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Food transition requires the replacement in human diet of animal-based proteins by alternative sources of proteins including plant-based proteins. This calls for a detailed knowledge of the functional properties of plant-based proteins, including their surface activity. In this framework, we provide here a comparative study of the interfacial properties of two plant proteins, extracted respectively from wheat and sunflower. We combine time- and concentration-dependent measurements of the surface tension and the surface rheology, as measured with a pendant-drop set-up, and of the surface excess concentration, as measured by ellipsometry, of plant protein interfacial films. We demonstrate a time-concentration superposition principle for the surface pressure and surface excess concentration, showing that the kinetics for the building of the interfacial films is essentially governed by the diffusion of the proteins from the bulk to the interface. We find that the rheological and structural properties of the interfacial protein films show markedly different behaviors for the two classes of protein, which is encoded in the structural features of the individual proteins: wheat proteins are more surface active than sunflower proteins, are keen to compress and re-arrange at an air-water interface, whereas sunflower proteins do not. This work provides qualitative and quantitative analysis of the comparative interfacial behavior of flexible and rigid plant proteins extracted respectively from wheat and sunflower, and demonstrates that a combination of several experimental techniques is necessary to obtain insightful information on the interfacial properties of any species.

Understanding the origin of the unique rheological properties of wheat gluten, the protein fraction of wheat grain, is crucial in bread-making processes and has raised questions of scientists for decades. Gluten is a complex mixture of two families of proteins, monomeric gliadins and polymeric glutenins. To better understand the respective role of the different classes of proteins in the supramolecular structure of gluten and its link to the material properties, we investigate here concentrated dispersions of gluten proteins in water with a fixed total protein concentration but variable composition in gliadin and glutenin. Linear viscoelasticity measurements show a gradual increase in the viscosity of the samples as the glutenin mass content increases from 7 to 66%. While the gliadin-rich samples are microphase-separated viscous fluids, homogeneous and transparent pre-gel and gels are obtained with the replacement of gliadin by glutenin. To unravel the flow properties of the gluten samples, we perform shear startup experiments at different shear-rates. In accordance with the linear viscoelastic signature, three classes of behavior are evidenced depending on the protein composition. As samples get depleted in gliadin and enriched in glutenin, distinctive features are measured: (i) viscosity undershoot suggesting droplet elongation for microphase-separated dispersions, (ii) stress overshoot and partial structural relaxation for near-critical pre-gels, and (iii) strain hardening and flow instabilities of gels. We discuss the experimental results by analogy with the behavior of model systems, including viscoelastic emulsions, branched polymer melts, and critical gels, and provide a consistent physical picture of the supramolecular features of the three classes of protein dispersions.

International audience; Wheat gluten includes two major proteins classes, gliadin (25–60 kg/mol) and glutenin polymers (100 to > 2,000 kg/mol) each comprising several polypeptides routinely identified by size-exclusion chromatography and electrophoresis. Gluten proteins are rich in glutamine (30%) and contain several repeated sequences, linking them to the wide class of intrinsically disordered protein (IDP). Here we showed that an ethanol/water (EtOH/W, 50/50, v/v) extract of an industrial gluten, comprising 1/3 of glutenin polymers and 2/3 of gliadin, underwent liquid-liquid phase separation (LLPS) below 14 °C, leading to two coexisting phases, respectively rich and poor in protein. As the quenching depth increased, proteins of lower and lower molecular weight joined the rich phase, akin to what would have been obtained for a polydisperse polymer sample. Within the rich phase the mass ratio of glutenin over gliadin decreased from 2.5 to 0.5 as the temperature dropped from 14 °C to −0.8 °C. Concomitantly the concentration in glutenin polymers increased up to 143 ± 6 g/L (at 9 °C) and then stopped to evolve, suggesting that the binodal line intersected the gelation line below this temperature. Applying the Flory-Huggins (FH) lattice model for each gluten protein classes, we demonstrated that their partitioning in the coexisting phases followed a same temperature dependency. However, some gliadin species joined the rich phase above their critical temperature. Here, specific interactions with the glutenin polymers through weak forces were exemplified. The study demonstrated the relevance of the Flory-Huggins (FH) lattice model in predicting phase behavior even when applied to complex protein mixtures.

In this Special Issue, we celebrate the connection between the physics of fluids, food science and education. We received over 30 articles concerning all types of kitchen flows, from simple to complex, from small to large scale. They include research articles, short reviews, educational articles, methods papers and expert tutorials. In the remainder of this editorial, we will summarise these works and highlight some exciting new opportunities in this effervescently growing branch of science.

