112 research outputs found
Competition between shear banding and wall slip in wormlike micelles
The interplay between shear band (SB) formation and boundary conditions (BC)
is investigated in wormlike micellar systems (CPyCl--NaSal) using ultrasonic
velocimetry coupled to standard rheology in Couette geometry. Time-resolved
velocity profiles are recorded during transient strain-controlled experiments
in smooth and sand-blasted geometries. For stick BC standard SB is observed,
although depending on the degree of micellar entanglement temporal fluctuations
are reported in the highly sheared band. For slip BC wall slip occurs only for
shear rates larger than the start of the stress plateau. At low entanglement,
SB formation is shifted by a constant , while for more
entangled systems SB constantly "nucleate and melt." Micellar orientation
gradients at the walls may account for these original features.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
The yielding dynamics of a colloidal gel
Attractive colloidal gels display a solid-to-fluid transition as shear
stresses above the yield stress are applied. This shear-induced transition is
involved in virtually any application of colloidal gels. It is also crucial for
controlling material properties. Still, in spite of its ubiquity, the yielding
transition is far from understood, mainly because rheological measurements are
spatially averaged over the whole sample. Here, the instrumentation of creep
and oscillatory shear experiments with high-frequency ultrasound opens new
routes to observing the local dynamics of opaque attractive colloidal gels. The
transition proceeds from the cell walls and heterogeneously fluidizes the whole
sample with a characteristic time whose variations with applied stress suggest
the existence of an energy barrier linked to the gelation process. The present
results provide new test grounds for computer simulations and theoretical
calculations in the attempt to better understand the yielding transition. The
versatility of the technique should also allow extensive mesoscopic studies of
rupture mechanisms in soft solids ranging from crystals to glassy materials.Comment: 8 pages, 5 figure
Shear-induced fragmentation of Laponite suspensions
Simultaneous rheological and velocity profile measurements are performed in a
smooth Couette geometry on Laponite suspensions seeded with glass microspheres
and undergoing the shear-induced solid-to-fluid (or yielding) transition. Under
these slippery boundary conditions, a rich temporal behaviour is uncovered, in
which shear localization is observed at short times, that rapidly gives way to
a highly heterogeneous flow characterized by intermittent switching from
plug-like flow to linear velocity profiles. Such a temporal behaviour is linked
to the fragmentation of the initially solid sample into blocks separated by
fluidized regions. These solid pieces get progressively eroded over time scales
ranging from a few minutes to several hours depending on the applied shear rate
. The steady-state is characterized by a homogeneous flow with
almost negligible wall slip. The characteristic time scale for erosion is shown
to diverge below some critical shear rate and to scale as
with above
. A tentative model for erosion is discussed together with
open questions raised by the present results.Comment: 19 pages, 13 figures, submitted to Soft Matte
Wall slip across the jamming transition of soft thermoresponsive particles
Flows of suspensions are often affected by wall slip, that is the fluid
velocity in the vicinity of a boundary differs from the wall velocity
due to the presence of a lubrication layer. While the slip velocity
robustly scales linearly with the stress
at the wall in dilute suspensions, there is no consensus regarding denser
suspensions that are sheared in the bulk, for which slip velocities have been
reported to scale as a with exponents inconsistently
ranging between 0 and 2. Here we focus on a suspension of soft thermoresponsive
particles and show that actually scales as a power law of the viscous
stress , where denotes the yield stress of the bulk
material. By tuning the temperature across the jamming transition, we further
demonstrate that this scaling holds true over a large range of packing
fractions on both sides of the jamming point and that the exponent
increases continuously with , from in the case of dilute
suspensions to for jammed assemblies. These results allow us to
successfully revisit inconsistent data from the literature and paves the way
for a continuous description of wall slip above and below jamming.Comment: 6 pages, 4 figures - accepted for publication as a Rapid
Communication in Phys. Rev.
Comment on ``Large Slip of Aqueous Liquid Flow over a Nanoengineered Superhydrophobic Surface'' by C-H Choi and C Kim
In a recent Letter (Phys. Rev. Lett. vol 96, 066001 (2006), ref [1]), Choi
and Kim reported slip lengths of a few tens of microns for water on
nanoengineered superhydrophobic surfaces, on the basis of rheometry
(cone-and-plate) measurements. We show that the experimental uncertainty in the
experiment of Ref. [1], expressed in term of slip lengths, lies in the range 20
- 100 micrometers, which is precisely the order of magnitude of the reported
slip lengths. Moreover we point out a systematic bias expected on the
superhydrophobic surfaces. We thus infer that it is not possible to draw out
any conclusion concerning the existence of huge slip lengths in the system
studied by Choi and Kim.Comment: to appear in Physical Review Letter
Transient Shear Banding in a Simple Yield Stress Fluid
We report a large set of experimental data which demonstrates that a simple
yield stress fluid, i.e. which does not present aging or thixotropy, exhibits
transient shear banding before reaching a steady state characterized by a
homogeneous, linear velocity profile. The duration of the transient regime
decreases as a power law with the applied shear rate . This power
law behavior, observed here in carbopol dispersions, does not depend on the gap
width and on the boundary conditions for a given sample preparation. For
s, heterogeneous flows could be observed for as
long as 10 s. These local dynamics account for the ultraslow stress
relaxation observed at low shear rates.Comment: 4 pages, 4 figure
Local Oscillatory Rheology from Echography
Local Oscillatory Rheology from Echography (LORE) consists in a traditional
rheology experiment synchronized with high-frequency ultrasonic imaging which
gives access to the local material response to oscillatory shear. Besides
classical global rheological quantities, this method provides quantitative
time-resolved information on the local displacement across the entire gap of
the rheometer. From the local displacement response, we compute and decompose
the local strain in its Fourier components and measure the spatially-resolved
viscoelastic moduli. After benchmarking our method on homogeneous Newtonian
fluids and soft solids, we demonstrate that this technique is well suited to
characterize spatially heterogeneous samples, wall slip, and the emergence of
nonlinearity under large amplitude oscillatory stress in soft materials.Comment: 10 pages, 5 figures, submitted to Phys. Rev. Applie
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