137 research outputs found
The Johnson-Segalman model with a diffusion term in Couette flow
We study the Johnson-Segalman (JS) model as a paradigm for some complex
fluids which are observed to phase separate, or ``shear-band'' in flow. We
analyze the behavior of this model in cylindrical Couette flow and demonstrate
the history dependence inherent in the local JS model. We add a simple gradient
term to the stress dynamics and demonstrate how this term breaks the degeneracy
of the local model and prescribes a much smaller (discrete, rather than
continuous) set of banded steady state solutions. We investigate some of the
effects of the curvature of Couette flow on the observable steady state
behavior and kinetics, and discuss some of the implications for metastability.Comment: 14 pp, to be published in Journal of Rheolog
High-Angular-Resolution Camera Coupled with an Undulator Source at the European Synchrotron Radiation Facility High-Brilliance Beamline
Shear-banding in a lyotropic lamellar phase, Part 2: Temporal fluctuations
We analyze the temporal fluctuations of the flow field associated to a
shear-induced transition in a lyotropic lamellar phase: the layering transition
of the onion texture. In the first part of this work [Salmon et al., submitted
to Phys. Rev. E], we have evidenced banded flows at the onset of this
shear-induced transition which are well accounted for by the classical picture
of shear-banding. In the present paper, we focus on the temporal fluctuations
of the flow field recorded in the coexistence domain. These striking dynamics
are very slow (100--1000s) and cannot be due to external mechanical noise.
Using velocimetry coupled to structural measurements, we show that these
fluctuations are due to a motion of the interface separating the two
differently sheared bands. Such a motion seems to be governed by the
fluctuations of , the local stress at the interface between the
two bands. Our results thus provide more evidence for the relevance of the
classical mechanical approach of shear-banding even if the mechanism leading to
the fluctuations of remains unclear
Shear-banding in a lyotropic lamellar phase, Part 1: Time-averaged velocity profiles
Using velocity profile measurements based on dynamic light scattering and
coupled to structural and rheological measurements in a Couette cell, we
present evidences for a shear-banding scenario in the shear flow of the onion
texture of a lyotropic lamellar phase. Time-averaged measurements clearly show
the presence of structural shear-banding in the vicinity of a shear-induced
transition, associated to the nucleation and growth of a highly sheared band in
the flow. Our experiments also reveal the presence of slip at the walls of the
Couette cell. Using a simple mechanical approach, we demonstrate that our data
confirms the classical assumption of the shear-banding picture, in which the
interface between bands lies at a given stress . We also outline
the presence of large temporal fluctuations of the flow field, which are the
subject of the second part of this paper [Salmon {\it et al.}, submitted to
Phys. Rev. E]
Rheology of Lamellar Liquid Crystals in Two and Three Dimensions: A Simulation Study
We present large scale computer simulations of the nonlinear bulk rheology of
lamellar phases (smectic liquid crystals) at moderate to large values of the
shear rate (Peclet numbers 10-100), in both two and three dimensions. In two
dimensions we find that modest shear rates align the system and stabilise an
almost regular lamellar phase, but high shear rates induce the nucleation and
proliferation of defects, which in steady state is balanced by the annihilation
of defects of opposite sign. The critical shear rate at onset of this second
regime is controlled by thermodynamic and kinetic parameters; we offer a
scaling analysis that relates the critical shear rate to a critical "capillary
number" involving those variables. Within the defect proliferation regime, the
defects may be partially annealed by slowly decreasing the applied shear rate;
this causes marked memory effects, and history-dependent rheology. Simulations
in three dimensions show instead shear-induced ordering even at the highest
shear rates studied here. This suggests that the critical shear rate shifts
markedly upward on increasing dimensionality. This may in part reflect the
reduced constraints on defect motion, allowing them to find and annihilate each
other more easily. Residual edge defects in the 3D aligned state mostly point
along the flow velocity, an orientation impossible in two dimensions.Comment: 18 pages, 12 figure
Phase Separation of Rigid-Rod Suspensions in Shear Flow
We analyze the behavior of a suspension of rigid rod-like particles in shear
flow using a modified version of the Doi model, and construct diagrams for
phase coexistence under conditions of constant imposed stress and constant
imposed strain rate, among paranematic, flow-aligning nematic, and log-rolling
nematic states. We calculate the effective constitutive relations that would be
measured through the regime of phase separation into shear bands. We calculate
phase coexistence by examining the stability of interfacial steady states and
find a wide range of possible ``phase'' behaviors.Comment: 23 pages 19 figures, revised version to be published in Physical
Review
Structure of nanoparticles embedded in micellar polycrystals
We investigate by scattering techniques the structure of water-based soft
composite materials comprising a crystal made of Pluronic block-copolymer
micelles arranged in a face-centered cubic lattice and a small amount (at most
2% by volume) of silica nanoparticles, of size comparable to that of the
micelles. The copolymer is thermosensitive: it is hydrophilic and fully
dissolved in water at low temperature (T ~ 0{\deg}C), and self-assembles into
micelles at room temperature, where the block-copolymer is amphiphilic. We use
contrast matching small-angle neuron scattering experiments to probe
independently the structure of the nanoparticles and that of the polymer. We
find that the nanoparticles do not perturb the crystalline order. In addition,
a structure peak is measured for the silica nanoparticles dispersed in the
polycrystalline samples. This implies that the samples are spatially
heterogeneous and comprise, without macroscopic phase separation, silica-poor
and silica-rich regions. We show that the nanoparticle concentration in the
silica-rich regions is about tenfold the average concentration. These regions
are grain boundaries between crystallites, where nanoparticles concentrate, as
shown by static light scattering and by light microscopy imaging of the
samples. We show that the temperature rate at which the sample is prepared
strongly influence the segregation of the nanoparticles in the
grain-boundaries.Comment: accepted for publication in Langmui
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