International audience; When a drop of fluid hits a small solid target of comparable size, it expands radially until reaching a maximum diameter and subsequently recedes. In this work, we show that the expansion process of liquid sheets is controlled by a combination of shear (on the target) and biaxial extensional (in the air) deformations. We propose an approach toward a rational description of the phenomenon for Newtonian and viscoelastic fluids by evaluating the viscous dissipation due to shear and extensional deformations, yielding a prediction of the maximum expansion factor of the sheet as a function of the relevant viscosity. For Newtonian systems, biaxial extensional and shear viscous dissipation are of the same order of magnitude. On the contrary, for thinning solutions of supramolecular polymers, shear dissipation is negligible compared to biaxial extensional dissipation and the biaxial thinning extensional viscosity is the appropriate quantity to describe the maximum expansion of the sheets. Moreover, we show that the rate-dependent biaxial extensional viscosities deduced from drop impact experiments are in good quantitative agreement with previous experimental data and theoretical predictions for various viscoelastic liquids.

We investigate the structure of gluten polymer-like gels in a binary mixture of water/ethanol, $50/50$ v/v, a good solvent for gluten proteins. Gluten comprises two main families of proteins, monomeric gliadins and polymer glutenins. In the semi-dilute regime, scattering experiments highlight two classes of behavior, akin to standard polymer solution and polymer gel, depending on the protein composition. We demonstrate that these two classes are encoded in the structural features of the proteins in very dilute solution, and are correlated with the presence of proteins assemblies of typical size tens of nanometers. The assemblies only exist when the protein mixture is sufficiently enriched in glutenins. They are found directly associated to the presence in the gel of domains enriched in non-exchangeable H-bonds and of size comparable to that of the protein assemblies. The domains are probed in neutron scattering experiments thanks to their unique contrast. We show that the sample visco-elasticity is also directly correlated to the quantity of domains enriched in H-bonds, showing the key role of H-bonds in ruling the visco-elasticity of polymer gluten gels., accepted for publication in Journal of Physics: Condensed Matter, Special issue on "Glasses and gels: a crossroad of molecular liquids, polymers and colloids"

We investigate the delayed rupture of biopolymer gels under a constant shear load by simultaneous dynamic light scattering and rheology measurements. We unveil the crucial role of normal stresses built up during gelation: all samples that eventually fracture self-weaken during the gelation process, as revealed by a partial relaxation of the normal stress concomitant to a burst of microscopic plastic rearrangements. Upon applying a shear stress, weakened gels exhibit in the creep regime distinctive signatures in their microscopic dynamics, which anticipate macroscopic fracture by up to thousands of seconds. The dynamics in fracturing gels are faster than those of non-fracturing gels and exhibit large spatio-temporal fluctuations. A spatially localized region with significant plasticity eventually nucleates, expands progressively, and finally invades the whole sample triggering macroscopic failure., Comment: accepted for publication in Physical Review Letters

International audience; Wheat gluten, one of the most complex viscoelastic protein networks in nature, is unique to get the specific texture of bread. Due to its complex protein composition, its insolubility in most solvents and the very high molar mass of half of the proteins (glutenin, the other half being gliadin), the architecture of the network is still not well understood. In this work, we have investigated model gluten protein extracts with contrasted compositions in glutenin and gliadin solubilized in a mild chaotropic solvent: ethanol/water (50/50 v/v). The samples display a liquid-liquid phase separation with an upper critical solution temperature that depends on the protein composition. The phase diagrams are consistent with the presence of supramolecular assemblies of proteins. To confirm the presence of these assemblies and fully characterize the objects dispersed in ethanol/water, we have used an asymmetrical flow field-flow fractionation (AsFlFFF) setup coupled with differential refractive index, multi-angle light scattering and dynamic light scattering detections to probe very dilute protein suspensions. We have identified three classes of objects, with distinctive molar mass, characteristic size and conformation: protein monomers, polymeric structures, and very loose protein assemblies with molar mass larger than 2.10 6 g/mol. A molecular characterization of the species by size exclusion chromatography in a denaturing solvent shows that polymers and assemblies are mainly composed of glutenin and ω-gliadin. The high content of ω-gliadin, devoid of cysteines, indicates the importance of non-covalent interactions involved in protein assemblies and might play a major role in gluten rheology

