518 research outputs found
Tailoring the frictional properties of granular media
A method of modifying the roughness of soda-lime glass spheres is presented,
with the purpose of tuning inter-particle friction. The effect of chemical
etching on the surface topography and the bulk frictional properties of grains
is systematically investigated. The surface roughness of the grains is measured
using white light interferometry and characterised by the lateral and vertical
roughness length scales. The underwater angle of repose is measured to
characterise the bulk frictional behaviour. We observe that the co-efficient of
friction depends on the vertical roughness length scale. We also demonstrate a
bulk surface roughness measurement using a carbonated soft drink.Comment: 10 pages, 17 figures, submitted to Phys. Rev.
Rheology of granular flows across the transition from soft to rigid particles
International audienc
Multiscale Analysis of the Stress State in a Granular Slope in Transition to Failure
By means of contact dynamics simulations, we analyze the stress state in a
granular bed slowly tilted towards 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 dilatation in the packing. We map this transition
into a percolation transition of the overloaded grains, and we argue that in
the presence of long-range correlations above the transition angle, the
granular slope is metastable.Comment: 11 pages, 14 Fig, submitted to PR
Ion Microscopy: A New Approach for Subcellular Localization of Labelled Molecules
Secondary ion mass spectroscopy (SIMS) was used to obtain images representing the intracellular distribution of molecules labelled with carbon 14.
Deoxyadenosine labelled with carbon 14 was added to a cultured human fibroblast cell medium, and the intracellular distribution of this molecule was studied using three different SIMS instruments: the CAMECA IMS 3F and SMI 300 ion microscopes and the UC-HRL scanning ion microprobe.
Carbon 14 distribution images obtained by this method show that deoxyadenosine U-C14 is present in the cytoplasm as well as the nucleus, with a higher concentration in\u27 the nucleoli. Our study clearly demonstrates that ion microscopy is well suited for carbon 14 detection and localization at the subcellular level, permitting a wide variety of microanalytical tracer experiments
Diphasic non-local model for granular surface flows
Considering recent results revealing the existence of multi-scale rigid
clusters of grains embedded in granular surface flows, i.e. flows down an
erodible bed, we describe here the surface flows rheology through a non-local
constitutive law. The predictions of the resulting model are compared
quantitatively to experimental results: The model succeeds to account for the
counter-intuitive shape of the velocity profile observed in experiments, i.e. a
velocity profile decreasing exponentially with depth in the static phase and
remaining linear in the flowing layer with a velocity gradient independent of
both the flowing layer thickness, the angle between the flow and the
horizontal, and the coefficient of restitution of the grains. Moreover, the
scalings observed in rotating drums are recovered, at least for small rotating
speed.Comment: 7 pages, submitted to Europhys. Let
Gravity-driven Dense Granular Flows
We report and analyze the results of numerical studies of dense granular
flows in two and three dimensions, using both linear damped springs and
Hertzian force laws between particles. Chute flow generically produces a
constant density profile that satisfies scaling relations suggestive of a
Bagnold grain inertia regime. The type of force law has little impact on the
behavior of the system. Bulk and surface flows differ in their failure criteria
and flow rheology, as evidenced by the change in principal stress directions
near the surface. Surface-only flows are not observed in this geometry.Comment: 4 pages, RevTeX 3.0, 4 PostScript figures (5 files) embedded with
eps
Logarithmic rate dependence in deforming granular materials
Rate-independence for stresses within a granular material is a basic tenet of
many models for slow dense granular flows. By contrast, logarithmic rate
dependence of stresses is found in solid-on-solid friction, in geological
settings, and elsewhere. In this work, we show that logarithmic rate-dependence
occurs in granular materials for plastic (irreversible) deformations that occur
during shearing but not for elastic (reversible) deformations, such as those
that occur under moderate repetitive compression. Increasing the shearing rate,
\Omega, leads to an increase in the stress and the stress fluctuations that at
least qualitatively resemble what occurs due to an increase in the density.
Increases in \Omega also lead to qualitative changes in the distributions of
stress build-up and relaxation events. If shearing is stopped at t=0, stress
relaxations occur with \sigma(t)/ \sigma(t=0) \simeq A \log(t/t_0). This
collective relaxation of the stress network over logarithmically long times
provides a mechanism for rate-dependent strengthening.Comment: 4 pages, 5 figures. RevTeX
Stress in frictionless granular material: Adaptive Network Simulations
We present a minimalistic approach to simulations of force transmission
through granular systems. We start from a configuration containing cohesive
(tensile) contact forces and use an adaptive procedure to find the stable
configuration with no tensile contact forces. The procedure works by
sequentially removing and adding individual contacts between adjacent beads,
while the bead positions are not modified. In a series of two-dimensional
realizations, the resulting force networks are shown to satisfy a linear
constraint among the three components of average stress, as anticipated by
recent theories. The coefficients in the linear constraint remain nearly
constant for a range of shear loadings up to about .6 of the normal loading.
The spatial distribution of contact forces shows strong concentration along
``force chains". The probability of contact forces of magnitude f shows an
exponential falloff with f. The response to a local perturbing force is
concentrated along two characteristic rays directed downward and laterally.Comment: 8 pages, 8 figure
Static Friction Phenomena in Granular Materials: Coulomb Law vs. Particle Geometry
The static as well as the dynamic behaviour of granular material are
determined by dynamic {\it and} static friction. There are well known methods
to include static friction in molecular dynamics simulations using scarcely
understood forces. We propose an Ansatz based on the geometrical shape of
nonspherical particles which does not involve an explicit expression for static
friction. It is shown that the simulations based on this model are close to
experimental results.Comment: 11 pages, Revtex, HLRZ-33/9
A model for collisions in granular gases
We propose a model for collisions between particles of a granular material
and calculate the restitution coefficients for the normal and tangential motion
as functions of the impact velocity from considerations of dissipative
viscoelastic collisions. Existing models of impact with dissipation as well as
the classical Hertz impact theory are included in the present model as special
cases. We find that the type of collision (smooth, reflecting or sticky) is
determined by the impact velocity and by the surface properties of the
colliding grains. We observe a rather nontrivial dependence of the tangential
restitution coefficient on the impact velocity.Comment: 11 pages, 2 figure
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