Ref HAL: hal-01110034_v1
PMID 25060535
DOI: 10.1039/c4sm01095g
WoS: 000341025500010
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29 Citations
Résumé:

The formation of aggregates in simplified industrial styrene–butadiene nanocomposites with silica filler hasbeen studied using a recent model based on a combination of electron microscopy, computer simulations,and small-angle X-ray scattering. The influence of the chain mass (40 to 280 kg/mol, PI < 1.1), which setsthe linear rheology of the samples, was investigated for a low (9.5 vol%) and high (19 vol%) silica volumefraction. 50% of the chains bear a single graftable end-group, and it is shown that the (chain-massdependent) grafting density is the structure-determining parameter. A model unifying all available dataon this system is proposed and used to determine a critical aggregate grafting density. The latter is foundto be closely related to the mushroom-to-brush transition of the grafted layer. To our best knowledge,this is the first comprehensive evidence for the control of the complex nanoparticle aggregate structurein nanocomposites of industrial relevance by the physical parameters of the grafted layer.

Ref HAL: hal-01104008_v1
Ref Arxiv: 1412.3930
DOI: 10.1017/jfm.2014.714
WoS: 000348128700022
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32 Citations
Résumé:

The collision of a liquid drop against a small target results in the formation of a thin liquid sheet that extends radially until it reaches a maximum diameter. The subsequent retraction is due to the air-liquid surface tension. We have used a time- and space-resolved technique to measure the thickness field of this class of liquid sheet, based on the grey level measurement of the image of a dyed liquid sheet recorded using a fast camera. This method enables a precise measurement of the thickness in the range $(10-450) \, \mathrm{\mu m}$, with a temporal resolution equal to that of the camera. We have measured the evolution with time since impact, $t$, and radial position, $r$, of the thickness, $h(r,t)$, for various drop volumes and impact velocities. Two asymptotic regimes for the expansion of the sheet are evidenced. The scalings of the thickness with $t$ and $r$ measured in the two regimes are those that were predicted in \citet{Rozhkov2004} fort the short-time regime and \citet{Villermaux2011} for the long time regime, but never experimentally measured before. Interestingly, our experimental data also evidence the existence of a maximum of the film thickness $h_{\rm{max}}(r)$ at a radial position $r_{\rm{h_{max}}}(t)$ corresponding to the crossover of these two asymptotic regimes. The maximum moves with a constant velocity of the order of the drop impact velocity, as expected theoretically. Thanks to our visualization technique, we also evidence an azimuthal thickness modulation of the liquid sheets.

Commentaires: accepted for publication in Journal of Fluid Mechanics. Réf Journal: J. Fluid Mech. (2015), vol. 764, pp. 428-444

Ref HAL: hal-01071076_v1
Ref Arxiv: 1409.0744
DOI: 10.1021/jp5047134
WoS: 000342396000005
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34 Citations
Résumé:

The supramolecular organization of wheat gluten proteins is largely unknown due to the intrinsic complexity of this family of proteins and their insolubility in water. We fractionate gluten in a water/ethanol (50/50 v/v) and obtain a protein extract which is depleted in gliadin, the monomeric part of wheat gluten proteins, and enriched in glutenin, the polymeric part of wheat gluten proteins. We investigate the structure of the proteins in the solvent used for extraction over a wide range of concentration, by combining X-ray scattering and multi-angle static and dynamic light scattering. Our data show that, in the ethanol/water mixture, the proteins display features characteristic of flexible polymer chains in a good solvent. In the dilute regime, the protein form very loose structures of characteristic size 150 nm, with an internal dynamics which is quantitatively similar to that of branched polymer coils. In more concentrated regimes, data highlight a hierarchical structure with one characteristic length scale of the order of a few nm, which displays the scaling with concentration expected for a semi-dilute polymer in good solvent, and a fractal arrangement at much larger length scale. This structure is strikingly similar to that of polymeric gels, thus providing some factual knowledge to rationalize the viscoelastic properties of wheat gluten proteins and their assemblies.

