2,536 research outputs found
Rheopexy and tunable yield stress of carbon black suspensions
We show that besides simple or thixotropic yield stress fluids there exists a
third class of yield stress fluids. This is illustrated through the rheological
behavior of a carbon black suspension, which is shown to exhibit a viscosity
bifurcation effect around a critical stress along with rheopectic trends, i.e.,
after a preshear at a given stress the fluid tends to accelerate when it is
submitted to a lower stress. Viscosity bifurcation displays here original
features: the yield stress and the critical shear rate depend on the previous
flow history. The most spectacular property due to these specificities is that
the material structure can be adjusted at will through an appropriate flow
history. In particular it is possible to tune the material yield stress to
arbitrary low values. A simple model assuming that the stress is the sum of one
component due to structure deformation and one component due to hydrodynamic
interactions predicts all rheological trends observed and appears to well
represent quantitatively the data.Comment: submitted to Soft Matte
Shear induced drainage in foamy yield-stress fluids
Shear induced drainage of a foamy yield stress fluid is investigated using
MRI techniques. Whereas the yield stress of the interstitial fluid stabilizes
the system at rest, a fast drainage is observed when a horizontal shear is
imposed. It is shown that the sheared interstitial material behaves as a
viscous fluid in the direction of gravity, the effective viscosity of which is
controlled by shear in transient foam films between bubbles. Results provided
for several bubble sizes are not captured by the R^2 scaling classically
observed for liquid flow in particulate systems, such as foams and thus
constitute a remarkable demonstration of the strong coupling of drainage flow
and shear induced interstitial flow. Furthermore, foam films are found to be
responsible for the unexpected arrest of drainage, thus trapping irreversibly a
significant amount of interstitial liquid.Comment: Published in Physical Review Letters.
http://prl.aps.org/abstract/PRL/v104/i12/e12830
Shear-induced sedimentation in yield stress fluids
Stability of coarse particles against gravity is an important issue in dense
suspensions (fresh concrete, foodstuff, etc.). On the one hand, it is known
that they are stable at rest when the interstitial paste has a high enough
yield stress; on the other hand, it is not yet possible to predict if a given
material will remain homogeneous during a flow. Using MRI techniques, we study
the time evolution of the particle volume fraction during the flows in a
Couette geometry of model density-mismatched suspensions of noncolloidal
particles in yield stress fluids. We observe that shear induces sedimentation
of the particles in all systems, which are stable at rest. The sedimentation
velocity is observed to increase with increasing shear rate and particle
diameter, and to decrease with increasing yield stress of the interstitial
fluid. At low shear rate ('plastic regime'), we show that this phenomenon can
be modelled by considering that the interstitial fluid behaves like a viscous
fluid -- of viscosity equal to the apparent viscosity of the sheared fluid --
in the direction orthogonal to shear. The behavior at higher shear rates, when
viscous effects start to be important, is also discussed. We finally study the
dependence of the sedimentation velocity on the particle volume fraction, and
show that its modelling requires estimating the local shear rate in the
interstitial fluid
Flows and heterogeneities with a vane tool: Magnetic resonance imaging measurements
We study the local flow properties of various materials in a vane-in-cup
geometry. We use magnetic resonance imaging techniques to measure velocities
and particle concentrations in flowing Newtonian fluid, yield stress fluid, and
in a concentrated suspension of noncolloidal particles in a yield stress fluid.
In the Newtonian fluid, we observe that the -averaged strain rate
component decreases as the inverse squared radius in the gap, in
agreement with a Couette analogy. This allows direct comparison (without
end-effect corrections) of the resistances to shear in vane and Couette
geometries. Here, the mean shear stress in the vane-in-cup geometry is slightly
lower than in a Couette cell of same dimensions, and a little higher than when
the vane is embedded in an infinite medium. We also observe that the flow
enters deeply the region between the blades, leading to significant extensional
flow. In the yield stress fluid, in contrast with the usually accepted picture
based on simulation results from the literature, we find that the layer of
material that is sheared near the blades at low velocity is not cylindrical.
