99 research outputs found
Isostatic phase transition and instability in stiff granular materials
In this letter, structural rigidity concepts are used to understand the
origin of instabilities in granular aggregates. It is shown that: a) The
contact network of a noncohesive granular aggregate becomes exactly isostatic
in the limit of large stiffness-to-load ratio. b) Isostaticity is responsible
for the anomalously large susceptibility to perturbation of these systems, and
c) The load-stress response function of granular materials is critical
(power-law distributed) in the isostatic limit. Thus there is a phase
transition in the limit of intinitely large stiffness, and the resulting
isostatic phase is characterized by huge instability to perturbation.Comment: RevTeX, 4 pages w/eps figures [psfig]. To appear in Phys. Rev. Let
Growing length scale in gravity-driven dense granular flow
We report simulations of a two-dimensional, dense, bidisperse system of
inelastic hard disks falling down a vertical tube under the influence of
gravity. We examine the approach to jamming as the average flow of particles
down the tube is slowed by making the outlet narrower. Defining coarse-grained
velocity and stress fields, we study two-point temporal and spatial correlation
functions of these fields in a region of the tube where the time-averaged
velocity is spatially uniform. We find that fluctuations in both velocity and
stress become increasingly correlated as the system approaches jamming. We
extract a growing length scale and time scale from these correlations.Comment: 21 pages, 13 figure
The statistics of particle velocities in dense granular flows
We present measurements of the particle velocity distribution in the flow of
granular material through vertical channels. Our study is confined to dense,
slow flows where the material shears like a fluid only in thin layers adjacent
to the walls, while a large core moves without continuous deformation, like a
solid. We find the velocity distribution to be non-Gaussian, anisotropic, and
to follow a power law at large velocities. Remarkably, the distribution is
identical in the fluid-like and solid-like regions. The velocity variance is
maximum at the core, defying predictions of hydrodynamic theories. We show
evidence of spatially correlated motion, and propose a mechanism for the
generation of fluctuational motion in the absence of shear.Comment: Submitted to Phys. Rev. Let
QUALITATIVE ANALYSIS AND ANTHELMINTIC ACTIVITY OF HYDRO-ALCOHOLIC EXTRACT OF TABERNAEMONTANA DIVARICATA
Tabernaemontana divaricata is a common shrub found in the tropical regions and is often used for medicinal purposes, particularly the flowers of the plant. The present study is conducted to compare and identify the phytochemical constituents by Thin Layer Chromatography (TLC) and Qualitative Phytochemical analysis and to determine the anthelmentic activity of fresh and dried flower extract of Tabernaemontana divaricata. The extract is obtained using two different methods like cold maceration and hot solvent extraction by using soxhlet apparatus, first with petroleum ether followed by hydroalcohol as solvents. The preliminary phytochemical analysis of the extract indicated the presence of Alkaloids, Flavanoids, Steroids, Proteins, Carbohydrates and Tannins. The Rf value of TLC is calculated and compared with standard values and analysis proved the presence of the phytochemical constituents. The anthelmentic activity studies are performed using Indian earth worms. For this, the concentrated extract is diluted to various concentrations, and the effect of each solution is studied by measuring the time taken for paralysis and death of the earth worms. It is found to show significant anthelmentic activity at various concentrations compared with that of the standard drug Metronidazole
Force Distribution in a Granular Medium
We report on systematic measurements of the distribution of normal forces
exerted by granular material under uniaxial compression onto the interior
surfaces of a confining vessel. Our experiments on three-dimensional, random
packings of monodisperse glass beads show that this distribution is nearly
uniform for forces below the mean force and decays exponentially for forces
greater than the mean. The shape of the distribution and the value of the
exponential decay constant are unaffected by changes in the system preparation
history or in the boundary conditions. An empirical functional form for the
distribution is proposed that provides an excellent fit over the whole force
range measured and is also consistent with recent computer simulation data.Comment: 6 pages. For more information, see http://mrsec.uchicago.edu/granula
Stresses in isostatic granular systems and emergence of force chains
Progress is reported on several questions that bedevil understanding of
granular systems: (i) are the stress equations elliptic, parabolic or
hyperbolic? (ii) how can the often-observed force chains be predicted from a
first-principles continuous theory? (iii) How to relate insight from isostatic
systems to general packings? Explicit equations are derived for the stress
components in two dimensions including the dependence on the local structure.
