119 research outputs found
Influence of boundary conditions on yielding in a soft glassy material
The yielding behavior of a sheared Laponite suspension is investigated within
a 1 mm gap under two different boundary conditions. No-slip conditions, ensured
by using rough walls, lead to shear localization as already reported in various
soft glassy materials. When apparent wall slip is allowed using a smooth
geometry, the sample is shown to break up into macroscopic solid pieces that
get slowly eroded by the surrounding fluidized material up to the point where
the whole sample is fluid. Such a drastic effect of boundary conditions on
yielding suggests the existence of some macroscopic characteristic length that
could be connected to cooperativity effects in jammed materials under shear.Comment: 4 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
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
Yielding dynamics of a Herschel-Bulkley fluid: a critical-like fluidization behaviour
The shear-induced fluidization of a carbopol microgel is investigated during
long start-up experiments using combined rheology and velocimetry in Couette
cells of varying gap widths and boundary conditions. As already described in
[Divoux et al., {\it Phys. Rev. Lett.}, 2010, {\bf 104}, 208301], we show that
the fluidization process of this simple yield stress fluid involves a transient
shear-banding regime whose duration decreases as a power law of the
applied shear rate \gp. Here we go one step further by an exhaustive
investigation of the influence of the shearing geometry through the gap width
and the boundary conditions. While slip conditions at the walls seem to
have a negligible influence on the fluidization time , different
fluidization processes are observed depending on \gp and : the shear band
remains almost stationary for several hours at low shear rates or small gap
widths before strong fluctuations lead to a homogeneous flow, whereas at larger
values of \gp or , the transient shear band is seen to invade the whole
gap in a much smoother way. Still, the power-law behaviour appears as very
robust and hints to critical-like dynamics. To further discuss these results,
we propose (i) a qualitative scenario to explain the induction-like period that
precedes full fluidization and (ii) an analogy with critical phenomena that
naturally leads to the observed power laws if one assumes that the yield point
is the critical point of an underlying out-of-equilibrium phase transition.Comment: 16 pages, 14+2 figures, published in Soft Matte
Yield stress and elasticity influence on surface tension measurements
We have performed surface tension measurements on carbopol gels of different
concentrations and yield stresses. Our setup, based on the force exerted by a
capillary bridge on two parallel plates, allows to measure an effective surface
tension of the complex fluid and to investigate the influence of flow history.
More precisely the effective surface tension measured after stretching the
bridge is always higher than after compressing it. The difference between the
two values is due to the existence of a yield stress in the fluid. The
experimental observations are successfully reproduced with a simple
elasto-plastic model. The shape of successive stretching-compression cycles can
be described by taking into account the yield stress and the elasticity of the
gel. We show that the surface tension of yield stress fluids is
the mean of the effective surface tension values only if the elastic modulus is
high compared to the yield stress. This work highlights that thermodynamical
quantities measurements are challenged by the fluid out-of-equilibrium state
implied by jamming, even at small scales where the shape of the bridge is
driven by surface energy. Therefore setups allowing deformation in opposite
directions are relevant for measurements on yield stress fluids.Comment: 12 pages, 16 figures in Soft Matter 201
Scaling laws for slippage on superhydrophobic fractal surfaces
We study the slippage on hierarchical fractal superhydrophobic surfaces, and
find an unexpected rich behavior for hydrodynamic friction on these surfaces.
We develop a scaling law approach for the effective slip length, which is
validated by numerical resolution of the hydrodynamic equations. Our results
demonstrate that slippage does strongly depend on the fractal dimension, and is
found to be always smaller on fractal surfaces as compared to surfaces with
regular patterns. This shows that in contrast to naive expectations, the value
of effective contact angle is not sufficient to infer the amount of slippage on
a fractal surface: depending on the underlying geometry of the roughness,
strongly superhydrophobic surfaces may in some cases be fully inefficient in
terms of drag reduction. Finally, our scaling analysis can be directly extended
to the study of heat transfer at fractal surfaces, in order to estimate the
Kapitsa surface resistance on patterned surfaces, as well as to the question of
trapping of diffusing particles by patchy hierarchical surfaces, in the context
of chemoreception
Wall slip regimes in jammed suspensions of soft microgels
International audienceWe characterize microfluidic flows of jammed suspensions of soft microgels (Carbopol) behaving as yield-stress fluids. We quantify the wall slip friction, i.e. the slip velocity V versus the tangential stress at the wall σw. We demonstrate a transition in slip regimes, from a non-linear behavior (V ∝ σ 2 w) to a linear one, as the stress at the wall is increased, as expected from scaling arguments. Using fluorescent imaging to characterize the microgel size, we rationalize the two friction regimes for various samples by estimating viscous and elastic forces at the scale of the microgel particle. Only local arguments are thus necessary to predict wall slip friction, in contrast to other complex flow features such as fluidity or shear banding where bulk and surface properties appear to be strongly related
Descriminalização: uma politica criminal alternativa
Dissertação (mestrado - Universidade Federal de Santa Catarina, Centro de Ciencias JuridicasAnálise da política criminal alternativa descriminalizante e suas conseqüências. Ressalta-se a necessidade de tornar mais eficientes e adequadas as normas jurídico-penais para uma conseqüente diminuição da violência punitiva. Violência esta resultante da operacionalidade dos sistemas penais latinoamericanos
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