There is thus a significant extensional component of shear that should be taken
into account in the analysis. Finally and surprisingly, in the suspension, we
observe that a thin non-cylindrical slip layer made of the pure interstitial
yield stress fluid appears quickly at the interface between the sheared
material and the material that moves as a rigid body between the blades. This
feature can be attributed to the non-symmetric trajectories of the noncolloidal
particles around the edges of the blades. This new important observation is in
sharp contradiction with the common belief that the vane tool prevents slippage
and may preclude the use of the vane tool for studying the flows of pasty
materials with large particles
Shear thickening and migration in granular suspensions
We study the emergence of shear thickening in dense suspensions of
non-Brownian particles. We combine local velocity and concentration
measurements using Magnetic Resonance Imaging with macroscopic rheometry
experiments. In steady state, we observe that the material is heterogeneous,
and we find that that the local rheology presents a continuous transition at
low shear rate from a viscous to a shear thickening, Bagnoldian, behavior with
shear stresses proportional to the shear rate squared, as predicted by a
scaling analysis. We show that the heterogeneity results from an unexpectedly
fast migration of grains, which we attribute to the emergence of the Bagnoldian
rheology. The migration process is observed to be accompanied by macroscopic
transient discontinuous shear thickening, which is consequently not an
intrinsic property of granular suspensions
EEG inverse problem solution with minimal influence of the conductivity
In this paper, we propose a novel method that improves the accuracy of the estimation of neural electrical dipoles when solving the EEG inverse problem. A spherical head model is used where we limit the influence of the unknown conductivity brain-skull ratio on the inverse problem.We redefine the cost function that is used in the EEG problem where only useful information is used as input in the inverse problem. In contrast to previous approaches, weighting factors are used where the electrodes are strategically chosen so to reduce the error made on EEG dipole source localization. The proposed method enhances the source localization accuracy from approximately 9mm to 1mm for dipoles near the edge and from 2.1mm to 0.4mm for dipoles near the center of the brain
Numerical studies of space filling designs: optimization of Latin Hypercube Samples and subprojection properties
International audienceQuantitative assessment of the uncertainties tainting the results of computer simulations is nowadays a major topic of interest in both industrial and scientific communities. One of the key issues in such studies is to get information about the output when the numerical simulations are expensive to run. This paper considers the problem of exploring the whole space of variations of the computer model input variables in the context of a large dimensional exploration space. Various properties of space filling designs are justified: interpoint-distance, discrepancy, minimum spanning tree criteria. A specific class of design, the optimized Latin Hypercube Sample, is considered. Several optimization algorithms, coming from the literature, are studied in terms of convergence speed, robustness to subprojection and space filling properties of the resulting design. Some recommendations for building such designs are given. Finally, another contribution of this paper is the deep analysis of the space filling properties of the design 2D-subprojections
Local determination of the constitutive law of a dense suspension of non-colloidal particles through MRI
We investigate the flowing behavior of dense suspensions of non-colloidal
particles, by coupling macroscopic rheometric experiments and local velocity
and concentration measurements through MRI techniques. We find that the flow is
localized at low velocities, and that the material is inhomogeneous; the local
laws inferred from macroscopic rheometric observations must then be
reinterpreted in the light of these local observations. We show that the short
time response to a velocity step allows to characterize dense suspensions
locally: they have a purely viscous behavior, without any observable influence
of friction. In the jammed zone, there may be a contact network, whereas in the
sheared zone there are only hydrodynamic interactions: localization consists in
a change in configuration at the grain scale. From the concentration and
velocity profiles, we have provided for the first time local measurements of
the concentration dependence of viscosity; we find a Krieger-Dougherty law
. Shear induced migration is almost
instantaneous and seems inconsistent with most observations: it would imply
that the diffusion coefficients strongly depend on the concentration. We
finally propose a simple constitutive law for dense suspensions, based on a
purely viscous behavior, that accounts for all the macroscopic and local
observations.Comment: Submitted to the Journal of Rheolog
Yield stress and shear-banding in granular suspensions
We study the emergence of a yield stress in dense suspensions of non-Brownian
particles, by combining local velocity and concentration measurements using
Magnetic Resonance Imaging with macroscopic rheometric experiments. We show
that the competition between gravity and viscous stresses is at the origin of
the development of a yield stress in these systems at relatively low volume
fractions. Moreover, it is accompanied by a shear banding phenomenon that is
the signature of this competition. However, if the system is carefully density
matched, no yield stress is encountered until a volume fraction of 62.7 0.3%
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