Green hydrogen production by photocatalytic water splitting offers a promising way to solve environment and energy issues. Conjugated polymer nanostructures (CPNs), with highly π-conjugated structures, have been demonstrated as a new class of photocatalysts. However, pristine CPNs show poor photocatalytic activity for hydrogen generation owing to fast charge carrier recombination and sluggish kinetics. The investigation of application of polypyrrole in photocatalytic hydrogen generation is also rare. Here, we report the surface modification of polypyrrole nanostructures (NSs) with Pt- and Ni-based nanoparticles for photocatalytic hydrogen evolution. PPy nanostructures (NSs) were synthesized using lamellar mesophases as soft templates. The PPy-NSs were modified with mono- and bimetallic (Pt, Ni, Pt–Ni) co-catalyst nanoparticles induced by radiolysis. The prepared Pt-PPy, Ni-PPy and PtNi-PPy exhibit excellent photocatalytic activity for H2 generation and a synergistic effect is obtained with co-modification with Pt and Ni. The effects of the nature of the metal precursors and the loading ratio were studied. We show that the loading rate of co-catalysts is crucial for H2 generation, and an excess of co-catalyst can drastically decrease the activity. The composite photocatalyst Pt-Ni-PPy nanostructures are very active and stable with cycling. Our results indicate that modified CPNs with mono- and bimetallic NPs are promising photocatalysts for hydrogen evolution.

We investigate the linear viscoelasticity of polymer gels produced by the dispersion of gluten proteins in water:ethanol binary mixtures with various ethanol contents, from pure water to 60% v/v ethanol. We show that the complex viscoelasticity of the gels exhibits a time/solvent composition superposition principle, demonstrating the self-similarity of the gels produced in different binary solvents. All gels can be regarded as near critical gels with characteristic rheological parameters, elastic plateau and characteristic relaxation time, which are related one to another, as a consequence of self-similarity, and span several orders of magnitude when changing the solvent composition. Thanks to calorimetry and neutron scattering experiments, we evidencea co-solvency effect with a better solvation of the complex polymer-like chains of the gluten proteins as the amount of ethanol increases. Overall the gel viscoelasticity can be accounted for by a unique characteristic length characterizing the crosslink density of the supramolecular network, which is solvent composition-dependent. On a molecular level, these findings could be interpreted as a transition of the supramolecular interactions, mainly H-bonds, from intra- to interchains, which would be facilitated by the disruption of hydrophobic interactions by ethanol molecules. This work provides new insight for tailoring the gelation process of complex polymer gels., Comment: Macromolecules. In press 2020

The mechanical properties of soft matter are of great importance in countless applications, in addition of being an active field of academic research. Given the relative ease with which soft materials can be deformed, their non-linear behavior is of particular relevance. Large loads eventually result in material failure. In this Perspective article, we discuss recent work aiming at detecting precursors of failure by scrutinizing the microscopic structure and dynamics of soft systems under various conditions of loading. In particular, we show that the microscopic dynamics is a powerful indicator of the ultimate fate of soft materials, capable of unveiling precursors of failure up to thousands of seconds before any macroscopic sign of weakening., 12 pages, 7 figures

International audience; Natural protein sources often display a huge complexity, being composed of a blend of polypeptides of various molecular weight, pHi and charge density. Gluten, the proteins extracted from wheat flour is one of such. Gluten is widely used for its viscoelastic properties as an improver of cereal products (bread, pastry, etc.). It is composed of two classes of proteins, named gliadin and glutenin, similar in their amounts in glutamine (30%) and proline (10%). The more than 25 different polypeptides belonging to the gliadin class are hard to fractionate into individual components because of high redundancy in the primary sequences. Glutenin are in the form of polymers made from several distinct polypeptides concatenated through inter-chain disulfide bonds. Their molecular weights are evenly distributed from 100 kg/mol to 7,000 kg/mol.While it is well established that gliadin confers viscosity to gluten whereas glutenin polymers are at the origin of its elastic resistance, the interactions existing between both classes of gluten protein remain unknown. We previously showed by SLS and multi-angle DLS that gluten proteins suspended in ethanol/water (50/50, v/v), a theta solvent, includes large proteins assemblies (26,000 kg/mol, Rh 100nm) displaying an internal dynamic1. To get a better insight of the composition of those assemblies, we combined biochemical and physicochemical approaches. On the one hand, gluten proteins suspended in ethanol/water were fractionated by Asymmetrical- Flow-Field-Flow Fractionation (A4F) coupled to UV, SLS and QELS detectors. On the other hand, gluten proteins were partitioned by liquid-phase decomposition in respect with temperature. Protein composition of partitioned phases and eluting fractions recovered from A4F were characterized by size-exclusion chromatography. A specific interaction between omega-gliadin and glutenin polymers was evidenced.This result strengthens a very recent study that demonstrates a significant positive correlation between glutenin polymeric proteins, the ω‐gliadin fraction, and dough properties.

