CiteULike: dchen's flow

APPLIED PHYSICS: Droplet Control for Microfluidics

Mathieu Joanicot — 2008-07-25T23:23:37-00:00

Science, Vol. 309, No. 5736. (5 August 2005), pp. 887-888.

10.1126/science.1112615

Multiphase Flow in Porous Media

PM Adler — 2008-07-25T17:25:15-00:00

Annual Review of Fluid Mechanics, Vol. 20, No. 1. (1988), pp. 35-59.

Foam Flows

AM Kraynik — 2008-07-25T16:18:51-00:00

Annual Review of Fluid Mechanics, Vol. 20, No. 1. (1988), pp. 325-357.

Emulsion drops in external flow fields -- The role of liquid interfaces

Peter Fischer — 2008-07-24T20:48:20-00:00

Current Opinion in Colloid & Interface Science, Vol. 12, No. 4-5. (October 2007), pp. 196-205.

We review the flow of emulsion drops, focusing on recent work involving complex interfaces, which may include the presence of surfactants, particles, surface-active polymers, or solid-like membrane layers. En route, important phenomena in multiphase flow associated with emulsion rheology are considered, including drop coalescence and breakup, surfactant transport, or the mechanics of composite interfaces.

Simple model for heterogeneous flows of yield stress fluids

Guillemette Picard — 2008-07-19T21:30:03-00:00

Physical Review E, Vol. 66, No. 5. (5 November 2002), 051501.

Slip and Flow in Soft Particle Pastes

Steven Meeker — 2008-07-19T21:18:42-00:00

Physical Review Letters, Vol. 92, No. 19. (11 May 2004), 198302.

Spatial cooperativity in soft glassy flows

J Goyon — 2008-07-03T06:17:56-00:00

Nature, Vol. 454, No. 7200., pp. 84-87.

Amorphous glassy materials of diverse nature—concentrated emulsions, granular materials, pastes, molecular glasses—display complex flow properties, intermediate between solid and liquid, which are at the root of their use in many applications1, 2, 3. A general feature of such systems, well documented yet not really understood, is the strongly nonlinear nature of the flow rule relating stresses and strain rates4, 5. Here we use a microfluidic velocimetry technique to characterize the flow of thin layers of concentrated emulsions, confined in gaps of different thicknesses by surfaces of different roughnesses. We find evidence for finite-size effects in the flow behaviour and the absence of an intrinsic local flow rule. In contrast to the classical nonlinearities of the rheological behaviour of amorphous materials, we show that a rather simple non-local flow rule can account for all the velocity profiles. This non-locality of the dynamics is quantified by a length, characteristic of cooperativity within the flow at these scales, that is unobservable in the liquid state (lower emulsion concentrations) and that increases with concentration in the jammed state. Beyond its practical importance for applications involving thin layers (for example, coatings), these non-locality and cooperativity effects have parallels in the behaviour of other glassy, jammed and granular systems, suggesting a possible fundamental universality.

Flow of Wet Granular Materials

N Huang — 2008-06-22T00:55:36-00:00

Physical Review Letters, Vol. 94, No. 2. (2005)

The transition from frictional to lubricated flows of a dense suspension of non-Brownian particles is studied. The pertinent parameter characterizing this transition is the Leighton number Le = , the ratio of lubrication to frictional forces. Le defines a critical shear rate below which no steady flow without localization exists. In the frictional regime the shear flow is localized. The lubricated regime is not simply viscous: the ratio of shear to normal stresses remains constant and the velocity profile has a universal form in both frictional and lubricated regimes. Finally, a discrepancy between local and global measurements of viscosity is identified, which suggests inhomogeneity of the material under flow.

Velocity distribution and the effect of wall roughness in granular Poiseuille flow

KC Vijayakumar — 2008-06-11T16:09:47-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 5. (2007)

From event-driven simulations of a gravity-driven channel flow of inelastic hard disks, we show that the velocity distribution function remains close to a Gaussian for a wide range densities (even when the Knudsen number is of order 1) if the walls are smooth and the particle collisions are nearly elastic. For dense flows, a transition from a Gaussian to a power-law distribution for the high-velocity tails occurs with increasing dissipation in the center of the channel, irrespective of wall roughness. For a rough wall, the near-wall distribution functions are distinctly different from those in the bulk, even in the quasielastic limit.

