghunter's flow

Studies of granular shear flows: Wall slip velocities, 'layering' and self-diffusion

SB Savage — 2008-07-29T18:58:07-00:00

Mechanics of Materials, Vol. 16 (1993)

The physics of traffic jams

Takashi Nagatani — 2008-06-20T05:55:04-00:00

Reports on Progress in Physics, Vol. 65, No. 9. (2002), pp. 1331-1386.

Traffic flow is a kind of many-body system of strongly interacting vehicles. Traffic jams are a typical signature of the complex behaviour of vehicular traffic. Various models are presented to understand the rich variety of physical phenomena exhibited by traffic. Analytical and numerical techniques are applied to study these models. Particularly, we present detailed results obtained mainly from the microscopic car-following models. A typical phenomenon is the dynamical jamming transition from the free traffic (FT) at low density to the congested traffic at high density. The jamming transition exhibits the phase diagram similar to a conventional gas-liquid phase transition: the FT and congested traffic correspond to the gas and liquid phases, respectively. The dynamical transition is described by the time-dependent Ginzburg-Landau equation for the phase transition. The jamming transition curve is given by the spinodal line. The metastability exists in the region between the spinodal and phase separation lines. The jams in the congested traffic reveal various density waves. Some of these density waves show typical nonlinear waves such as soliton, triangular shock and kink. The density waves are described by the nonlinear wave equations: the Korteweg-de-Vries (KdV) equation, the Burgers equation and the Modified KdV equation. Subjects like the traffic flow such as bus-route system and pedestrian flow are touched as well. The bus-route system with many buses exhibits the bunching transition where buses bunch together with proceeding ahead. Such dynamic models as the car-following model are proposed to investigate the bunching transition and bus delay. A recurrent bus exhibits the dynamical transition between the delay and schedule-time phases. The delay transition is described in terms of the nonlinear map. The pedestrian flow also reveals the jamming transition from the free flow at low density to the clogging at high density. Some models are presented to study the pedestrian flow. When the clogging occurs, the pedestrian flow shows the scaling behaviour.

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.

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.

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.

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.

Avalanche dynamics in a pile of rice

Vidar Frette — 2008-06-06T14:06:25-00:00

Nature, Vol. 379, No. 6560. (4 January 1996), pp. 49-52.

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.

Particle transport at low pressure: particle deposition in a tube with an abrupt contraction

Shintaro Sato — 2008-05-28T11:38:34-00:00

Journal of Aerosol Science, Vol. 33, No. 4. (April 2002), pp. 659-671.

Particle inertial deposition in a tube with an abrupt contraction was studied experimentally and numerically at low pressure. Measurements were performed for particle deposition onto an orifice plate (orifice diameter: 1.16 cm) placed in a tube (inner diameter: 3.48 cm). The system pressure ranged from 27 to 37 Pa (0.2-0.28 Torr) and the flow Reynolds number, Re, in the tube was 3.0. Spatially uniform aerosols were produced in the low-pressure environment by the method developed by Sato et al. (Aerosol Sci. Technol., 2001). The observed deposition-efficiency curve as a function of the Stokes number, Stk, is different from that obtained for Re[greater-or-equal, slanted]100 at atmospheric pressure (J. Aerosol Sci. 21 (1990) 29; J. Aerosol Sci. 26 (1995) 563). The difference was confirmed by numerical studies. The numerical results for Re=0.1-30 revealed that the deposition curve shifts to a larger Stokes number as the Reynolds number decreases due to flow-pattern changes near the orifice plate. An empirical deposition-efficiency curve was obtained as a function of the Stokes number, the Reynolds number, and the contraction ratio (the ratio of the tube inner diameter to the orifice diameter), R.

Deposition of particles in a convergent channel

RY Chen — 2008-05-28T11:36:02-00:00

Powder Technology, Vol. 87, No. 1. (April 1996), pp. 83-86.

