336 research outputs found
Elastic turbulence in shear banding wormlike micelles
We study the dynamics of the Taylor-Couette flow of shear banding wormlike
micelles. We focus on the high shear rate branch of the flow curve and show
that for sufficiently high Weissenberg numbers, this branch becomes unstable.
This instability is strongly sub-critical and is associated with a shear stress
jump. We find that this increase of the flow resistance is related to the
nucleation of turbulence. The flow pattern shows similarities with the elastic
turbulence, so far only observed for polymer solutions. The unstable character
of this branch led us to propose a scenario that could account for the recent
observations of Taylor-like vortices during the shear banding flow of wormlike
micelles
Interface instability in shear banding flow
We report on the spatio-temporal dynamics of the interface in shear-banding
flow of a wormlike micellar system (cetyltrimethylammonium bromide and sodium
nitrate in water) during a start-up experiment. Using the scattering properties
of the induced structures, we demonstrate the existence of an instability of
the interface between bands along the vorticity direction. Different regimes of
spatio-temporal dynamics of the interface are indentified along the stress
plateau. We build a model based on the flow symetry which qualitatively
describes the observed patterns
Time scales in shear banding of wormlike micelles
Transient stress and birefringence measurements are performed on wormlike micellar solutions that "shear band", i.e. undergo flow-induced coexistence of states of different viscosities along a constant stress "plateau". Three well-defined relaxation times are found after a strain rate step between two banded flow states on the stress plateau. Using the Johnson-Segalman model, we relate these time scales to three qualitatively different stages in the evolution of the bands and the interface between them: band destabilization, reconstruction of the interface, and travel of the fully formed interface. The longest timescale is then used to estimate the magnitude of the (unknown) "gradient" terms that must be added to constitutive relations to explain the history independence of the steady flow and the plateau stress selection
Chitosan-based hydrogels supplemented with gelatine and Link N enhance extracellular matrix deposition by encapsulated cells in a degenerative intervertebral disc environment
Injectable therapies for intervertebral disc (IVD) repair are gaining much interest. Recently, a chitosan (CH)-based injectable scaffold has been developed that has similar mechanical properties to human nucleus pulposus (NP) and provides a suitable environment for encapsulated NP cell survival and proteoglycan production. The hypothesis of the study was that the biological response of the encapsulated cells can be further increased by adding gelatine and Link N (LN, a naturally occurring peptide present in cartilage and IVD extracellular matrix), known to increase cell adhesion and proteoglycan production, respectively. The effect of gelatine on the mechanical properties of a CH hydrogel was evaluated through rheological and compressive mechanical tests. Production of proteoglycan [assessed as glycosaminoglycan (GAG)] by encapsulated NP cells was determined in the presence or absence of gelatine in normal or degenerative medium supplemented with LN. Normal and degenerative media replicate the healthy and degenerative disc environment, respectively. Gelatine slightly reduced the gelation rate of CH hydrogel but improved its final mechanical properties in compression. LN had a minimal effect in normal medium but induced significantly more GAG production in degenerative medium (p < 0.001, 4.7-fold superior to the control), reaching similar results to transforming growth factor (TGF)-β (used as a positive control). GAG production was further increased in CH-gelatine hydrogels, confirming an additive effect of LN and gelatine in a degenerative environment. The results supported the concept that CH-gelatine hydrogels supplemented with LN can help restore the function of the NP during the early stages of IVD degeneration
Simulating Plasmon Resonances of Gold Nanoparticles with Bipyramidal Shapes by Boundary Element Methods
Computational modeling and accurate simulations of localized surface plasmon resonance (LSPR) absorption properties are reported for gold nanobipyramids (GNBs), a class of metal nanoparticle that features highly tunable, geometry-dependent optical properties. GNB bicone models with spherical tips performed best in reproducing experimental LSPR spectra while the comparison with other geometrical models provided a fundamental understanding of base shapes and tip effects on the optical properties of GNBs. Our results demonstrated the importance of averaging all geometrical parameters determined from transmission electron microscopy images to build representative models of GNBs. By assessing the performances of LSPR absorption spectra simulations based on a quasi-static approximation, we provided an applicability range of this approach as a function of the nanoparticle size, paving the way to the theoretical study of the coupling between molecular electron densities and metal nanoparticles in GNB-based nanohybrid systems, with potential applications in the design of nanomaterials for bioimaging, optics and photocatalysis
An elasto-visco-plastic model for immortal foams or emulsions
A variety of complex fluids consist in soft, round objects (foams, emulsions,
assemblies of copolymer micelles or of multilamellar vesicles -- also known as
onions). Their dense packing induces a slight deviation from their prefered
circular or spherical shape. As a frustrated assembly of interacting bodies,
such a material evolves from one conformation to another through a succession
of discrete, topological events driven by finite external forces. As a result,
the material exhibits a finite yield threshold. The individual objects usually
evolve spontaneously (colloidal diffusion, object coalescence, molecular
diffusion), and the material properties under low or vanishing stress may alter
with time, a phenomenon known as aging. We neglect such effects to address the
simpler behaviour of (uncommon) immortal fluids: we construct a minimal, fully
tensorial, rheological model, equivalent to the (scalar) Bingham model.