We investigate by time-resolved Synchrotron ultra-small X-ray scattering the dynamics of liquid-liquid phase-separation (LLPS) of gluten protein suspensions following a temperature quench. Samples at a fixed concentration (237 mg/ml) but with different protein compositions are investigated. In our experimental conditions, we show that fluid viscoelastic samples depleted in polymeric glutenin phase-separate following a spinodal decomposition process. We quantitatively probe the late stage coarsening that results from a competition between thermodynamics that speeds up the coarsening rate as the quench depth increases, and transport that slows downs the rate. For even deeper quenches, the even higher viscoelasticity of the continuous phase leads to a "quasi" arrested phase separation. Anomalous phase-separation dynamics is by contrast measured for a gel sample rich in glutenin, due to elastic constraints. This work illustrates the role of viscoelasticity in the dynamics of LLPS in protein dispersions., Comment: accepted for publication in Soft Matter

International audience; Gliadins are edible wheat storage proteins well known for their surface active properties. In this paper, we present experimental results on the interfacial properties of acidic solutions of gliadin studied over 5 decades of concentrations, from 0.001 to 110 g/L. Dynamic pendant drop tensiometry reveals that the surface pressure  of gliadin solutions builds up in a multistep process. The series of curves of the time evolution of collected at different bulk protein concentrations C can be merged onto a single master curve when  is plotted as a function of t where t is the time elapsed since the formation of the air/water interface and  is a shift parameter that varies with C as a power law with an exponent 2. The existence of such time-concentration superposition, which we evidence for the first time, indicates that the same mechanisms govern the surface tension evolution at all concentrations and are accelerated by an increase of the bulk concentration. The scaling of  with C is consistent with a kinetic of adsorption controlled by the diffusion of the proteins in the bulk. Moreover, we show that the proteins adsorption at the air/water interface is kinetically irreversible. Correlated evolutions of the optical and elastic properties of the interfaces, as probed by ellipsometry and surface dilatational rheology respectively, provide a consistent physical picture of the building up of the protein interfacial layer. A progressive coverage of the interface by the proteins occurs at low . This stage is followed, at higher , by conformational rearrangements of the protein film, which are identified by a strong increase of the dissipative viscoelastic properties of the film concomitantly with a peculiar evolution of its optical profile that we have rationalized. In the last stage, at even higher surface pressure, the adsorption is arrested; the optical profile is not modified while the elasticity of the interfacial layer dramatically increases with the surface pressure, presumably due to the film ageing.

Power law rheology is of widespread occurrence in complex materials that are characterized by the presence of a very broad range of microstructural length and time scales. Although phenomenological models able to reproduce the observed rheological features exist, in general a well-established connection with the microscopic origin of this mechanical behavior is still missing. As a model system, this work focuses on a fractal colloidal gel. We thoroughly characterize the linear power law rheology of the sample and its age dependence. We show that at all sample ages and for a variety of rheological tests, the gel linear viscoelasticity is very accurately described by a fractional Maxwell (FM) model, characterized by a power law behavior. Thanks to a unique setup that couples small-angle static and dynamic light scattering to rheological measurements, we show that in the linear regime, shear induces reversible nonaffine rearrangements which might be at the origin of the power law rheology and we discuss the possible relationship between the FM model and the microscopic structure of the gel.Power law rheology is of widespread occurrence in complex materials that are characterized by the presence of a very broad range of microstructural length and time scales. Although phenomenological models able to reproduce the observed rheological features exist, in general a well-established connection with the microscopic origin of this mechanical behavior is still missing. As a model system, this work focuses on a fractal colloidal gel. We thoroughly characterize the linear power law rheology of the sample and its age dependence. We show that at all sample ages and for a variety of rheological tests, the gel linear viscoelasticity is very accurately described by a fractional Maxwell (FM) model, characterized by a power law behavior. Thanks to a unique setup that couples small-angle static and dynamic light scattering to rheological measurements, we show that in the linear regime, shear induces reversible nonaffine rearrangements which might be at the origin of the power law rheology and we discuss the ...

International audience; Model gluten gels

Réf Journal: Journal of Rheology 61(6), 1267-1275, 2017; International audience; We study the fracture of reversible double transient networks, constituted of water suspensions of entangled surfactant wormlike micelles reversibly linked by various amounts of telechelic polymers. We provide a state diagram that delineates the regime of fracture without necking of the filament from the regime where no fracture or break-up has been observed. We show that filaments fracture when stretched at a rate larger than the inverse of the slowest relaxation time of the networks. We quantitatively demonstrate that dissipation processes are not relevant in our experimental conditions and that, depending on the density of nodes in the networks, fracture occurs in the linear viscoelastic regime or in a non-linear regime. In addition, analysis of the crack opening profiles indicates deviations from a parabolic shape close to the crack tip for weakly connected networks. We demonstrate a direct correlation between the amplitude of the deviation from the parabolic shape and the amount of non linear viscoelasticity.