Particle trajectory and deposition in a convergent two-dimensional channel are analyzed for the uniform and developing laminar flow with a dilute particle concentration. The governing equations of particle motion are given under the effect of inertia force, viscous force, gravity force and electrostatic force, which were solved with the fourth-order Runge-Kutta method. Depositions were calculated based on the distance traveled by a particle to reach the wall. Many computations for deposition are performed on a combination of different parameters, and the results are then discussed in detail.

Particle motions in low-Reynolds number pressure-driven flows through converging–diverging microchannels

Xiangchun Xuan — 2008-05-28T11:27:33-00:00

Journal of Micromechanics and Microengineering, Vol. 16, No. 1. (2006), pp. 62-69.

Particle motions were experimentally studied in low-Reynolds number pressure-driven flows through converging-diverging microchannels. The whole process of particle acceleration and deceleration close to the bottom channel wall through the converging and diverging sections was visualized. We find that the ratio of the particle velocity in the throat to that in the straight channel is decreased when particles move closer to either sidewall. This ratio is, however, insensitive to the particle size. Both phenomena are well explained with the ratio of fluid velocity at the particle centers in the two regions. It is also found that the particle velocity ratio increases when particles move slower, and the underlying reason may need further investigation.

Pressure drop enhancement in a concentrated suspension flowing through an abrupt axisymmetric contraction-expansion

Tracey Moraczewski — 2008-05-23T16:52:54-00:00

Physics of Fluids, Vol. 19, No. 10. (2007)

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Stream Spreading in Multilayer Microfluidic Flows of Suspensions

MU Larsen — 2008-05-23T16:50:44-00:00

Anal. Chem., Vol. 79, No. 5. (1 March 2007), pp. 1947-1953.

Abstract: The goal of this experimental study is to quantify the spreading of parallel streams with viscosity contrast in multilayer microfluidic flows. Three streams converge into one channel where a test fluid is sheathed between two layers of a Newtonian reference fluid. The test fluids are Newtonian fluids with viscosities ranging from 1.1 to 48.2 cP and suspensions of 10-m-diameter PMMA particles with particle volume fractions = 0.16-0.30. The fluid interface locations are identified through fluorescence microscopy. The steady-state width of the center stream is strongly dependent on the viscosity ratio between the adjacent fluids and exhibits a near power-law relationship. This dependence occurs for both the Newtonian fluids and the suspensions, although the slopes differ. The high-concentration suspension ( = 0.30) diverges from Newtonian behavior, while the low-concentration suspensions ( = 0.16, 0.22) closely approximate that of the Newtonian fluids. The observed suspension behavior can be attributed to shear-induced particle migration.

The effect of inlet conditions on concentrated suspension flows in abrupt expansions

Tracey Moraczewski — 2008-05-23T16:48:27-00:00

Physics of Fluids, Vol. 18, No. 12. (2006)

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Flow of a concentrated suspension through an abrupt axisymmetric expansion measured by nuclear magnetic resonance imaging

T Moraczewski — 2008-05-23T16:46:12-00:00

Journal of Rheology, Vol. 49, No. 6. (2005)

Complex flow transitions in a homogeneous, concentrated emulsion

Nina Shapley — 2008-05-23T16:37:25-00:00

Physics of Fluids, Vol. 15, No. 4. (2003), pp. 881-891.

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Existence of a critical particle concentration in plugging of a packed bed

VB Pandya — 2008-05-23T13:13:48-00:00

American Institute of Chemical Engineers. AIChE Journal, Vol. 44, No. 4. (1998), pp. 978-981.

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.

Jamming during the discharge of granular matter from a silo

I Zuriguel — 2008-05-21T19:44:44-00:00

PHYSICAL REVIEW E, Vol. 71 (2005), pp. 051303-1-051303-9.

