395 research outputs found
The mechanics of yield stress fluids: similarities, specificities and open questions
A wide range of materials encountered in our everyday life, such as clay suspensions, foams, concentrated emulsions, cement pastes, paints, glues, purees, creams, can flow like simple liquids under certain conditions and behave like solids under other conditions. This is the specificity of yield stress fluids which makes them so useful in various applications. In their “liquid regime” these materials exhibit typical flow properties of simple fluids such as a transition to turbulence, the roll wave instability, the hydraulic jump, etc. The specific properties occur when the solid regime is involved, either in a part of the material or as a whole. In that case one may for example observe plug flow, flow stoppage over steep slopes, no sedimentation of dense particles, cylindrical drips, Saffman-Taylor instability at vanishing velocity, etc. In addition yield stress fluids are often thixotropic, i.e. their viscosity may vary in time. The physical origin of this phenomenon and the mechanical model appropriate for describing it remain the most challenging aspects of these fluids
Transition from a simple yield stress fluid to a thixotropic material
From MRI rheometry we show that a pure emulsion can be turned from a simple
yield stress fluid to a thixotropic material by adding a small fraction of
colloidal particles. The two fluids have the same behavior in the liquid regime
but the loaded emulsion exhibits a critical shear rate below which no steady
flows can be observed. For a stress below the yield stress, the pure emulsion
abruptly stops flowing, whereas the viscosity of the loaded emulsion
continuously increases in time, which leads to an apparent flow stoppage. This
phenomenon can be very well represented by a model assuming a progressive
increase of the number of droplet links via colloidal particles.Comment: Published in Physical Review E.
http://pre.aps.org/abstract/PRE/v76/i5/e05140
Flow of wet granular materials
The transition from frictional to lubricated flow of a dense suspension of
non-Brownian particles is studied. The pertinent parameter characterizing this
transition is the Leighton number ,
which represents the ratio of lubrication to frictional forces. The Leighton
number defines a critical shear rate below which no steady flow without
localization exists. In the frictional regime the shear flow is localized. The
lubricated regime is not simply viscous: the ratio of shear to normal stresses
remains constant, as in the frictional regime; moreover the velocity profile
has a single universal form in both frictional and lubricated regimes. Finally,
a discrepancy between local and global measurements of viscosity is identified,
which suggests inhomogeneity of the material under flow.Comment: Accepted for publication by Physical Review Letters (december 2004
Motion of a sphere through an aging system
We have investigated the drag on a sphere falling through a clay suspension
that has a yield stress and exhibits rheological aging. The drag force
increases with both speed and the rest time between preparation of the system
and the start of the experiment, but there exists a nonzero minimum speed below
which steady motion is not possible. We find that only a very thin layer of
material around the sphere is fluidized when it moves, while the rest of
suspension is deformed elastically. This is in marked contrast to what is found
for yield-stress fluids that do not age.Comment: latex, 4 figure
Unified study of glass and jamming rheology in soft particle systems
We explore numerically the shear rheology of soft repulsive particles at
large volume fraction. The interplay between viscous dissipation and thermal
motion results in multiple rheological regimes encompassing Newtonian,
shear-thinning and yield stress regimes near the `colloidal' glass transition
when thermal fluctuations are important, crossing over to qualitatively similar
regimes near the `jamming' transition when dissipation dominates. In the
crossover regime, glass and jamming sectors coexist and give complex flow
curves. Although glass and jamming limits are characterized by similar
macroscopic flow curves, we show that they occur over distinct time and stress
scales and correspond to distinct microscopic dynamics. We propose a simple
rheological model describing the glass to jamming crossover in the flow curves,
and discuss the experimental implications of our results.Comment: 5 pages, 3 figs; v2 accepted to publication to Phys. Rev. Let
Internal relaxation time in immersed particulate materials
We study the dynamics of the solid to liquid transition for a model material
made of elastic particles immersed in a viscous fluid. The interaction between
particle surfaces includes their viscous lubrication, a sharp repulsion when
they get closer than a tuned steric length and their elastic deflection induced
by those two forces. We use Soft Dynamics to simulate the dynamics of this
material when it experiences a step increase in the shear stress and a constant
normal stress. We observe a long creep phase before a substantial flow
eventually establishes. We find that the typical creep time relies on an
internal relaxation process, namely the separation of two particles driven by
the applied stress and resisted by the viscous friction. This mechanism should
be relevant for granular pastes, living cells, emulsions and wet foams
Yield stress and elastic modulus of suspensions of noncolloidal particles in yield stress fluids
We study experimentally the behavior of isotropic suspensions of noncolloidal
particles in yield stress fluids. This problem has been poorly studied in the
literature, and only on specific materials. In this paper, we manage to develop
procedures and materials that allow us to focus on the purely mechanical
contribution of the particles to the yield stress fluid behavior, independently
of the physicochemical properties of the materials. This allows us to relate
the macroscopic properties of these suspensions to the mechanical properties of
the yield stress fluid and the particle volume fraction, and to provide results
applicable to any noncolloidal particle in any yield stress fluid. We find that
the elastic modulus-concentration relationship follows a Krieger-Dougherty law,
and we show that the yield stress-concentration relationship is related to the
elastic modulus-concentration relationship through a very simple law, in
agreement with results from a micromechanical analysis
Inhomogeneous shear flows in soft jammed materials with tunable attractive forces
We perform molecular dynamics simulations to characterize the occurrence of
inhomogeneous shear flows in soft jammed materials. We use rough walls to
impose a simple shear flow and study the athermal motion of jammed assemblies
of soft particles, both for purely repulsive interactions and in the presence
of an additional short-range attraction of varying strength. In steady state,
pronounced flow inhomogeneities emerge for all systems when the shear rate
becomes small. Deviations from linear flow are stronger in magnitude and become
very long-lived when the strength of the attraction increases, but differ from
permanent shear-bands. Flow inhomogeneities occur in a stress window bounded by
the dynamic and static yield stress values. Attractive forces enhance the flow
heterogeneities because they accelerate stress relaxation, thus effectively
moving the system closer to the yield stress regime where inhomogeneities are
most pronounced. The present scenario for understanding the effect of particle
adhesion on shear localization, which is based on detailed molecular dynamics
simulations with realistic particle interactions, differs qualitatively from
previous qualitative explanations and ad-hoc theoretical modelling.Comment: 12 pages, 12 figure
Three-dimensional jamming and flows of soft glassy materials
Various disordered dense systems such as foams, gels, emulsions and colloidal
suspensions, exhibit a jamming transition from a liquid state (they flow) to a
solid state below a yield stress. Their structure, thoroughly studied with
powerful means of 3D characterization, exhibits some analogy with that of
glasses which led to call them soft glassy materials. However, despite its
importance for geophysical and industrial applications, their rheological
behavior, and its microscopic origin, is still poorly known, in particular
because of its nonlinear nature. Here we show from two original experiments
that a simple 3D continuum description of the behaviour of soft glassy
materials can be built. We first show that when a flow is imposed in some
direction there is no yield resistance to a secondary flow: these systems are
always unjammed simultaneously in all directions of space. The 3D jamming
criterion appears to be the plasticity criterion encountered in most solids. We
also find that they behave as simple liquids in the direction orthogonal to
that of the main flow; their viscosity is inversely proportional to the main
flow shear rate, as a signature of shear-induced structural relaxation, in
close similarity with the structural relaxations driven by temperature and
density in other glassy systems.Comment: http://www.nature.com/nmat/journal/v9/n2/abs/nmat2615.htm
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
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