Importantly, the model consistently describes the ability of such soft
materials to deform substantially in the elastic regime (be it compressible or
not) before they undergo (incompressible) plastic creep -- or viscous flow
under even higher stresses.Comment: 69 pages, 29 figure
Taylor-like vortices in the shear-banding flow of giant micelles
Using flow visualizations in Couette geometry, we demonstrate the existence
of Taylor-like vortices in the shear-banding flow of a giant micelles system.
We show that vortices stacked along the vorticity direction develop
concomitantly with interfacial undulations. These cellular structures are
mainly localized in the induced band and their dynamics is fully correlated
with that of the interface. As the control parameter increases, we observe a
transition from a steady vortex flow to a state where pairs of vortices are
continuously created and destroyed. Normal stress effects are discussed as
potential mechanisms driving the three-dimensional flow.Comment: 5 pages, 4 figure
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]
Unstable Flow and Non-Monotonic Constitutive Equation of Transient Networks
We have measured the nonlinear rheological response of a model transient
network over a large range of steady shear rates. The system is built up from
an oil in water droplet microemulsion into which a telechelic polymer is
incorporated. The phase behaviour is characterized which comprises a liquid-gas
phase separation and a percolation threshold. The rheological measurements are
performed in the one phase region above the percolation line. Shear thinning is
observed for all samples, leading in most cases to an unstable stress response
at intermediate shear rates. We built up a very simple mean field model which
involves the reduction of the residence time of the stickers in the droplets
due to the chain tensions at high shear. The computed constitutive equation is
non-monotonic with a range where the stress is a decreasing function of the
rate, a feature that indeed makes homogeneous flows unstable. The computed the
flow curves compare well to the experiments.Comment: mai 200
The synthesis of the rhamnogalacturonan II component 3-deoxy-D-manno-2-octulosonic acid (Kdo) is required for pollen tube growth and elongation
Despite a very complex structure, the sugar composition of the rhamnogalacturonan II (RG-II) pectic fraction is extremely conserved. Among its constituting monosaccharides is the seldom-observed eight-carbon sugar 3-deoxy-D-manno-octulosonic acid (Kdo), whose phosphorylated precursor is synthesized by Kdo-8-P synthase. As an attempt to alter specifically the RG-II structure in its sugar composition and assess the consequences on the function of RG-II in cell wall and its relationship with growth, Arabidopsis null mutants were sought in the genes encoding Kdo-8-P synthase. Here, the isolation and characterization of one null mutant for the isoform 1 (AtkdsA1-S) and two distinct null mutants for the isoform 2 of Arabidopsis Kdo-8-P synthase (AtkdsA2-V and AtkdsA2-S) are described. Evidence is provided that AtkdsA2 gene expression is preferentially associated with plantlet organs displaying a meristematic activity, and that it accounts for 75% of the mRNAs to be translated into Kdo-8-P synthase. Furthermore, this predominant expression of AtKDSA2 over AtKDSA1 was confirmed by quantification of the cytosolic Kdo content in the mutants, in a variety of ecotypes. The inability to identify a double knockout mutant originated from pollen abortions, due to the inability of haploid pollen of the AtkdsA1- AtkdsA2- genotype to form an elongated pollen tube properly and perform fertilization
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