Stability diagram for dense suspensions of model colloidal Al[sub 2]O[sub 3] particles in shear flow

Martin Hecht — 2008-06-11T14:33:11-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 5. (2007)

In Al2O3 suspensions, depending on the experimental conditions, very different microstructures can be found, comprising fluidlike suspensions, a repulsive structure, and a clustered microstructure. For technical processing in ceramics, the knowledge of the microstructure is of importance, since it essentially determines the stability of a workpiece to be produced. To enlighten this topic, we investigate these suspensions under shear by means of simulations. We observe cluster formation on two different length scales: the distance of nearest neighbors and on the length scale of the system size. We find that the clustering behavior does not depend on the length scale of observation. If interparticle interactions are not attractive the particles form layers in the shear flow. The results are summarized in a stability diagram.

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.

Simulated three-component granular segregation in a rotating drum

DC Rapaport — 2008-03-07T17:08:22-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 4. (2007)

Discrete particle simulations are used to model segregation in granular mixtures of three different particle species in a horizontal rotating drum. Axial band formation is observed, with medium-size particles tending to be located between alternating bands of big and small particles. Partial radial segregation also appears; it precedes the axial segregation and is characterized by an inner core region richer in small particles. Axial bands are seen to merge during the long simulation runs, leading to a coarsening of the band pattern; the relocation of particles involved in one such merging event is examined. Overall, the behavior is similar to experiment and represents a generalization of what occurs in the simpler two-component mixture.

Effects of fluid viscosity on band segregation dynamics in bidisperse granular slurries

Stanley Fiedor — 2008-06-10T20:53:58-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 4. (2007)

Parallel experiments in long, axially rotated cylinders are used to study the influence of interstitial fluid viscosity on particle segregation in bidisperse granular slurries. A uniformly mixed initial state segregates into surface bands, which alternate between regions of large particles and regions composed of a mixture of small and large particles. As the tumbler rotates, the relative area of the mixed particle bands increases and saturates, while the number of bands reaches a peak and then decreases logarithmically in time for all viscosities studied. With increasing interstitial fluid viscosity, the asymptotic mixed band area increases proportionally, the time for bands to appear at the surface decreases, and the peak number of bands goes through a maximum at a viscosity of ~3 cP. Extrapolation to the low-viscosity limit matches the data for dry granular systems; at the high-end viscosity there is a value beyond which no axial banding occurs. A heuristic mechanism based on the coexistence of pure and mixed particle phases and their dependence on viscosity is presented to rationalize key aspects of the results.

Response to perturbations for granular flow in a hopper

John Wambaugh — 2008-06-10T18:55:24-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 5. (2007)

We experimentally investigate the response to perturbations of circular symmetry for dense granular flow inside a three-dimensional right-conical hopper. These experiments consist of particle tracking velocimetry for the flow at the outer boundary of the hopper. We are able to test commonly used constitutive relations and observe granular flow phenomena that we can model numerically. Unperturbed conical hopper flow has been described as a radial velocity field with no azimuthal component. Guided by numerical models based upon continuum descriptions, we find experimental evidence for secondary, azimuthal circulation in response to perturbation of the symmetry with respect to gravity by tilting. For small perturbations we can discriminate between constitutive relations, based upon the agreement between the numerical predictions they produce and our experimental results. We find that the secondary circulation can be suppressed as wall friction is varied, also in agreement with numerical predictions. For large tilt angles we observe the abrupt onset of circulation for parameters where circulation was previously suppressed. Finally, we observe that for large tilt angles the fluctuations in velocity grow, independent of the onset of circulation.

Interplay of air and sand: Faraday heaping unravelled

Henk van Gerner — 2008-06-10T18:47:37-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 5. (2007)

We report on numerical simulations of a vibrated granular bed including the effect of the ambient air, generating the famous Faraday heaps known from experiment. A detailed analysis of the forces shows that the heaps are formed and stabilized by the airflow through the bed while the gap between bed and vibrating bottom is growing, confirming the pressure gradient mechanism found experimentally by Thomas and Squires [Phys. Rev. Lett. 81, 574 (1998)], with the addition that the airflow is partly generated by isobars running parallel to the surface of the granular bed. Importantly, the simulations also explain the heaping instability of the initially flat surface and the experimentally observed coarsening of a number of small heaps into a larger one.

