51 research outputs found
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)
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
Couette Flow of Two-Dimensional Foams
We experimentally investigate flow of quasi two-dimensional disordered foams
in Couette geometries, both for foams squeezed below a top plate and for freely
floating foams. With the top-plate, the flows are strongly localized and rate
dependent. For the freely floating foams the flow profiles become essentially
rate-independent, the local and global rheology do not match, and in particular
the foam flows in regions where the stress is below the global yield stress. We
attribute this to nonlocal effects and show that the "fluidity" model recently
introduced by Goyon {\em et al.} ({\em Nature}, {\bf 454} (2008)) captures the
essential features of flow both with and without a top plate.Comment: 6 pages, 5 figures, revised versio
Some Applications of Magnetic Resonance Imaging in Fluid Mechanics: Complex Flows and Complex Fluids
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
Next-generation care pathways for allergic rhinitis and asthma multimorbidity: A model for multimorbid non-communicable diseases—Meeting Report (Part 2)
15-epi-lipoxin A4 reduces the mortality of prematurely born pups in a mouse model of infection-induced preterm birth
Preterm birth remains the leading cause of neonatal mortality and morbidity worldwide. There are currently few effective therapies and therefore an urgent need for novel treatments. Although there is much focus on trying to alter gestation of delivery, the primary aim of preterm birth prevention therapies should be to reduce prematurity related mortality and morbidity. Given the link between intrauterine infection and inflammation and preterm labour (PTL), we hypothesized that administration of lipoxins, key anti-inflammatory and pro-resolution mediators, could be a useful novel treatment for PTL. Using a mouse model of infection-induced PTL, we investigated whether 15-epi-lipoxin A(4) could delay lipopolysaccharide (LPS)-induced PTL and reduce pup mortality. On D17 of gestation mice (n = 9–12) were pretreated with vehicle or 15-epi-lipoxin A(4) prior to intrauterine administration of LPS or PBS. Although pretreatment with 15-epi-lipoxin A(4) did not delay LPS-induced PTL, there was a significant reduction in the mortality amongst prematurely delivered pups (defined as delivery within 36 h of surgery) in mice treated with 15-epi-lipoxin A(4) prior to LPS treatment, compared with those receiving LPS alone (P < 0.05). Quantitative real-time (QRT)-PCR analysis of utero-placental tissues harvested 6 h post-treatment demonstrated that 15-epi-lipoxin A(4) treatment increased Ptgs2 expression in the uterus, placenta and fetal membranes (P < 0.05) and decreased 15-Hpgd expression (P < 0.05) in the placenta and uterus, suggesting that 15-epi-lipoxin A(4) may regulate the local production and activity of prostaglandins. These data suggest that augmenting lipoxin levels could be a useful novel therapeutic option in the treatment of PTL, protecting the fetus from the adverse effects of infection-induced preterm birth
Boundary layer (shear-band) in frustrated viscoplastic flows
We show that frustrated creep flows of yield stress fluids give rise to a boundary layer, which takes the form of a liquid region of uniform significant thickness separating two solid regions. In this boundary layer the shear rate is approximately constant for a given flow rate and the layer thickness varies extremely slowly with the flow rate
Some applications of magnetic resonance imaging in fluid mechanics: Complex flows and complex fluids
International audienceThe review deals with applications of magnetic resonance imaging (MRI) techniques to study flow. We first briefly discuss the principles of flow measurement by MRI and give examples of some applications, such as multiphase flows, the MRI rheology of complex fluid flows, and blood flows in the human body
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