Commentaires: J. Phys. Chem. B 2014, 118, 11065

Ref HAL: hal-01064084_v1
PMID 24910076
DOI: 10.1039/c4sm00482e
WoS: 000341025700014
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17 Citations
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Here multiple angle of incidence ellipsometry was successfully applied to in situ assess the contact angle and surface coverage of gold nanoparticles as small as 18 nm, coated with stimuli-responsive polymers, at water-oil and water-air interfaces in the presence of NaCl and NaOH, respectively. The interfacial adsorption of the nanoparticles was found very slow and took days to reach a fairly low surface coverage. For water-oil interfaces, in-situ nanoparticle contact angles agree with the macroscopic equilibrium contact angles of planar gold surfaces with the same polymer coatings; whilst for water-air interfaces, significant differences have been observed.

Ref HAL: hal-01058961_v1
DOI: 10.1122/1.4866592
WoS: 000342206200022
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13 Citations
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Upon addition of small nonadsorbing linear polymers, colloidal glasses composed of large hard spheres melt and eventually revitrify into the so-called attractive glass regime. We show that, when replacing the hard spheres by star polymers representing model soft particles, a reentrant gel is formed. This is the result of compression and depletion of the stars due to the action of the osmotic pressure from the linear homopolymers. The viscoelastic properties of the soft dense gel were studied with emphasis on the shear-induced yielding process, which involved localized breaking of elements with a size of the order of the correlation length. Based on these results, a phenomenological attempt was made at describing the universal rheological features of colloid/nonadsorbing polymer mixtures of varying softness. The star gel was found to undergo thermoreversible melting, despite the fact that conventional hard-sphere depletion gels are invariant to heating. This phenomenon is attributed to the hybrid internal microstructure of the stars, akin to a dry-to-wet brush transition, and is characterized by slow kinetics, on the time scale of the osmotic gel formation process. These results may be useful in finding generic features in colloidal gelation, as well as in the molecular design of new soft composite materials.

Ref HAL: hal-01058850_v1
Ref Arxiv: 1311.1996
DOI: 10.1103/PhysRevLett.113.078301
WoS: 000341115700027
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25 Citations
Résumé:

We use confocal microscopy and time-resolved light scattering to investigate plasticity in a colloidal polycrystal, following the evolution of the network of grain boundaries as the sample is submitted to thousands of shear deformation cycles. The grain boundary motion is found to be ballistic, with a velocity distribution function exhibiting non-trivial power law tails. The shear- induced dynamics initially slow down, similarly to the aging of the spontaneous dynamics in glassy materials, but eventually reach a steady state. Surprisingly, the cross-over time between the ini- tial aging regime and the steady state decreases with increasing probed length scale, hinting at a hierarchical organization of the grain boundary dynamics.

Commentaires: main paper + supplementary materials Journal: Phys. Rev. Lett., 113, 078301 (2014)

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Ref Arxiv: 1408.6085
DOI: 10.1021/ac501115y
WoS: 000338488800043
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23 Citations
Résumé:

Taylor dispersion analysis is an increasingly popular characterization method that measures the diffusion coefficient, and hence the hydrodynamic radius, of (bio)polymers, nanoparticles or even small molecules. In this work, we describe an extension to current data analysis schemes that allows size polydispersity to be quantified for an arbitrary sample, thereby significantly enhancing the potentiality of Taylor dispersion analysis. The method is based on a cumulant development similar to that used for the analysis of dynamic light scattering data. Specific challenges posed by the cumulant analysis of Taylor dispersion data are discussed, and practical ways to address them are proposed. We successfully test this new method by analyzing both simulated and experimental data for solutions of moderately polydisperse polymers and polymer mixtures.

Commentaires: 41 pages (including Supporting Information), 5 figures (+ 7 figs. in the Supporting Information)