Experiments and simulations of a gravitational granular flow instability

Jan Vinningland — 2008-06-10T18:44:30-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 5. (2007)

An instability is observed as a layer of dense granular material positioned above a layer of air falls in a gravitational field [Phys. Rev. Lett. 99, 048001 (2007)]. A characteristic pattern of fingers emerges along the interface defined by the grains, and a transient coarsening of the structure is caused by a coalescence of neighboring fingers. The coarsening is limited by the production of new fingers as the separation of the existing fingers reaches a certain distance. The experiments and simulations presented are shown to be comparable both qualitatively and quantitatively. The characteristic inverse length scale of the structures, obtained as the mean of the solid fraction power spectrum, relaxes toward a stable value shared by the numerical and experimental data. Further, the response of the numerical model to changes in various model parameters is investigated. These parameters include the density of the grains, the shape of the initial air-grain interface, and the dissipation of the granular phase. Also, the growth rates of the bulk solid fraction and the air-grain interface are obtained from Fourier power spectra of the numerical data. This analysis reveals that the instability is never in a linear regime, not even initially.

Spontaneous formation of dual stratification patterns in a large quasi-two-dimensional sand pile

Michiko Shimokawa — 2008-06-10T15:12:31-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 1. (2008)

By pouring a mixture of two types of grains into a large vertical cell with a narrow space, we discovered a dual stratification pattern consisting of two different wavelengths at the upper and lower regions of the resulting sand pile. In the formation of this pattern, we also observed an additional type of kink—a wave that moves toward the top of the sand pile along the slope. The kink, herein called a trapped kink, is essential for the formation of dual stratification patterns. Asymmetric probability distributions are obtained from measurement of the position where the kinks are generated. We proposed a phenomenological model to describe the kink generation process. The results analyzed by this model agree with experimental distributions.

Interplay between hydrodynamic and Brownian fluctuations in sedimenting colloidal suspensions

JT Padding — 2008-01-23T07:40:30-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 1. (2008)

We apply a hybrid molecular dynamics and mesoscopic simulation technique to study the steady-state sedimentation of hard sphere particles for Peclet number (Pe) ranging from 0.08 to 12. Hydrodynamic backflow causes a reduction of the average sedimentation velocity relative to the Stokes velocity. We find that this effect is independent of Pe number. Velocity fluctuations show the expected effects of thermal fluctuations at short correlation times. At longer times, nonequilibrium hydrodynamic fluctuations are visible, and their character appears to be independent of the thermal fluctuations. The hydrodynamic fluctuations dominate the diffusive behavior even for modest Pe number, while conversely the short-time fluctuations are dominated by thermal effects for surprisingly large Pe numbers. Inspired by recent experiments, we also study finite sedimentation in a horizontal planar slit. In our simulations distinct lateral patterns emerge, in agreement with observations in the experiments.

Profile blunting and flow blockage in a yield-stress fluid: A molecular dynamics study

F Varnik — 2008-06-09T22:44:55-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 1. (2008)

The flow of a simple glass forming system (a 80:20 binary Lennard-Jones mixture) through a planar channel is studied via molecular dynamics simulations. The flow is driven by an external body force similar to gravity. Previous studies show that the model exhibits both a static [F. Varnik et al., J. Chem. Phys. 120, 2788 (2004)] and a dynamic [F. Varnik and O. Henrich, Phys. Rev. B 73, 174209 (2006)] yield stress in the glassy phase. These observations are corroborated by the present work, where we investigate how the presence of a yield stress may affect the system behavior in a Poiseuille-type flow geometry. In particular, we observe a blunted velocity profile across the channel: A relatively wide region in the channel center flows with a constant velocity (zero shear rate) followed by a nonlinear change of the shear rate as the walls are approached. The observed velocity gradients are compared to those obtained from the knowledge of the shear stress across the channel and the flow curves (stress versus shear rate), the latter being determined in our previous simulations of homogeneous shear flow. Furthermore, using the value of the (dynamic) yield stress known from previous simulations, we estimate the threshold body force for a complete arrest of the flow. Indeed, a blockage is observed as the imposed force falls below this threshold value. Small but finite shear rates are observed at stresses above the dynamic but below the static yield stress. We discuss the possible role of the stick-slip-like motion for this observation.

