184 research outputs found
Influence of shear stress applied during flow stoppage and rest period on the mechanical properties of thixotropic suspensions
We study the solid mechanical properties of several thixotropic suspensions
as a function of the shear stress history applied during their flow stoppage
and their aging in their solid state. We show that their elastic modulus and
yield stress depend strongly on the shear stress applied during their
solid-liquid transition (i.e., during flow stoppage) while applying the same
stress only before or only after this transition may induce only second-order
effects: there is negligible dependence of the mechanical properties on the
preshear history and on the shear stress applied at rest. We also found that
the suspensions age with a structuration rate that hardly depends on the stress
history. We propose a physical sketch based on the freezing of a microstructure
whose anisotropy depends on the stress applied during the liquid-solid
transition to explain why the mechanical properties depend strongly on this
stress. This sketch points out the role of the internal forces in the colloidal
suspensions' behavior. We finally discuss briefly the macroscopic consequences
of this phenomenon and show the importance of using a controlled-stress
rheometer
Elastic medium confined in a column versus the Janssen experiment
We compute the stresses in an elastic medium confined in a vertical column,
when the material is at the Coulomb threshold everywhere at the walls.
Simulations are performed in 2 dimensions using a spring lattice, and in 3
dimensions, using Finite Element Method. The results are compared to the
Janssen model and to experimental results for a granular material. The
necessity to consider elastic anisotropy to render qualitatively the
experimental findings is discussed
Particle Diffusion in Slow Granular Bulk Flows
We probe the diffusive motion of particles in slowly sheared three
dimensional granular suspensions. For sufficiently large strains, the particle
dynamics exhibits diffusive Gaussian statistics, with the diffusivity
proportional to the local strain rate - consistent with a local, quasi static
picture. Surprisingly, the diffusivity is also inversely proportional to the
depth of the particles within the flow - at the free surface, diffusivity is
thus ill defined. We find that the crossover to Gaussian displacement
statistics is governed by the same depth dependence, evidencing a non-trivial
strain scale in three dimensional granular flows.Comment: 6 page
Shear thickening of cornstarch suspensions as a re-entrant jamming transition
We study the rheology of cornstarch suspensions, a dense system of
non-Brownian particles that exhibits shear thickening, i.e. a viscosity that
increases with increasing shear rate. Using MRI velocimetry we show that the
suspension has a yield stress. From classical rheology it follows that as a
function of the applied stress the suspension is first solid (yield stress),
then liquid and then solid again when it shear thickens. The onset shear rate
for thickening is found to depend on the measurement geometry: the smaller the
gap of the shear cell, the lower the shear rate at which thickening occurs.
Shear thickening can then be interpreted as the consequence of the Reynolds
dilatancy: the system under flow wants to dilate but instead undergoes a
jamming transition because it is confined, as confirmed by measurement of the
dilation of the suspension as a function of the shear rate
A giant overshoot effect in the Janssen granular column
We present new experimental results on the mechanical behavior of static
granular assemblies confined in a vertical column. Our measurements confirm,
for the first time, the universal Janssen's scaling for the stress saturation
curve. We show consequently, in the context of isotropic elasticity, a relation
between the Poisson ratio and granular packing fraction. Moreover, using a
systematic study of the overshoot effect created by a top mass equal to the
saturation mass, we show behaviors reproduced qualitatively by isotropic
elastic materials but in the case of a granular assembly of a spectacular
amplitude. These experimental results are strong tests for any theory of
granular matter
Macroscopic Discontinuous Shear Thickening vs Local Shear Jamming in Cornstarch
We study the emergence of discontinuous shear-thickening (DST) in cornstarch,
by combining macroscopic rheometry with local Magnetic Resonance Imaging (MRI)
measurements. We bring evidence that macroscopic DST is observed only when the
flow separates into a low-density flowing and a high-density jammed region. In
the shear-thickened steady state, the local rheology in the flowing region, is
not DST but, strikingly, is often shear-thinning. Our data thus show that the
stress jump measured during DST, in cornstach, does not capture a secondary,
high-viscosity branch of the local steady rheology, but results from the
existence of a shear jamming limit at volume fractions quite significantly
below random close packing.Comment: To be published in PR
Slow dynamics and aging of a confined granular flow
We present experimental results on slow flow properties of a granular
assembly confined in a vertical column and driven upwards at a constant
velocity V. For monodisperse assemblies this study evidences at low velocities
() a stiffening behaviour i.e. the stress necessary to obtain
a steady sate velocity increases roughly logarithmically with velocity. On the
other hand, at very low driving velocity (), we evidence a
discontinuous and hysteretic transition to a stick-slip regime characterized by
a strong divergence of the maximal blockage force when the velocity goes to
zero. We show that all this phenomenology is strongly influenced by surrounding
humidity. We also present a tentative to establish a link between the granular
rheology and the solid friction forces between the wall and the grains. We base
our discussions on a simple theoretical model and independent grain/wall
tribology measurements. We also use finite elements numerical simulations to
confront experimental results to isotropic elasticity. A second system made of
polydisperse assemblies of glass beads is investigated. We emphasize the onset
of a new dynamical behavior, i.e. the large distribution of blockage forces
evidenced in the stick-slip regime
Three-dimensional foam flow resolved by fast X-ray tomographic microscopy
Thanks to ultra fast and high resolution X-ray tomography, we managed to
capture the evolution of the local structure of the bubble network of a 3D foam
flowing around a sphere. As for the 2D foam flow around a circular obstacle, we
observed an axisymmetric velocity field with a recirculation zone, and
indications of a negative wake downstream the obstacle. The bubble
deformations, quantified by a shape tensor, are smaller than in 2D, due to a
purely 3D feature: the azimuthal bubble shape variation. Moreover, we were able
to detect plastic rearrangements, characterized by the neighbor-swapping of
four bubbles. Their spatial structure suggest that rearrangements are triggered
when films faces get smaller than a characteristic area.Comment: 5 pages, 5 figure
Effect of an electric field on an intermittent granular flow
Granular gravity driven flows of glass beads have been observed in a silo
with a flat bottom. A DC high electric field has been applied perpendicularly
to the silo to tune the cohesion. The outlet mass flow has been measured. An
image subtraction technique has been applied to visualize the flow geometry and
a spatiotemporal analysis of the flow dynamics has been performed. The outlet
mass flow is independent of voltage, but a transition from funnel flow to
rathole flow is observed. This transition is of probabilistic nature and an
intermediate situation exists between the funnel and the rathole situations. At
a given voltage, two kinds of flow dynamics can occur : a continuous flow or an
intermittent flow. The electric field increases the probability to observe an
intermittent flow.Comment: Accepted for publication in PRE on Apr 9, 201
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