14 research outputs found
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
Wide-gap Couette flows of dense emulsions: Local concentration measurements, and comparison between macroscopic and local constitutive law measurements through magnetic resonance imaging
Flows of dense emulsions show many complex features among which long range
nonlocal effects pose a problem for macroscopic characterization. In order to
get around this problem, we study the flows of several dense emulsions in a
wide-gap Couette geometry. We couple macroscopic rheometric experiments and
local velocity measurements through MRI techniques. As concentration
heterogeneities can be expected, we designed a method to measure the local
droplet concentration in emulsions with a MRI device. In contrast to dense
suspensions of rigid particles where very fast migration occurs under shear, we
show that no migration takes place in dense emulsions even for strains as large
as 100 000 in our systems. As a result of the absence of migration and of
finite size effect, we are able to determine very precisely the local
rheological behavior of several dense emulsions. As the materials are
homogeneous, this behavior can also be inferred from purely macroscopic
measurements. We thus suggest that properly analyzed purely macroscopic
measurements in a wide-gap Couette geometry can be used as a tool to study the
local constitutive laws of dense emulsions. All behaviors are basically
consistent with Herschel-Bulkley laws of index 0.5, but discrepancies exist at
the approach of the yield stress due to slow shear flows below the apparent
yield stress in the case of a strongly adhesive emulsion. The existence of a
constitutive law accounting for all flows contrasts with previous results
obtained within a microchannel by Goyon et al. (2008): the use of a wide-gap
Couette geometry is likely to prevent here from nonlocal finite size effects;
it also contrasts with the observations of B\'ecu et al. (2006)
Phenomenology and physical origin of shear-localization and shear-banding in complex fluids
We review and compare the phenomenological aspects and physical origin of
shear-localization and shear-banding in various material types, namely
emulsions, suspensions, colloids, granular materials and micellar systems. It
appears that shear-banding, which must be distinguished from the simple effect
of coexisting static-flowing regions in yield stress fluids, occurs in the form
of a progressive evolution of the local viscosity towards two significantly
different values in two adjoining regions of the fluids in which the stress
takes slightly different values. This suggests that from a global point of view
shear-banding in these systems has a common physical origin: two physical
phenomena (for example, in colloids, destructuration due to flow and
restructuration due to aging) are in competition and, depending on the flow
conditions, one of them becomes dominant and makes the system evolve in a
specific direction.Comment: The original publication is available at http://www.springerlink.co
Recent experimental probes of shear banding
Recent experimental techniques used to investigate shear banding are
reviewed. After recalling the rheological signature of shear-banded flows, we
summarize the various tools for measuring locally the microstructure and the
velocity field under shear. Local velocity measurements using dynamic light
scattering and ultrasound are emphasized. A few results are extracted from
current works to illustrate open questions and directions for future research.Comment: Review paper, 23 pages, 11 figures, 204 reference