Granular flow in a rapidly rotated system with fixed walls

Eric Corwin — 2008-06-09T19:28:36-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 3. (2008)

The flow properties in a granular system confined by a rapidly rotated bottom surface and fixed walls are investigated. Above sufficiently high rotation rates the system enters a state in which the flow is independent of the driving rate. Further, a nearly constant shear-strain rate is measured throughout the entire system. Optical and particle imaging velocimetry methods are used to measure the surface flows, and large-scale molecular dynamics simulations are performed to model the flows both at the surface and in the interior of the material. The analysis shows a regime of granular flow in which shear banding is absent.

Granular flow in rotating cylinders with noncircular cross sections

DVN Prasad — 2008-06-08T23:50:12-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

An experimental and theoretical study is carried out of the flow of granular material in cylinders with different cross-sectional shapes rotated about their axes. The flow of particles in such geometries is confined to a shallow layer at the free surface. The length and thickness of the layer shrink and expand periodically with rotation of the cylinder, resulting in chaotic advection and improved mixing of passive tracers. Experimental results obtained by flow visualization are reported for quasi-two-dimensional mixers half filled with glass beads. A depth-averaged flow model to predict the time-varying layer thickness profile is presented, along with a perturbation solution in terms of a small parameter k, which is the ratio of the maximum layer thickness to the half length of the layer (L), at the cross-section orientation when the length is minimum. To the lowest order [O(k0)], the model predicts that the layer profiles scaled with L() at different mixer orientation angles () are identical and the same as that for a circle. The measured layer thickness profiles averaged over different orientations of noncircular mixers match reasonably well with the theory, but the standard deviations are larger for the noncircular cylinders compared to the circle. The O(k) perturbation solution and the full theory both predict that the scaled layer thickness varies periodically; the deviations are proportional to the rate of change of the length with orientation. The perturbation solution gives results close to those from the numerical solution except at cylinder orientations when the length of the flowing layer changes sharply. The measured variation of the scaled midlayer thickness with orientation for all geometries is well predicted by the theory.

Anomalous behavior of a single particle falling through a funnel

Yuan Fang — 2008-06-08T23:48:50-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

We show several surprising phenomena that occur in an extremely simple system of a single frictionless, inelastic, spherical particle falling under gravity through a symmetric funnel. One might naively expect that particles would fall through funnels with steeper sides more quickly, exert a smaller total impulse on the funnel walls, and lose less energy. However, we show that there are special ranges of angles of the funnel walls for which exactly the opposite occurs. Typically, the particle will experience a sequence of collisions that is highly sensitive to the location at which it enters the funnel and nearby particle trajectories become widely dispersed. However, in the special angular ranges this is not the case and the particle can experience sequences of collisions that have a highly coherent structure. We provide a theoretical analysis that can predict and explain this surprising behavior.

Start and stop of an avalanche in a granular medium subjected to an inner water flow

P Philippe — 2008-06-08T23:35:29-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

We report experimental results on the maximum angle of stability, i.e., the so-called avalanche angle, of a granular medium subjected to an inner water flow controlled by a constant pressure drop. A unique avalanche threshold is derived by two alternative theoretical developments, namely a continuum and a discrete approach, and is successfully confronted to many measurements in a large experimental range. A qualitative analysis of the instability triggering reveals different dynamical behaviors depending on whether the water flow is downward or upward in the granular layer, namely stabilizing versus destabilizing regime. Contrary to the purely hydrostatic situation, the free surface following an avalanche departs from a linear shape because the dynamical pressure gradient is no longer constant in the medium. A simple model is proposed that can satisfactorily predict the postavalanche height profile as well as its subsequent evolution for higher inclinations.

Stripe formation in an immiscible polymer blend under electric and shear-flow fields

Yang Na — 2008-06-08T22:59:03-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

We found a stripe formation in an emulsion of a liquid crystalline polymer (LCP) and a machine oil (OIL) in electric and shear fields. Through the simultaneous measurement with a confocal scanning laser microscope and a rheometer, it was clearly shown that the formation of stripes, which are periodically arrayed, leads to the increase of the shear stress. The droplets, which are one component of the emulsion, start to be connected at low electric fields and then change into the stripes with the increase of electric field. Finally, a three-dimensional network is formed at high electric fields. The period and fluctuation of the stripe structure were also investigated in detail.