In this work, we present an experimental study of the jamming that stops the free flow of grains from a silo discharging by gravity. When the outlet size is not much bigger than the beads, granular material jams the outlet of the container due to the formation of an arch. Statistical data from the number of grains fallen between consecutive jams are presented. The information that they provide can help one to understand the jamming phenomenon. As the ratio between the size of the orifice and the size of the beads is increased, the probability that an arch blocks the outlet decreases. We show here that there is a power-law divergence of the mean avalanche size for a finite critical radius. Beyond this critical radius, no jamming can occur and the flow is never stopped. The dependence of the arch formation on the shape and the material of the grains has been explored. It has been found that the material properties of the grains do not affect the arch formation probability. On the contrary, the shape of the grains deeply influences it. A simple model to interpret the results is also discussed.

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.

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.

Particle transport in a nonuniform flow field: Retardation and clogging

Julio Valdes — 2008-05-15T15:56:26-00:00

Applied Physics Letters, Vol. 90, No. 24. (2007)

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Plugging by hydrodynamic bridging during flow of stable colloidal particles within cylindrical pores

Venkatachalam Ramachandran — 2008-05-15T15:50:14-00:00

Journal of Fluid Mechanics, Vol. 385, No. -1. (2000), pp. 129-156.

On dense granular flows

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.

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.

Flocculation in laminar tube flow

J Gregory — 2008-05-13T22:49:43-00:00

Chemical Engineering Science, Vol. 36, No. 11. (1981), pp. 1789-1794.

Ink-jet printed nanoparticle microelectromechanical systems

SB Fuller — 2008-05-13T22:46:21-00:00

Microelectromechanical Systems, Journal of, Vol. 11, No. 1. (2002), pp. 54-60.

Reports a method to additively build three-dimensional (3-D) microelectromechanical systems (MEMS) and electrical circuitry by ink-jet printing nanoparticle metal colloids. Fabricating metallic structures from nanoparticles avoids the extreme processing conditions required for standard lithographic fabrication and molten-metal-droplet deposition. Nanoparticles typically measure 1 to 100 nm in diameter and can be sintered at plastic-compatible temperatures as low as 300°C to form material nearly indistinguishable from the bulk material. Multiple ink-jet print heads mounted to a computer-controlled 3-axis gantry deposit the 10% by weight metal colloid ink layer-by-layer onto a heated substrate to make two-dimensional (2-D) and 3-D structures. We report a high-Q resonant inductive coil, linear and rotary electrostatic-drive motors, and in-plane and vertical electrothermal actuators. The devices, printed in minutes with a 100 μm feature size, were made out of silver and gold material with high conductivity,and feature as many as 400 layers, insulators, 10:1 vertical aspect ratios, and etch-released mechanical structure. These results suggest a route to a desktop or large-area MEMS fabrication system characterized by many layers, low cost, and data-driven fabrication for rapid turn-around time, and represent the first use of ink-jet printing to build active MEMS

Experimental Investigation of Liquid and Particle-laden Flows in Microtubes

K Sharp — 2008-05-13T22:42:08-00:00

(2001)

Shear Zones and Wall Slip in the Capillary Flow of Concentrated Colloidal Suspensions

Lucio Isa — 2008-03-18T22:57:29-00:00

Physical Review Letters, Vol. 98, No. 19. (2007)

We image the flow of a nearly random close packed, hard-sphere colloidal suspension (a “paste”) in a square capillary using confocal microscopy. The flow consists of a “plug” in the center while shear occurs localized adjacent to the channel walls, reminiscent of yield-stress fluid behavior. However, the observed scaling of the velocity profiles with the flow rate strongly contrasts yield-stress fluid predictions. Instead, the velocity profiles can be captured by a theory of stress fluctuations originally developed for chute flow of dry granular media. We verified this both for smooth and rough walls.

Direct Flow Visualization of Colloidal Gels in Microfluidic Channels

MT Roberts — 2008-05-08T21:05:46-00:00

Langmuir, Vol. 23, No. 17. (14 August 2007), pp. 8726-8731.