We develop and thoroughly test a stress-controlled, parallel plates shear cell that can be coupled to an optical microscope or a small angle light scattering setup, for simultaneous investigation of the rheological properties and the microscopic structure of soft materials under an imposed shear stress. In order to minimize friction, the cell is based on an air bearing linear stage, the stress is applied through a contactless magnetic actuator, and the strain is measured through optical sensors. We discuss the contributions of inertia and of the small residual friction to the measured signal and demonstrate the performance of our device in both oscillating and step stress experiments on a variety of viscoelastic materials., 14 pages, 10 figures

Structuring wheat gluten proteins into gels with tunable mechanical properties would provide more versatility for the production of plant protein-rich food products. Gluten, a strongly elastic protein material insoluble in water, is hardly processable. We use a novel fractionation procedure allowing the isolation from gluten of a water/ethanol soluble protein blend, enriched in glutenin polymers at an unprecedented high ratio (50%). We investigate here the viscoelasticity of suspensions of the protein blend in a water/ethanol (50/50 v/v) solvent, and show that, over a wide range of concentrations, they undergo a spontaneous gelation driven by hydrogen bonding. We successfully rationalize our data using percolation models and relate the viscoelasticity of the gels to their fractal dimension measured by scattering techniques. The gluten gels display self-healing properties and their elastic plateaus cover several decades, from 0.01 to 10000 Pa. In particular very soft gels as compared to standard hydrated gluten can be produced., Comment: Food Hydrocolloids, in press

We investigate the dynamics of freely expanding liquid sheets prepared with fluids with different rheological properties, (i) viscous fluids with a zero-shear viscosity $\eta_0$ in the range $(1-1000) \, \mathrm{mPa.s}$ and (ii) viscoelastic Maxwell fluids whose elastic modulus, $G_0$, characteristic relaxation time, $\tau$, and zero-shear viscosity, $\eta_0=G_0 \tau$, can be tuned over several orders of magnitude. The sheets are produced by impacting a drop of fluid on a small cylindrical solid target. For viscoelastic fluids, we show that, when $\tau$ is shorter than the typical lifetime of the sheet ($\sim 10$ ms), the dynamics of the sheet is similar to that of Newtonian viscous liquids with equal zero-shear viscosity. In that case, for little viscous samples ($\eta_0, Comment: 10 pages, 9 figures

The structure of model gluten protein gels prepared in ethanol/water is investigated by small angle X-ray (SAXS) and neutrons (SANS) scattering. We show that gluten gels display radically different SAXS and SANS profiles when the solvent is (at least partially) deuterated. The detailed analysis of the SANS signal as a function of the solvent deuteration demonstrates heterogeneities of sample deuteration at different length scales. The progressive exchange between the protons (H) of the proteins and the deuteriums (D) of the solvent is inhomogeneous and 60 nm large zones that are enriched in H are evidenced. In addition, at low protein concentration, in the sol state, solvent deuteration induces a liquid/liquid phase separation. Complementary biochemical and structure analyses show that the denser protein phase is more protonated and specifically enriched in glutenin, the polymeric fraction of gluten proteins. These findings suggest that the presence of H-rich zones in gluten gels would arise from the preferential interaction of glutenin polymers through a tight network of non-exchangeable intermolecular hydrogen bonds., Comment: Soft Matter, Royal Society of Chemistry, 2016

The development of visible-light responsive photocatalysts would permit more efficient use of solar energy and thus would bring sustainable solutions to many environmental issues. Conductive polymers appear as a new class of very active photocatalysts under visible light. Among them poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most promising conjugated polymer with a wide range of applications. PEDOT nanostructures synthesized in soft templates via chemical oxidative polymerization demonstrate unprecedented photocatalytic activities for water treatment without the assistance of sacrificial reagents or noble metal co-catalysts and turn out to be better than TiO2 as benchmark catalyst. The PEDOT nanostructures exhibit a narrow band gap (E = 1.69 eV) and are characterized by excellent ability to absorb light in visible and near infrared region. The novel PEDOT-based photocatalysts are very stable with cycling and can be reused without appreciable loss of activity. Interestingly, hollow micrometric vesicular structures of PEDOT are not effective photocatalysts as compared to nanometric spindles suggesting size and shape dependent photocatalytic properties. The visible-light active photocatalytic properties of the polymer nanostructures present promising applications in solar light harvesting and broader fields.