Slip boundary conditions for shear flow of polymer melts past atomically flat surfaces

Anoosheh Niavarani — 2008-06-08T21:49:42-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

Molecular dynamics simulations are carried out to investigate the dynamic behavior of the slip length in thin polymer films confined between atomically smooth thermal surfaces. For weak wall-fluid interactions, the shear rate dependence of the slip length acquires a distinct local minimum followed by a rapid growth at higher shear rates. With increasing fluid density, the position of the local minimum is shifted to lower shear rates. We found that the ratio of the shear viscosity to the slip length, which defines the friction coefficient at the liquid/solid interface, undergoes a transition from a nearly constant value to power law decay as a function of the slip velocity. In a wide range of shear rates and fluid densities, the friction coefficient is determined by the product of the value of the surface-induced peak in the structure factor and the contact density of the first fluid layer near the solid wall.

Passive Oscillations of Two Tandem Flexible Filaments in a Flowing Soap Film

Lai Jia — 2008-06-08T18:18:59-00:00

Physical Review Letters, Vol. 100, No. 22. (2008)

The passive oscillations of flexible filaments in a flowing soap film were investigated to learn the serial interaction between them. When arranged in tandem, the downstream filament flaps at the same frequency as that of the upstream one, but with a larger amplitude, whereas the upstream one is almost unaffected compared to the single filament case. The data analysis shows the downstream filament indeed extracts energy from the vortex street and receives greater force than the upstream one or a single filament in a uniform flow.

Capillarylike Fluctuations at the Interface of Falling Granular Jets

Yacine Amarouchene — 2008-06-05T20:44:07-00:00

Physical Review Letters, Vol. 100, No. 21. (2008)

We study the interface fluctuations of a granular jet falling under gravity and show that for small scales they are the analog of the thermally induced capillary waves. Experimental results from radial height and velocity fluctuations, static correlation functions and capillary ripple velocities allow us to estimate a granular surface tension. The ultralow interfacial tensions measured (of the order of 100 µN/m) can be rationalized using a simple model.

Analysis of DNA Elasticity

RP Linna — 2008-05-05T21:17:56-00:00

Physical Review Letters, Vol. 100, No. 16. (2008)

With a model that incorporates hydrodynamics directly, we show that flow experiments can be used for detecting some characteristics of the DNA elasticity which manifest themselves clearly at large length scales but cannot be observed by mechanical forcing experiments even at very small length scales. By systematic analysis, the conclusiveness of different experimental methods is evaluated. For the wormlike chain, confirmed as the correct model for DNA, we find an underlying scaling relation between its extension and flow velocity of the form Lp~v0.155, which emphasizes the significance of hydrodynamics.

Delaying Transition to Turbulence by a Passive Mechanism

Jens Fransson — 2008-05-05T16:29:56-00:00

Physical Review Letters, Vol. 96, No. 6. (2006)

Reducing skin friction is important in nature and in many technological applications. This reduction may be achieved by reducing stresses in turbulent boundary layers, for instance tailoring biomimetic rough skins. Here we take a second approach consisting of keeping the boundary layer laminar as long as possible by forcing small optimal perturbations. Because of the highly non-normal nature of the underlying linearized operator, these perturbations are highly amplified and able to modify the mean velocity profiles at leading order. We report results of wind-tunnel experiments in which we implement this concept by using suitably designed roughness elements placed on the skin to enforce nearly optimal perturbations. We show that by using this passive control technique it is possible to sensibly delay transition to turbulence.

High-Rayleigh-Number Convection in a Vertical Channel

M Gibert — 2008-05-05T16:13:23-00:00

Physical Review Letters, Vol. 96, No. 8. (2006)

We measure the relation between convective heat flux and temperature gradient in a vertical channel filled with water, the average vertical mass flux being zero. Compared to the classical Rayleigh-Bénard case, this situation has the advantage of avoiding plates and, thus, their neighborhood, in which is usually concentrated most of the temperature gradient. Consequently, inertial processes should control the convection, with poor influence of the viscosity. This idea gives a good account of our observations, if we consider that a natural vertical length, different from the channel width, appears. Our results also suggest that heat fluxes can be deduced from velocity measurements in free convective flows. This confers to our results a wide range of applications.