Abstract: The behavior of colloidal gels under pressure-driven flow in square microchannels is quantified by microscopic particle image velocimetry (PIV) and compared to predictions of available rheological models. The gels consist of hydrophobically modified silica microspheres ( = 0.15-0.33) suspended in a refractive index-matched fluid along with fluorescent tracers to aid visualization. Measured velocity flow profiles show a transition from plug flow to more fluid-like behavior with increasing volumetric flow rate (Q) at all . This transition is not captured by theoretical predictions of the flow profile based on the Herschel-Bulkley model. Rather, a model that accounts for gel breakup into a suspension of clusters at elevated shear rates by assuming a finite viscosity at infinite shear is needed to accurately predict the flow behavior of colloidal gels at large Q.

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.

Mechanism for clogging of microchannels

Hans Wyss — 2007-09-22T19:57:03-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 74, No. 6. (2006)

We investigate clogging of microchannels at the single-pore level using microfluidic devices as model porous media. The process of clogging is studied at low volume fractions and high flow rates, a technologically important regime. We show that clogging is independent of particle flow rate and volume fraction, indicating that collective effects do not play an important role. Instead, the average number of particles that can pass through a pore before it clogs scales with the ratio of pore to particle size. We present a simple model that accounts for the data.

Lattice boltzmann simulation on particle suspensions in a two–dimensional symmetric stenotic artery

HB Li — 2008-04-28T12:59:25-00:00

Physical Review E, Vol. 69 (2004)

BLOOD FLOW IN ARTERIES

David Ku — 2008-04-28T12:52:48-00:00

Annual Review of Fluid Mechanics, Vol. 29, No. 1. (1997), pp. 399-434.

Abstract Blood flow in arteries is dominated by unsteady flow phenomena. The cardiovascular system is an internal flow loop with multiple branches in which a complex liquid circulates. A nondimensional frequency parameter, the Womersley number, governs the relationship between the unsteady and viscous forces. Normal arterial flow is laminar with secondary flows generated at curves and branches. The arteries are living organs that can adapt to and change with the varying hemodynamic conditions. In certain circumstances, unusual hemodynamic conditions create an abnormal biological response. Velocity profile skewing can create pockets in which the direction of the wall shear stress oscillates. Atherosclerotic disease tends to be localized in these sites and results in a narrowing of the artery lumen--a stenosis. The stenosis can cause turbulence and reduce flow by means of viscous head losses and flow choking. Very high shear stresses near the throat of the stenosis can activate platelets and thereby induce thrombosis, which can totally block blood flow to the heart or brain. Detection and quantification of stenosis serve as the basis for surgical intervention. In the future, the study of arterial blood flow will lead to the prediction of individual hemodynamic flows in any patient, the development of diagnostic tools to quantify disease, and the design of devices that mimic or alter blood flow. This field is rich with challenging problems in fluid mechanics involving three-dimensional, pulsatile flows at the edge of turbulence.

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.

Funneling of Flow into Grain-to-grain Contacts Drives Colloid−Colloid Aggregation in the Presence of an Energy Barrier

Meiping Tong — 2008-04-28T10:59:04-00:00

Environ. Sci. Technol., Vol. 42, No. 8. (15 April 2008), pp. 2826-2832.

Abstract: Deposition behaviors of carboxylate-modified polystyrene latex microspheres (five sizes ranging from 0.1 to 2.0 µm) were examined in packed porous media, impinging jet, and porous media-packed flow chamber systems under a variety of environmentally relevant ionic strength and flow conditions in the presence of an energy barrier to deposition. Temporally constant deposition rate coefficients were observed for all microsphere sizes under baseline conditions, whereas temporal increases in colloid deposition rate coefficients (ripening) occurred for all microsphere sizes in response to slight increases in solution ionic strength and slight decreases in fluid velocity. This transition from clean bed deposition to ripening was triggered by relatively subtle changes in solution chemistry and fluid velocity. Direct observation of colloid deposition in a flow chamber packed with porous media revealed that colloidal aggregates formed at grain-to-grain contacts in the porous media. The absence of ripening in an impinging jet system (unbounded flat surface) examined under equivalent conditions to the packed porous media further indicated that colloid aggregation was driven by the funneling of fluid into the grain-to-grain contacts. Comparison of colloid breakthrough in porous media comprised of smooth-spherical versus angular grains demonstrated that the propensity to trigger ripening increased with the number and length of grain-to-grain contacts.