We study the destabilization mechanism of thin liquid sheets expanding in air and show that dilute oil-in-water emulsion-based sheets disintegrate through the nucleation and growth of holes that perforate the sheet. The velocity and thickness fields of the sheet outside the holes are not perturbed by holes and hole opening follows a Taylor-Culick law. We find that a pre-hole, which widens and thins out the sheet with time, systematically precedes the hole nucleation. The growth dynamics of the pre-hole follows the law theoretically predicted for a liquid spreading on another liquid of higher surface tension due to Marangoni stresses. Classical Marangoni spreading experiments quantitatively corroborate our findings., Physical Review Letters in press

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., Comment: to appear in Journal of Physical Chemistry B

International audience; We describe a single step preparation of nanostructures of poly(3,4-ethylenedioxythiophene), PEDOT, in the hydrophobic domains of cationic surfactant-based hexagonal mesophases via chemical oxidative polymerization of EDOT monomers using FeCl3 as an oxidizing agent. After polymerization, the hexagonal structure of the mesophases is preserved as demonstrated by polarized light microscopy and X-ray scattering measurements. After extraction from mesophases, the chemical structure of PEDOT is confirmed by Fourier transform infrared spectroscopy. Moreover, PEDOT morphology is checked by transmission and scanning electron microscopies. PEDOT nanostructures with spindle-like or vesicle-like shapes are obtained depending on the experimental conditions. In the original method, high resolution atomic force microscopy, coupled with infrared nanospectroscopy, is used to probe the local chemical composition of PEDOT nanostructures. Finally, the as-prepared PEDOT polymers are characterized by both good thermal stability up to 200 °C and a relatively high conductivity value up to 0.4 S cm−1 as determined by thermogravimetric analysis and four probe measurements respectively.

We use confocal microscopy and time-resolved light scattering to investigate plasticity in a col- loidal 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., Comment: main paper + supplementary materials

Journal: Advanced Functional Materials 22, 4900 (2012); International audience; We use hexagonal mesophases made of oil-swollen surfactant-stabilized tubes arranged on a triangular lattice in water and doped with metallic salts as templates for the radiolytic synthesis of nanostructures. The nanostructures formed in this type of soft matrix are bimetallic palladium-platinum porous nanoballs composed of 3D-connected nanowires, of typical thickness 2.5 nm, forming hexagonal cells. We demonstrate using electron microscopy and small-angle X-ray scattering that the pore size of the nanoballs is directly determined by the diameter of the oil tube of the doped mesophases, which we have varied in a controlled fashion from 10 to 55 nm. Bimetallic nanostructures comprising various proportions of palladium and platinum can be synthesized. Their alloy structure was evidenced by X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and high-angular dark field scanning transmission electron microscopy experiments. Our templating approach allows therefore the synthesis of bimetallic nanoballs of tunable porosity and composition.

International audience; Very long and thin monocrystalline Pd nanowires were synthesized by decomposition using hydrazine vapour treatment of Pd2(DBA)3 confined in the oil phase of a hexagonal mesophase. The mesophase is composed of toluene-swollen tubes, doped with the Pd precursor, stabilized by a monolayer of surfactant and cosurfactant molecules, and arranged on a triangular lattice in water. Our synthesis is selective and the experiments show that both the confinement and the nature of the surfactant are essential to obtain these Pd nanowires.

We study the crystallization of a colloidal model system in presence of secondary nanoparticles acting as impurities. Using confocal microscopy, we show that the nanoparticles segregate in the grain boundaries of the colloidal polycrystal. We demonstrate that the texture of the polycrystal can be tuned by varying independently the nanoparticle volume fraction and the crystallization rate, and quantify our findings using standard models for the nucleation and growth of crystalline materials. Remarkably, we find that the efficiency of the segregation of the nanoparticles in the grain-boundaries is determined solely by the typical size of the crystalline grains., accepted for publication in Soft Matter

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 Langmuir

International audience; We report on a new class of self-assembled transient networks made of surfactant micelles of tunable morphology (from spheres, to rodlike to wormlike) reversibly linked by telechelic polymers. Linear rheological measurements show that three distinct domains can be defined depending on the morphologies of the micelles: a domain where the micelles are isolated and not entangled, an intermediate domain where the micelles are partially entangled and a domain where the micelles are fully entangled. Flow curves of the transient networks of tunable morphology suggest that one can associate to the three domains distinct failures modes: a brittle mode, an intermediate mode and finally a ductile/shear banding mode, as the micelles grow. Thanks to this unique class of self-assembled networks, a continuous failure mode transition from brittle to ductile has been evidenced.

We use light microscopy to investigate the aging dynamics of a glass made of closely packed soft spheres, following a rapid transition from a fluid to a solid-like state. By measuring time-resolved, coarse-grained displacements fields, we identify two classes of dynamical events, corresponding to reversible and irreversible rearrangements, respectively. The reversible events are due to the small, experimentally unavoidable fluctuations of the temperature imposed to the sample, leading to transient thermal expansions and contractions that cause shear deformations. The irreversible events are plastic rearrangements, induced by the repeated shear cycles. We show that the displacement due to the irreversible rearrangements grows linearly with time, both on average and at a local level. The velocity associated with this ballistic motion decreases exponentially with sample age, accounting for the observed slowing down of the dynamics. The displacement field due to the irreversible rearrangements has a vortex-like structure and is spatially correlated over surprisingly long distances., Comment: 10 pages, 7 figures, submitted to PRE