Crystallization of a Quasi-Two-Dimensional Granular Fluid

PM Reis — 2007-12-10T22:23:17-00:00

Physical Review Letters, Vol. 96, No. 25. (2006)

We experimentally investigate the crystallization of a uniformly heated quasi-2D granular fluid as a function of the filling fraction. Our experimental results for the Lindemann melting criterion, the radial distribution function, the bond order parameter, and the statistics of topological changes at the particle level are the same as those found in simulations of equilibrium hard disks. This direct mapping suggests that the study of equilibrium systems can be effectively applied to study nonequilibrium steady states such as those found in our driven and dissipative granular system.

General Probabilistic Approach to the Filtration Process

N Roussel — 2008-05-04T18:21:59-00:00

Physical Review Letters, Vol. 98, No. 11. (2007)

We show experimentally that clogging is basically a matter of the probability of the presence of particles. We describe this process as a function of the main variables of the process, namely, the ratio of particle to mesh hole size, the solid fraction, and the number of grains arriving at each mesh hole during one test, with the help of a simple model, the predictions of which are in very good agreement with our experimental data.

Dynamics of the Solid and Liquid Phases in Dilute Sheared Brownian Suspensions: Irreversibility and Particle Migration

Jennifer Brown — 2008-05-04T18:06:26-00:00

Physical Review Letters, Vol. 99, No. 24. (2007)

Magnetic resonance measurements of migration and irreversible dynamics in the capillary shear flow of a Brownian suspension are presented. The results demonstrate the presence of phenomena typically associated with concentrated noncolloidal systems and indicate the role of many body hydrodynamics in dilute Brownian suspension transport. The application of concepts from chaos theory and nonequilibrium statistical mechanics is demonstrated.

Role of friction in compaction and segregation of granular materials

Yair Srebro — 2008-04-29T00:48:31-00:00

Physical Review E, Vol. 68, No. 6. (3 December 2003), 061301.

Activated hopping and dynamical fluctuation effects in hard sphere suspensions and fluids

Erica Saltzman — 2007-10-29T19:49:58-00:00

The Journal of Chemical Physics, Vol. 125, No. 4. (2006)

Single particle Brownian dynamics simulation methods are employed to establish the full trajectory level predictions of our nonlinear stochastic Langevin equation theory of activated hopping dynamics in glassy hard sphere suspensions and fluids. The consequences of thermal noise driven mobility fluctuations associated with the barrier hopping process are determined for various ensemble-averaged properties and their distributions. The predicted mean square displacements show classic signatures of transient trapping and anomalous diffusion on intermediate time and length scales. A crossover to a stronger volume fraction dependence of the apparent nondiffusive exponent occurs when the entropic barrier is of order the thermal energy. The volume fraction dependences of various mean relaxation times and rates can be fitted by empirical critical power laws with parameters consistent with ideal mode-coupling theory. However, the results of our divergence-free theory are largely a consequence of activated dynamics. The experimentally measurable alpha relaxation time is found to be very similar to the theoretically defined mean reaction time for escape from the barrier-dominated regime. Various measures of decoupling have been studied. For fluid states with small or nonexistent barriers, relaxation times obey a simple log-normal distribution, while for high volume fractions the relaxation time distributions become Poissonian. The product of the self-diffusion constant and mean alpha relaxation time increases roughly as a logarithmic function of the alpha relaxation time. The cage scale incoherent dynamic structure factor exhibits nonexponential decay with a modest degree of stretching. A nearly universal collapse of the different volume fraction results occurs if time is scaled by the mean alpha relaxation time. Hence, time-volume fraction superposition holds quite well, despite the presence of stretching and volume fraction dependent decoupling associated with the stochastic barrier hopping process. The relevance of other origins of dynamic heterogeneity (e.g., mesoscopic domains), and comparison of our results with experiments, simulations, and alternative theories, is discussed. ©2006 American Institute of Physics

