共查询到20条相似文献,搜索用时 31 毫秒
1.
We study the internal dynamical processes taking place in a granular packing below yield stress. At all packing fractions and down to vanishingly low applied shear, a logarithmic creep is observed. The experiments are analyzed using a viscoelastic model which introduces an internal, time-dependent, fluidity variable. For all experiments, the creep dynamics can be rescaled onto a unique curve which displays jamming at the random-close-packing limit. At each packing fraction, we measure a stress corresponding to the onset of internal granular reorganization and a slowing down of the creep dynamics before the final yield. 相似文献
2.
We present extensive computational results for the effective temperature,
defined by the fluctuation-dissipation relation between the mean square
displacement and the average displacement of grains, under the action of
a weak, external perturbation, of a sheared, bi-disperse granular packing
of compressible spheres. We study the dependence of this parameter on the
shear rate and volume fractions, the type of particle and the observable
in the fluctuation-dissipation relation. We find the same temperature
for different tracer particles in the system. The temperature becomes
independent on the shear rate for slow enough shear suggesting that it is
the effective temperature of the jammed packing. However, we also show
that the agreement of the effective temperature for different observables
is only approximate, for very long times, suggesting that this defintion
may not capture the full thermodynamics of the system. On the other hand,
we find good agreement between the dynamical effective temperature and a
compactivity calculated assuming that all jammed states are equiprobable.
Therefore, this definition of temperature may capture an instance
of the ergodic hypothesis for granular materials as proposed by theoretical
formalisms for jamming. Finally, our simulations indicate
that the average shear
stress and apparent shear viscosity follow the usual relation with the shear
rate for complex fluids. Our results show that the application of
shear induces jamming in packings whose particles interact
by tangential forces. 相似文献
3.
Azéma E Radjaï F Peyroux R Richefeu V Saussine G 《The European physical journal. E, Soft matter》2008,26(3):327-335
We analyze the dynamics of a 3D granular packing composed of particles of irregular polyhedral shape confined inside a rectangular box with a retaining wall subjected to horizontal harmonic forcing. The simulations are performed by means of the contact dynamics method for a broad set of loading parameters. We explore the vibrational dynamics of the packing, the evolution of solid fraction and the scaling of dynamics with the loading parameters. We show that the motion of the retaining wall is strongly anharmonic as a result of jamming and grain rearrangements. It is found that the mean particle displacement scales with inverse square of frequency, the inverse of the force amplitude and the square of gravity. The short-time compaction rate grows in proportion to frequency up to a characteristic frequency, corresponding to collective particle rearrangements between equilibrium states, and then it declines in inverse proportion to frequency. 相似文献
4.
Nicodemi M Coniglio A Herrmann HJ 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》1999,59(6):6830-6837
This paper presents the study of density fluctuations in a model for vibrated granular media. Their microscopic origin is shown to be linked to the microscopic disorder in grains packing. Varying vibrations amplitude and duration, several regimes are found for density relaxation. Its power spectrum is well described by power laws. 相似文献
5.
Z. M. Jakšić S. B. Vrhovac B. M. Panić Z. Nikolić B. M. Jelenković 《The European physical journal. E, Soft matter》2008,27(4):345-356
We study experimentally the creeping penetration of guest (percolating) grains through densely packed granular media in two dimensions. The evolution of the system of the guest grains during the penetration is studied by image analysis. To quantify the changes in the internal structure of the packing, we use Voronoï tessellation and a certain shape factor which is a clear indicator of the presence of different underlying substructures (domains). We first consider the impact of the effective gravitational acceleration on upward penetration of grains. It is found that the higher effective gravity increases the resistance to upward penetration and enhances structural organization in the system of the percolating grains. We also focus our attention on the dependence of the structural rearrangements of percolating grains on some parameters like polydispersity and the initial packing fraction of the host granular system. It is found that the anisotropy of penetration is larger in the monodisperse case than in the bidisperse one, for the same value of the packing fraction of the host medium. Compaction of initial host granular packing also increases anisotropy of penetration of guest grains. When a binary mixture of large and small guest grains is penetrated into the host granular medium, we observe size segregation patterns. 相似文献
6.
J. M. Luck A. Mehta 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,57(4):429-451
Granular media jam into a panoply of metastable states.