We have investigated the nonlinear rheology of a soft composite transient network made of a solution of surfactant wormlike micelles (WM) in the semidilute regime that are reversibly bridged by telechelic polymers. The samples are well described, in the linear regime, as two Maxwell fluids components blends, characterized by two markedly different characteristic times. The slow mode is mainly related to the transient network of entangled WM, and the fast mode to the network of telechelic chains. In this paper we investigate the nonlinear viscoelasticity and show that the nonlinear behavior reflects as well the behavior of two coupled networks. On one hand, stress relaxation experiments and time-resolved stress response following the application of a constant shear rate show that, in the weakly nonlinear regime, these novel composite networks stiffen. A fourfold increase of the elastic modulus with respect to the linear value is reached for strain amplitude of about 200%. This strain hardening is due to the nonlinear stretching of the telechelic polymer chains. On the other hand, the samples exhibit shear banding in the highly nonlinear regime, similarly to pure semidilute solutions of WM.

Palladium nanowires (of length a few tens of nanometers) are synthesized in a hexagonal mesophase formed by a quaternary system (Pd-doped water, surfactant, cosurfactant, and oil) by electron beam irradiation. The mesophases can be doped by high concentrations of palladium (0.1 M) without any disturbance of the structure of the mesophases which allows the quantitative synthesis of 1D Pd nanostructures. We found an increase in the average length of the nanowires with the amount of cosurfactant (pentanol) that assists the reduction/growth processes. The electrocatalytic oxidation of ethanol was selected as a test reaction in alkaline medium where Pd is known to be among the best electrode materials. We found that the Pd nanowires exhibit both a very important electrocatalytic activity for ethanol oxidation and a very high stability.

Ultrathin palladium hexagonal nanosheets were synthesized in emulsions constituted of droplets of toluene containing Pd complexes in water and stabilized by CTAB as surfactant and in quaternary mesophases formed by water, toluene containing Pd complexes, CTAB, and a cosurfactant. We have identified slow reduction and nucleation provided by CO and adsorption at the CTAB interface as key factors for the formation of Pd nanosheets. These nanosheets exhibit a broad absorption band from the visible to the near-infrared region.

International audience; We show that two types of complex fluids belonging to the same class of soft materials (transient networks), and having the same linear rheological properties (Maxwell fluids) exhibit two distinct fracture behaviors as solids do. We have designed a third experimental system that captures the distinct features of both former ones. We show that it could be a good candidate to investigate the brittle-ductile transition in complex fluids.

Three-dimensional connected Pd nanowires forming nanoballs have been synthesized by slow reduction of PdII in hexagonal mesophases made by a quaternary system (water/cyclohexane/surfactant/cosurfactant). Both confinement and slow reduction are necessary to obtain these new nanostructures. This material exhibits remarkable electrocatalytic behavior for ethanol oxidation in 1 M KOH and appears as a promising candidate for use in direct ethanol fuel cells.

International audience; We show quantitatively that tiny amounts of copolymer that decorate a oil/water interfaces can greatly enhance the stability of swollen surfactant hexagonal phases, comprising oil tubes regularly arranged in a water matrix. Such soft composite materials, whose both radius of the tubes and water channel between the tubes can be controlled independently over large ranges, offer a potential interest for the synthesis of mesoporous materials.

International audience; The thermodynamics and structural behavior of a new type of transient network are reported. The network is obtained by adding in a solution of entangled surfactant wormlike micelles a telechelic triblock copolymer whose hydrophobic ends anchor into the micelles and whose hydrophilic tails are swollen in the aqueous solvent and reversibly link the entangled cylindrical micelles. For comparison, we have also studied the same surfactant system decorated with an amphiphilic diblock copolymer which corresponds exactly to a triblock telechelic copolymer cut into two identical diblock copolymers. We find that the addition of telechelic polymers induces an effective attraction of entropic origin between the surfactant that may result, in equilibrium, in the coexistence of a dilute phase and a connected network, as predicted by theory. Small-angle neutron scattering experiments show first that the locally cylindrical structure of the micelles is maintained upon copolymer addition over a wide range of copolymer-to-surfactant molar ratio. On the other hand, the addition of telechelic polymer correlates with (i) the emergence of a broad peak in the structure factor at a finite scattering vector, q, due to the short-range steric repulsion induced by the polymer between the cylinders, and (ii) the large rise of the scattered intensity at low q's (for small enough surfactant concentration), which indicates an increase of the osmotic compressibility of the solution due to the effective attractive interaction between cylinders provoked by the telechelic polymers linking them.