Signatures of granular microstructure in dense shear flows

Daniel — 2008-04-28T16:13:37-00:00

Nature 406, 385-389 (27 July 2000)

Granular materials and ordinary fluids react differently to shear stresses. Rather than deforming uniformly, materials such as dry sand or cohesionless powders develop shear bands1, 2, 3, 4, 5—narrow zones of large relative particle motion, with essentially rigid adjacent regions. Because shear bands mark areas of flow, material failure and energy dissipation, they are important in many industrial, civil engineering and geophysical processes6. They are also relevant to lubricating fluids confined to ultrathin molecular layers7. However, detailed three-dimensional information on motion within a shear band, including the degree of particle rotation and interparticle slip, is lacking. Similarly, very little is known about how the microstructure of individual grains affects movement in densely packed material5. Here we combine magnetic resonance imaging, X-ray tomography and high-speed-video particle tracking to obtain the local steady-state particle velocity, rotation and packing density for shear flow in a three-dimensional Couette geometry. We find that key characteristics of the granular microstructure determine the shape of the velocity profile.

Liquid-induced transitions in granular media

P Tegzes — 2008-04-28T15:18:49-00:00

Physical Review E, Vol. 60, No. 5. (1 November 1999), 5823.

Viscous instabilities in flowing foams: a Cellular Potts Model approach

Soma Sanyal — 2006-10-18T18:43:28-00:00

J. Stat. Mech., Vol. 2006, No. 10. (October 2006), P10008.

FOAM FLOWS

Andrew Kraynik — 2008-04-28T13:59:32-00:00

Ann.Rev. Fluid Mech. 988.20 : 325-57

Suspended Particles in Fluid Flow Through Tubes

RG Cox — 2008-04-27T00:22:57-00:00

Annual Review of Fluid Mechanics, Vol. 3 (1971), pp. 291-316.

Not Available

Rapid Granular Flows

CS Campbell — 2008-03-06T06:04:18-00:00

Annual Review of Fluid Mechanics, Vol. 22, No. 1. (1990), pp. 57-90.

Pressure-driven flow of suspensions of liquid drops

Hua Zhou — 2008-04-27T00:15:54-00:00

Physics of Fluids, Vol. 6, No. 1. (1994), pp. 80-94.

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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.

Foam Mechanics at the Bubble Scale

DJ Durian — 2007-09-27T03:41:32-00:00

Physical Review Letters, Vol. 75, No. 26. (25 December 1995), 4780.

By focusing on entire bubbles rather than films or vertices; a simple model is proposed for the deformation and flow of foam in which dimensionality; polydispersity; and liquid content can easily be varied. Simulation results are presented for the linear elastic properties as a function of bubble volume fraction; showing a melting transition where the static shear modulus vanishes and the relaxation time scale peaks. Results are also presented for shear stress versus strain rate; showing intermittent flow via avalanchelike topological rearrangements and Bingham-plastic behavior.

Diffusion and Mixing in Gravity-Driven Dense Granular Flows

Jaehyuk Choi — 2008-01-23T19:08:39-00:00

Physical Review Letters, Vol. 92, No. 17. (27 April 2004), 174301.

We study the transport properties of particles draining from a silo using imaging and direct particle tracking. The particle displacements show a universal transition from superdiffusion to normal diffusion; as a function of the distance fallen; independent of the flow speed. In the superdiffusive (but sub-ballistic) regime; which occurs before a particle falls through its diameter; the displacements have fat-tailed and anisotropic distributions. In the diffusive regime; we observe very slow cage breaking and Péclet numbers of order 100; contrary to the only previous microscopic model (based on diffusing voids). Overall; our experiments show that diffusion and mixing are dominated by geometry; consistent with long-lasting contacts but not thermal collisions; as in normal fluids.

Three-dimensional imaging of multiphase flow in porous media

ML Turner — 2008-04-26T23:47:39-00:00

Physica A: Statistical Mechanics and its Applications, Vol. 339, No. 1-2. (1 August 2004), pp. 166-172.