The way in which these states are achieved depends on the nature
of local and global constraints on grains; here we investigate this issue by means of a non-equilibrium stochastic model of
a hindered granular column near its jamming limit. Grains feel the constraints of grains
above and below them differently, depending on their position.
A rich phase diagram with four dynamical phases (ballistic, activated, logarithmic and glassy) is revealed. The statistics
of the jamming time and of the metastable states reached as attractors of the zero-temperature dynamics
is investigated in each of these phases. Of particular interest is the glassy phase, where intermittency and a strong deviation
from Edwards' flatness are manifest. 相似文献
7.
8.
B. Bassetti M. Cosentino Lagomarsino P. Jona 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,15(3):483-492
We propose a two-dimensional model for the organization of stabilized microtubules driven by molecular motors in an unconfined
geometry. In this model two kinds of dynamics are competing. The first one is purely diffusive, with an interaction between
the rotational degrees of freedom, while the second one is a local drive, dependent on microtubule polarity. As a result,
there is a configuration dependent driving field. Applying a molecular field approximation, we are able to derive continuum
equations. A study on the solutions of these equations shows non-equilibrium inhomogeneous steady states in various regions
of the parameter space. The presence and stability of such self-organized states are investigated in terms of entropy production.
Numerical simulations confirm our analytic results.
Received 4 August 1999 and Received in final form 24 November 1999 相似文献
9.
We present a new approach to determine velocity distributions in granular gases to improve the Sonine polynomial expansion of the velocity distribution function, at higher inelasticities, for the homogeneous cooling regime of inelastic hard spheres. The perturbative consistency is recovered using a new set of dynamical variables based on the characteristic function and we illustrate our approach by computing the first four Sonine coefficients for moderate and high inelasticities. The analytical coefficients are compared with molecular dynamics simulations results and with a previous approach by Huthmann et al. 相似文献
10.
Vandewalle N Lumay G Gerasimov O Ludewig F 《The European physical journal. E, Soft matter》2007,22(3):241-248
This article is a review of our recent and new experimental works on granular compaction. The effects of various microscopic
parameters on the
compaction dynamics are addressed, in particular the influence
of the grain shape, the friction and the cohesion between the grains. Two
dimensionnal and three dimensionnal systems are analysed. And the role of
dimensionality will be emphasized. Theoretical and numerical investigations provide additional informations about that phenomenon.
Indeed numerical models permit us to study the influence of some parameters not easily accessible experimentally. Our results
show that the above mentioned parameters have a deep impact on the compaction dynamics. Anisotropic grains lead to two different
compaction regimes separated by a “burst" of the packing fraction. Friction is observed to modify how the grains are arranged
in the pile. This is confirmed by numerical simulations. Cohesive forces between particles inhibit compaction and lead to
extremely low values of the packing fraction. 相似文献
11.
Ribière P Richard P Bideau D Delannay R 《The European physical journal. E, Soft matter》2005,16(4):415-420
We report on experiments to measure the temporal and spatial evolution of packing arrangements of anisotropic and weakly confined granular material, using high-resolution -ray adsorption. In these experiments, the particle configurations start from an initially disordered, low-packing-fraction state and under vertical solicitations evolve to a dense state. We find that the packing fraction evolution is slowed by the grain anisotropy but, as for spherically shaped grains, can be well fitted by a stretched exponential. For a given type of grains, the characteristic times of relaxation and of convection are found to be of the same order of magnitude. On the contrary, compaction mechanisms in the media strongly depend on the grain anisotropy. 相似文献
12.
Shinbrot T 《The European physical journal. E, Soft matter》2007,22(3):209-217
Analysis of granular flows has been a significant theoretical challenge over
the past several decades. These flows are difficult to analyze largely
because they exhibit both solid-like and fluid-like behaviors side-by-side
in single experiments. In this paper, we examine two experiments in which
the co-existence between these states is especially marked and leads to
unique patterns that may serve as signatures for underlying granular
dynamics deserving of further scrutiny. In these experiments, we find that
when fluidization of grains is prolonged — as can be expected to occur for
example under reduced gravity environments or under conditions of strong
kinetic forcing (e.g. during earthquakes) — grains can produce residual
depositional patterns that are difficult to distinguish from fluvial
deposits. This suggests that geological landforms under low gravity (for
example on Mars) or influenced by strong forcing (for example during
earthquakes) may behave in a fluid-like manner despite being entirely dry. 相似文献
13.