Creep experiments on polycrystalline surfactant hexagonal columnar phases show a power law regime, followed by a drastic fluidization before reaching a final stationary flow. The scaling of the fluidization time with the shear modulus of the sample and stress applied suggests that the onset of flow involves a bulk reorganization of the material. This is confirmed by X-ray scattering under stress coupled to \textit{in situ} rheology experiments, which show a collective reorientation of all crystallites at the onset of flow. The analogy with the fracture of heterogeneous materials is discussed., to appear in Phys. Rev. Lett

We study mixtures of charged surfactants, which alone in solution form uni- and multilamellar vesicles, and oppositely charged polyelectrolytes (PEs). The phase behavior is investigated at fixed surfactant concentration as a function of the PE-to-surfactant charge ratio $x$. We find that, for $x>0$, aggregates form. Light microscopy and X-ray scattering experiments show that the isoelectric point plays a crucial role since the morphology and the microscopic structure of the aggregates are different before ($x\leq1$) and after the isoelectric point ($x>1$). To better understand the dynamics for the formation of PE/surfactant complexes, we perform light microscopy experiments where we follow in real-time the effect of a PE solution on one multilamellar vesicle (MLV). We find that the PE induces a peeling of the bilayers of the MLV one by one. The peeling is accompanied by strong shape fluctuations of the MLV and leads ultimately to a pile of small aggregates. This novel phenomenon is analyzed in detail and discussed in terms of PE-induced tension, and pore formation and growth in a surfactant bilayer., to appear in Langmuir

Measuring, characterizing and modelling the slow dynamics of glassy soft matter is a great challenge, with an impact that ranges from industrial applications to fundamental issues in modern statistical physics, such as the glass transition and the description of out-of-equilibrium systems. Although our understanding of these phenomena is still far from complete, recent simulations and novel theoretical approaches and experimental methods have shed new light on the dynamics of soft glassy materials. In this paper, we review the work of the last few years, with an emphasis on experiments in four distinct and yet related areas: the existence of two different glass states (attractive and repulsive), the dynamics of systems very far from equilibrium, the effect of an external perturbation on glassy materials, and dynamical heterogeneity.

Synthesis and self-assembly of nanomaterials can be controlled by the properties of soft matter. On one hand, dedicated nanoreactors such as reverse microemulsions or miniemulsions can be designed. On the other hand, direct shape control can be provided by the topology of liquid crystals that confine the reacting medium within a specific geometry. In the first case, the preparation of micro- or miniemulsions generally requires energetic mechanical stirring. The second approach uses thermodynamically stable systems, but it remains usually limited to binary (water + surfactant) systems. We report the preparation of different families of materials in highly ordered quaternary mediums that exhibit a liquid crystal structure with a high cell parameter. They were prepared with the proper ratios of salted water, nonpolar solvent, surfactant, and cosurfactants that form spontaneously swollen hexagonal phases. These swollen liquid crystals can be prepared from all classes of surfactants (cationic, anionic, and non...

We investigate by rheology and light scattering the influence of the elastic modulus, ${G}_{0}$, on the slow dynamics and the aging of a soft glass. We show that the slow dynamics and the aging can be entirely described by the evolution of an effective viscosity, ${\ensuremath{\eta}}_{\mathrm{eff}}$, defined as the characteristic time measured in a stress relaxation experiment times ${G}_{0}$. At all times, ${\ensuremath{\eta}}_{\mathrm{eff}}$ is found to be independent of ${G}_{0}$, of elastic perturbations, and of the rate at which the sample is quenched in the glassy phase. We propose a simple model that links ${\ensuremath{\eta}}_{\mathrm{eff}}$ to the internal stress built up at the fluid-to-solid transition.

International audience; We use the random phase approximation to compute the structure factor S(q) of a solution of chains interacting through a soft and short range repulsive potential V. Above a threshold polymer concentration, whose magnitude is essentially controlled by the range of the potential, S(q) exhibits a peak whose position depends on the concentration. We take advantage of the close analogy between polymers and wormlike micelles and apply our model, using a Gaussian function for V, to quantitatively analyze experimental small angle neutron scattering profiles of solutions of hairy wormlike micelles. These samples, which consist in surfactant self-assembled flexible cylinders decorated by amphiphilic copolymer, provide indeed an appropriate experimental model system to study the structure of sterically interacting polymer solutions.

A positively charged, mixed bilayer vesicle in the presence of negatively charged surfaces (for example, colloidal particles) can spontaneously partition into an adhesion zone of definite area, and another zone that repels additional negative objects. Although the membrane itself has nonnegative charge in the repulsive zone, negative counterions on the interior of the vesicle spontaneously aggregate there, and present a net negative charge to the exterior. Beyond the fundamental result that oppositely charged objects can repel, our mechanism helps explain recent experiments on surfactant vesicles., Latex using epsfig and afterpage; pdf available at http://www.physics.upenn.edu/~nelson/Mss/repel.pdf