This paper reports progress in the 3D pore-scale micro-CT imaging of multiple fluid phases during drainage experiments in porous materials. Experiments performed on a sintered monodisperse bead pack and a Berea sandstone sample are described. It is observed that the residual (trapped) wetting phase in the sintered bead pack is present as pendular rings, bridges between adjacent grains and lenses within pore throats. Estimates of the residual wetting phase saturation are in accord with previous experiments and predictions on model bead packs. Relative permeabilities computed directly on the digitised images of the fluid phases are in good agreement with experimentally measured values for Berea sandstone. Two simple numerical methods for estimating two phase drainage saturation distributions directly from images are compared. Both methods give good agreement with the bead pack experiment. The match for Berea sandstone is poorer; differences may be due to variable wettability.

Dynamics of random packings in granular flow

Chris Rycroft — 2008-04-26T23:45:29-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 73, No. 5. (2006)

We present a multiscale simulation algorithm for amorphous materials, which we illustrate and validate in a canonical case of dense granular flow. Our algorithm is based on the recently proposed spot model, where particles in a dense random packing undergo chainlike collective displacements in response to diffusing "spots" of influence, carrying a slight excess of interstitial free volume. We reconstruct the microscopic dynamics of particles from the "coarse grained" dynamics of spots by introducing a localized particle relaxation step after each spot-induced block displacement, simply to enforce packing constraints with a (fairly arbitrary) soft-core repulsion. To test the model, we study to what extent it can describe the dynamics of up to 135 000 frictional, viscoelastic spheres in granular drainage simulated by the discrete-element method (DEM). With only five fitting parameters (the radius, volume, diffusivity, drift velocity, and injection rate of spots), we find that the spot simulations are able to largely reproduce not only the mean flow and diffusion, but also some subtle statistics of the flowing packings, such as spatial velocity correlations and many-body structural correlations. The spot simulations run over 100 times faster than the DEM and demonstrate the possibility of multiscale modeling for amorphous materials, whenever a suitable model can be devised for the coarse-grained spot dynamics.

DEBRIS-FLOW MOBILIZATION FROM LANDSLIDES

Richard Iverson — 2008-04-26T23:17:17-00:00

Annual Review of Earth and Planetary Sciences, Vol. 25, No. 1. (1997), pp. 85-138.

Abstract Field observations, laboratory experiments, and theoretical analyses indicate that landslides mobilize to form debris flows by three processes: (a) widespread Coulomb failure within a sloping soil, rock, or sediment mass, (b) partial or complete liquefaction of the mass by high pore-fluid pressures, and (c) conversion of landslide translational energy to internal vibrational energy (i.e. granular temperature). These processes can operate independently, but in many circumstances they appear to operate simultaneously and synergistically. Early work on debris-flow mobilization described a similar interplay of processes but relied on mechanical models in which debris behavior was assumed to be fixed and governed by a Bingham or Bagnold rheology. In contrast, this review emphasizes models in which debris behavior evolves in response to changing pore pressures and granular temperatures. One-dimensional infinite-slope models provide insight by quantifying how pore pressures and granular temperatures can influence the transition from Coulomb failure to liquefaction. Analyses of multidimensional experiments reveal complications ignored in one-dimensional models and demonstrate that debris-flow mobilization may occur by at least two distinct modes in the field.

Analysis of granular flow in a pebble-bed nuclear reactor

Chris Rycroft — 2008-04-26T23:14:59-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 74, No. 2. (2006)

Pebble-bed nuclear reactor technology, which is currently being revived around the world, raises fundamental questions about dense granular flow in silos. A typical reactor core is composed of graphite fuel pebbles, which drain very slowly in a continuous refueling process. Pebble flow is poorly understood and not easily accessible to experiments, and yet it has a major impact on reactor physics. To address this problem, we perform full-scale, discrete-element simulations in realistic geometries, with up to 440 000 frictional, viscoelastic 6-cm-diam spheres draining in a cylindrical vessel of diameter 3.5 m and height 10 m with bottom funnels angled at 30° or 60°. We also simulate a bidisperse core with a dynamic central column of smaller graphite moderator pebbles and show that little mixing occurs down to a 1:2 diameter ratio. We analyze the mean velocity, diffusion and mixing, local ordering and porosity (from Voronoi volumes), the residence-time distribution, and the effects of wall friction and discuss implications for reactor design and the basic physics of granular flow.

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

L Isa — 2006-06-07T11:46:33-00:00

Journal of Physics: Conference Series, Vol. 40, No. 1. (2006), 124.