The equations are shown to be hyperbolic and their general solutions, as well
as the Green function, are found. It is shown that the solutions give rise to
force chains and the explicit dependence of the force chains trajectories and
magnitudes on the local geometry is predicted. Direct experimental tests of the
predictions are proposed. Finally, a framework is proposed to relate the
analysis to non-isostatic and more realistic granular assemblies.Comment: 4 pages, 2 figures, Corrected typos and clkearer text, submitted to
Phys. Rev. Let
Development of Stresses in Cohesionless Poured Sand
The pressure distribution beneath a conical sandpile, created by pouring sand
from a point source onto a rough rigid support, shows a pronounced minimum
below the apex (`the dip'). Recent work of the authors has attempted to explain
this phenomenon by invoking local rules for stress propagation that depend on
the local geometry, and hence on the construction history, of the medium. We
discuss the fundamental difference between such approaches, which lead to
hyperbolic differential equations, and elastoplastic models, for which the
equations are elliptic within any elastic zones present .... This displacement
field appears to be either ill-defined, or defined relative to a reference
state whose physical existence is in doubt. Insofar as their predictions depend
on physical factors unknown and outside experimental control, such
elastoplastic models predict that the observations should be intrinsically
irreproducible .... Our hyperbolic models are based instead on a physical
picture of the material, in which (a) the load is supported by a skeletal
network of force chains ("stress paths") whose geometry depends on construction
history; (b) this network is `fragile' or marginally stable, in a sense that we
define. .... We point out that our hyperbolic models can nonetheless be
reconciled with elastoplastic ideas by taking the limit of an extremely
anisotropic yield condition.Comment: 25 pages, latex RS.tex with rspublic.sty, 7 figures in Rsfig.ps.
Philosophical Transactions A, Royal Society, submitted 02/9
Vector lattice model for stresses in granular materials
A vector lattice model for stresses in granular materials is proposed. A two
dimensional pile built by pouring from a point is constructed numerically
according to this model. Remarkably, the pile violates the Mohr Coulomb
stability criterion for granular matter, probably because of the inherent
anisotropy of such poured piles. The numerical results are also compared to the
earlier continuum FPA model and the (scalar) lattice -model
Properties of layer-by-layer vector stochastic models of force fluctuations in granular materials
We attempt to describe the stress distributions of granular packings using
lattice-based layer-by-layer stochastic models that satisfy the constraints of
force and torque balance and non-tensile forces at each site. The inherent
asymmetry in the layer-by-layer approach appears to lead to an asymmetric force
distribution, in disagreement with both experiments and general symmetry
considerations. The vertical force component probability distribution is robust
and in agreement with predictions of the scalar q model while the distribution
of horizontal force components is qualitatively different and depends on the
details of implementation.Comment: 18 pages, 12 figures (with subfigures), 1 table. Uses revtex,
epsfig,subfigure, and cite. Submitted to PRE. Plots have been bitmapped.
High-resolution version is available. Email [email protected] or
download from http://rainbow.uchicago.edu/~mbnguyen/research/vm.htm
Stress response function of a two-dimensional ordered packing of frictional beads
We study the stress profile of an ordered two-dimensional packing of beads in
response to the application of a vertical overload localized at its top
surface. Disorder is introduced through the Coulombic friction between the
grains which gives some indeterminacy and allows the choice of one constrained
random number per grain in the calculation of the contact forces. The so-called
`multi-agent' technique we use, lets us deal with systems as large as
grains. We show that the average response profile has a double
peaked structure. At large depth , the position of these peaks grows with
, while their widths scales like . and are analogous to
`propagation' and `diffusion' coefficients. Their values depend on that of the
friction coefficient . At small , we get and , with , which means that the peaks get
closer and wider as the disorder gets larger. This behavior is qualitatively
what was predicted in a model where a stochastic relation between the stress
components is assumed.Comment: 7 pages, 7 figures, accepted version to Europhys. Let
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