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.

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.

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.

Concentrated Suspensions Under Flow: Shear-Thickening and Jamming

P Hébraud — 2008-04-26T14:19:04-00:00

Modern physics letters. B, Vol. 19, No. 13/14. (2005), pp. 614-624.

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.

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.

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.

Particle migration in pressure-driven flow of a Brownian suspension

M Frank — 2008-04-26T13:59:20-00:00

J. Fluid Mech., Vol. 493 (2003), pp. 363-378.

Experimental studies of the flow of concentrated hard sphere suspensions into a constriction

L Isa — 2008-04-26T13:57:03-00:00

Journal of Physics: Conference Series, Vol. 40 (2006), pp. 124-132.

On flow-blocking particle structures in microtubes

K Sharp — 2008-04-25T19:10:12-00:00

Microfluidics and Nanofluidics, Vol. 1, No. 4. (17 October 2005), pp. 376-380.

Given their small size, microchannels are susceptible to being blocked by small amounts of solid matter. The lifetime of certain microfluidic devices depends on their ability to maintain flow without interruption, and certain microfluidic applications require devices that include the transport of either liquids that may contain impurities or liquids that intentionally contain particles. Under the conditions studied in the present experiments, flow-stopping blockages were observed for the flow of liquids with the surprisingly low volume concentrations of ϕ = 0.005–0.056 of polystyrene particles (dp of the order of tens of microns) through circular microtubes with diameters (D) of the order of 100 μm. The quantity of interest is the occurrence of a flow-stopping blockage in the flow of a suspension of hard spheres, over the range of from 0.2 to 0.5 and ϕ dp/D is presented. Given that higher concentrations of particles have been used in many previous studies of suspension flows, the occurrence of blockages at these concentrations was initially unexpected. Based upon the data and the parameters varied in the current study, the blockages are more likely to occur when the nominal particle-to-tube diameter ratios are approximately 0.3–0.4.

Flow and jam of granular particles in a two-dimensional hopper

Kiwing To — 2008-04-25T19:03:42-00:00

Physica A: Statistical Mechanics and its Applications, Vol. 315, No. 1-2. (15 November 2002), pp. 174-180.

The jamming phenomenon of granular flow of mono-disperse disks in a two-dimensional hopper is studied experimentally. When the opening of the hopper d is small, jamming occurs due to formation of an arch at the hopper opening. The jamming probability as a function of the hopper opening width is measured which show a sharp decrease at some range of d. By observing the disk configurations of the arch in the jamming events, the jamming probability can be explained quantitatively using a restricted random walker model. The average horizontal and vertical spans of the jamming arch are calculated from the restricted random walk model which compares favorably with experiments.

Jamming of Granular Flow in a Two-Dimensional Hopper

Kiwing To — 2007-09-27T03:12:29-00:00

Physical Review Letters, Vol. 86, No. 1. (1 January 2001), 71.

We study experimentally the jamming phenomenon of granular flow of monodisperse disks of D = 5 mm diameter in a two-dimensional hopper with opening R . The jamming probability J ( d ) is measured where d ≡ R / D . We found that J ( d ) decreases from 1 to zero when d increases from 2 to 5. From observing the disk configurations of the arch in the jamming events; the jamming probability can be explained quantitatively by treating the arch as the trajectory of a restricted random walker.

Shear thickening in a model colloidal suspension

Jerome Delhommelle — 2008-03-31T14:54:25-00:00

The Journal of Chemical Physics, Vol. 123, No. 7. (2005)

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Ionic colloidal crystals of oppositely charged particles

Mirjam Leunissen — 2005-09-07T17:30:20-00:00

Nature, Vol. 437, No. 7056., pp. 235-240.