The response of a granular material during a stop-and-go
shear experiment is investigated using an annular shear cell and
silicagel powders of different particle sizes. The experimental
results are examined on the basis of the Dieterich-Rice-Ruina
model for solid friction. In addition to making this analogy
with solid friction, we describe a new instability that is
observed when restarting shear, where the powder bed is found to
slip and compact for short hold times but only dilates for long
hold times. The minimum hold time to restore a non-slip
behaviour has been investigated for different size particles and
normal loadings. The observed dependencies show analogies
between this behaviour and the sliding rearrangements seen above
the stick-slip threshold. 相似文献
14.
Based on a novel interplay of wetting adhesion and concurrent flow concepts, we propose to drive open microfluidic systems by shear in a covering fluid layer, e.g., oil covering water-filled chemical channels. The advantages are simpler forcing and prevention of evaporation of volatile components. We calculate the expected throughput for straight channels and show that devices can be built with off-the-shelf technology. Molecular dynamics simulations suggest that this concept is scalable down to the nanoscale. 相似文献
15.
16.
Jacob X Aleshin V Tournat V Leclaire P Lauriks W Gusev VE 《Physical review letters》2008,100(15):158003
Experimentally determined dispersion relations for acoustic waves guided along the mechanically free surface of an unconsolidated granular packed structure provide information on the elasticity of granular media at very low pressures that are naturally controlled by the gravitational acceleration and the depth beneath the surface. The experiments confirm recent theoretical predictions that relaxation of the disordered granular packing through nonaffine motion leads to a peculiar scaling of shear rigidity with pressure near the jamming transition corresponding to zero pressure. 相似文献
17.
18.
By means of contact dynamics simulations, we analyze the stress state in a granular bed slowly tilted toward its angle of
repose. An increasingly large number of grains are overloaded in the sense that they are found to carry a stress ratio above
the Coulomb yield threshold of the whole packing. Using this property, we introduce a coarse-graining length scale at which
all stress ratios are below the packing yield threshold. We show that this length increases with the slope angle and jumps
to a length comparable to the depth of the granular bed at an angle below the angle of repose. This transition coincides with
the onset of dilation in the packing. We map this transition into a percolation transition of the overloaded grains, and discuss
it in terms of long-range correlations and granular slope metastability. 相似文献
19.
F. Q. Potiguar R. Dickman 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,52(1):83-90
We study the lattice gas with nearest-neighbor
exclusion on the square lattice and Kawasaki (hopping) dynamics, under the
influence of a nonuniform drive, via Monte Carlo simulation.
The drive, which favors motion along the +x
direction and inhibits motion in the opposite direction, varies linearly with
y. (The boundaries along the drive direction are periodic, so that
the system is not described by an equilibrium Gibbs distribution.) As in the
uniformly driven case [R. Dickman, Phys. Rev. E 64, 16124 (2001)], the
onset of sublattice ordering occurs at a lower density than in equilibrium, but
here an unexpected feature appears: particles migrate out of the high-drive
region. For intermediate system sizes (L ≃100),
the accumulation of particles is sufficient for the low-drive region to become
ordered at a global density of about 0.3. Above this density we observe a
surprising reversal in the density profile, with particles accumulating to the
high-drive region, due to jamming. For larger systems (L≥200) particles
quickly jam in the high-drive region, as occurs under uniform drive, and the
accumulation of particles in the low-field region is severely reduced. 相似文献
20.
Dijksman JA Wortel GH van Dellen LT Dauchot O van Hecke M 《Physical review letters》2011,107(10):108303
We establish that the rheological curve of dry granular media is nonmonotonic, both in the presence and absence of external mechanical agitations. In the presence of weak vibrations, the nonmonotonic flow curves govern a hysteretic transition between slow but steady and fast, inertial flows. In the absence of vibrations, the nonmonotonic flow curve governs the yielding behavior of granular media. Finally, we show that nonmonotonic flow curves can be seen in at least two different flow geometries and for several granular materials. 相似文献