3,181 research outputs found
Soft grain compression: beyond the jamming point
We present the experimental studies of highly strained soft bidisperse
granular systems made of hyperelastic and plastic particles. We explore the
behavior of granular matter deep in the jammed state from local field
measurement from the grain scale to the global scale. By mean of digital image
correlation and accurate image recording we measure for each compression step
the evolution of the particle geometries and their right Cauchy-Green strain
tensor fields. We analyze the evolution of the usual macroscopic observables
(stress, packing fraction, coordination, fraction of non-rattlers,
\textit{etc}.) along the compression process through the jamming point and far
beyond. We also analyze the evolution of the local strain statistics and
evidence a crossover in the material behavior deep in the jammed state. We show
that this crossover depends on the particle material. We argue that the strain
field is a reliable observable to describe the evolution of a granular system
through the jamming transition and deep in the dense packing state whatever is
the material behavior.Comment: 10 figure
Jamming Transition In Non-Spherical Particle Systems: Pentagons Versus Disks
We investigate the jamming transition in a quasi-2D granular material composed of regular pentagons or disks subjected to quasistatic uniaxial compression. We report six major findings based on experiments with monodisperse photoelastic particles with static friction coefficient μ≈1. (1) For both pentagons and disks, the onset of rigidity occurs when the average coordination number of non-rattlers, Znr, reaches 3, and the dependence of Znr on the packing fraction ϕ changes again when Znr reaches 4. (2) Though the packing fractions ϕc1 and ϕc2 at these transitions differ from run to run, for both shapes the data from all runs with different initial configurations collapses when plotted as a function of the non-rattler fraction. (3) The averaged values of ϕc1 and ϕc2 for pentagons are around 1% smaller than those for disks. (4) Both jammed pentagons and disks show Gamma distribution of the Voronoi cell area with same parameters. (5) The jammed pentagons have similar translational order for particle centers but slightly less orientational order for contacting pairs compared to jammed disks. (6) For jammed pentagons, the angle between edges at a face-to-vertex contact point shows a uniform distribution and the size of a cluster connected by face-to-face contacts shows a power-law distribution
Behavior of pressure and viscosity at high densities for two-dimensional hard and soft granular materials
The pressure and the viscosity in two-dimensional sheared granular assemblies
are investigated numerically. The behavior of both pressure and viscosity is
smoothly changing qualitatively when starting from a mono-disperse hard-disk
system without dissipation and moving towards a system of (i) poly-disperse,
(ii) soft particles with (iii) considerable dissipation.
In the rigid, elastic limit of mono-disperse systems, the viscosity is
approximately inverse proportional to the area fraction difference from
, but the pressure is still finite at . In
moderately soft, dissipative and poly-disperse systems, on the other hand, we
confirm the recent theoretical prediction that both scaled pressure (divided by
the kinetic temperature ) and scaled viscosity (divided by )
diverge at the same density, i.e., the jamming transition point , with the exponents -2 and -3, respectively. Furthermore, we observe
that the critical region of the jamming transition becomes invisible as the
restitution coefficient approaches unity, i.e. for vanishing dissipation.
In order to understand the conflict between these two different predictions
on the divergence of the pressure and the viscosity, the transition from soft
to hard particles is studied in detail and the dimensionless control parameters
are defined as ratios of various time-scales. We introduce a dimensionless
number, i.e. the ratio of dissipation rate and shear rate, that can identify
the crossover from the scaling of very hard, i.e. rigid disks to the scaling in
the soft, jamming regime.Comment: 23 pages, 20 figures, to appear in Progress of Theoretical Physics
Supplemen
Defining Temperatures of Granular Powders Analogously with Thermodynamics to Understand the Jamming Phenomena
For the purpose of applying laws or principles originated from thermal
systems to granular athermal systems, we may need to properly define the
critical temperature concept in granular powders. The conventional
environmental temperature in thermal systems is too weak to drive movements of
particles in granular powders and cannot function as a thermal energy
indicator. For maintaining the same functionality as in thermal systems, the
temperature in granular powders is defined analogously and uniformly in this
article. The newly defined granular temperature is utilized to describe and
explain one of the most important phenomena observed in granular powders, the
jamming transition, by introducing jamming temperature and jamming volume
fraction concepts. The predictions from the equations of the jamming volume
fractions for several cases like granular powders under shear or vibration are
in line with experimental observations and empirical solutions in powder
handlings. The goal of this article is to establish similar concepts in
granular powders, allowing granular powders to be described with common laws or
principles we are familiar with in thermal systems. Our intention is to build a
bridge between thermal systems and granular powders to account for many
similarities already found between these two systems.Comment: 34 pages,15 figure
Jamming Transition In Non-Spherical Particle Systems: Pentagons Versus Disks
We investigate the jamming transition in a quasi-2D granular material composed of regular pentagons or disks subjected to quasistatic uniaxial compression. We report six major findings based on experiments with monodisperse photoelastic particles with static friction coefficient μ≈1. (1) For both pentagons and disks, the onset of rigidity occurs when the average coordination number of non-rattlers, Znr, reaches 3, and the dependence of Znr on the packing fraction ϕ changes again when Znr reaches 4. (2) Though the packing fractions ϕc1 and ϕc2 at these transitions differ from run to run, for both shapes the data from all runs with different initial configurations collapses when plotted as a function of the non-rattler fraction. (3) The averaged values of ϕc1 and ϕc2 for pentagons are around 1% smaller than those for disks. (4) Both jammed pentagons and disks show Gamma distribution of the Voronoi cell area with same parameters. (5) The jammed pentagons have similar translational order for particle centers but slightly less orientational order for contacting pairs compared to jammed disks. (6) For jammed pentagons, the angle between edges at a face-to-vertex contact point shows a uniform distribution and the size of a cluster connected by face-to-face contacts shows a power-law distribution
Jamming and unjamming of concentrated colloidal dispersions in channel flow
We investigated the pressure driven flow of concentrated colloidal dispersions in a converging channel geometry. Optical microscopy and image analysis were used to track tracer particles mixed into dispersions of sterically stabilized poly(methyl methacrylate) (PMMA) spheres. The dispersions were drawn into a round \unit[0.5]{mm} capillary at one of two pump speeds ( applied pressure): v_1=\unit[0.245]{ml\,\, min^{-1}} and v_2=\unit[0.612]{ml\,\, min^{-1}}. We observed that the dispersions at particle volume fractions followed Hagen-Poiseuille flow for a simple fluid; i.e. the mean flow rate is approximately proportional to pressure drop (pump speed) and inversely proportional viscosity . Above this concentration (), the dispersions exhibit granular-like jamming behavior with becoming independent of the pressure drop. However, at the highest applied pressure (), the dispersions are able to unjam and switch from granular-like behaviour back to a simple hard-sphere liquid like system, due to the formation of rotating vortices in the spatial flow pattern. This mechanism is consistent with computer simulations of granular systems and supports for example proposed explanations of anomalously low friction in earthquake faults
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