Slip and friction mechanisms at polymer semi-dilute solutions / solid interfaces

The role of the polymer volume fraction, $\phi$, on steady state slippage and interfacial friction is investigated for a semi-dilute polystyrene solutions in diethyl phthalate in contact with two solid surfaces. Signicant slippage is evidenced for all samples, with slip lengths b obeying a power law dependence. The Navier's interfacial friction coecient, k, is deduced from the slip length measurements and from independent measurements of the solutions viscosity $\eta$. The observed scaling of k versus $\phi$ clearly excludes a molecular mechanism of friction based on the existence of a depletion layer. Instead, we show that the data of $\eta$($\phi$) and k($\phi$) are understood when taking into account the dependence of the solvent friction on $\phi$. Two models, based on the friction of blobs or of monomers on the solid surface, well describe our data. Both points out that the Navier's interfacial friction is a semi-local phenomenon

Polymer brush collapse under shear flow

Shear responsive surfaces offer potential advances in a number of applications. Surface functionalisation using polymer brushes is one route to such properties, particularly in the case of entangled polymers. We report on neutron reflectometry measurements of polymer brushes in entangled polymer solutions performed under controlled shear, as well as coarse-grained computer simulations corresponding to these interfaces. Here we show a reversible and reproducible collapse of the brushes, increasing with the shear rate. Using two brushes of greatly different chain lengths and grafting densities, we demonstrate that the dynamics responsible for the structural change of the brush are governed by the free chains in solution rather than the brush itself, within the range of parameters examined. The phenomenon of the brush collapse could find applications in the tailoring of nanosensors, and as a way to dynamically control surface friction and adhesion

Gum Arabic in solution: Composition and multi-scale structures

Gum Arabic is a natural acacia tree exudate containing hyperbranched polysaccharides and proteins. Here, we perform a dual chromatographic separation together with small-angle X-ray and neutron scattering structural characterizations. We show that the different species present in Gum Arabic can not be easily classified in distinct families. They are rather build from various combinations of two building blocks that are evidenced by a mismatch between small-angle X-ray and neutron scattering. One block corresponds to hyperbranched polysaccharides, which we describe as three-dimensional multi-scale porous colloids possessing three length scales of 7, 2 and 0.7 nm. The other block corresponds to protein chains that organize as Gaussian chains in solution and are prone to aggregation. A large array of polysaccharide/protein conjugates was identified, which differs in size, hydrophobicity and amino-acid content. Still, their structure is always the juxtaposition of the two building blocks structures. Additionally, small-angle neutron scattering reveals that large-scale structures are ubiquitous in Gum Arabic solutions and originate from the self-association of both free and conjugated polypeptide chains. Despite its compositional complexity, Gum Arabic solutions thus possess a robust multi-scale structure that is mainly impacted by concentration and ionic repulsions

Neutron reflectivity for soft matter

Specular neutron reflectivity is a technique enabling the measurement of coherent neutron scattering length density profile perpendicular to the plane of a surface or interface, and thereby the profile of chemical composition. The characteristic sizes that are probed range from around 5Å up 5000 Å. It is a scattering technique that averages information over the entire surface and it is therefore not possible to obtain information on correlations in the plane of the interface. The specific properties of neutrons (possibility of tuning the contrast by isotopic substitution, negligible absorption, low energy of the incident neutrons) makes it particularly interesting in the fields of soft matter and biophysics. This course is composed of three parts describing respectively its principle, the experimental aspects (diffractometers, samples), and some scientific examples of neutron reflectometry focusing on the use of contrast variation to probe polymeric systems

Exact resolution function for double-disk chopper neutron time-of-flight spectrometers : Application to reflectivity

International audienceThe exact resolution function in transfer vector for the reflectometer HERM\`ES at Laboratoire L\'eon Brillouin is calculated as an example of neutron time-of-fligh spectrometer with double-disk chopper. Calculation accounts for wavelength distribution of the incident beam, tilt of the chopper axis, collimation and gravity, without approximation of gaussian distributions or independence of these different contributions. Numerical implementation is provided that matches the sections of the paper. We show that data fitting using this exact resolution function allows us to reach much better results than its usual approximation by a gaussian profile

Duplex nanoporous alumina and polyelectrolyte adsorption: more insights from a combined neutron reflectivity and electron microscopy study

International audienceNeutron reflectivity (NR) is a powerful technique to investigate the incorporation of nanomaterials (polymers, nanoparticles, etc) into multilayer porous systems. Here we propose an experimental approach combining NR and scanning electron microscopy (SEM) to successfully characterize duplex nanoporous anodic aluminum oxides (nAAO) and to extract quantitative information about the entering and adsorption of polyelectrolytes (PEs) in nanopores. Duplex nAAO are promising systems to study the influence of geometrical constriction, i.e. the reduction of pore diameters along the pore channel, on the confinement of condensed matters

Comparison of the Slip of a PDMS Melt on Weakly Adsorbing Surfaces Measured by a New Photobleaching-Based Technique

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Improving structural features of nanoporous alumina using deuterated electrolytes

International audienceNanoporous anodic aluminum oxide (AAO) is a well-known nanoporous material with multiple applications. In the last years new experimental strategies have been developed to produce AAO in order to tune the pore morphology or ordering and discuss its growth mechanism. Here we describe an original approach to synthesize AAOs using deuterated electrolytes (i.e. sulfuric or oxalic acids in deuterated water D2O) leading to a pore diameter reduction of around 20 % and an increase in the pore ordering and pore growth. Better pore ordering in D2O is correlated to an increase in anion incorporation during the synthesis and the growth rate improvement is related to a decrease in the activation energy of the alumina formation. Moreover, the use of deuterated species allows to discuss the incorporation of hydroxyl groups during the AAO synthesis or the hydration after immersion in H2O or D2O by Small-Angle Neutron Scattering (SANS), a technique sensitive to deuteration. SANS reveals no changes between AAOs synthesized in H2O or D2O due to OH incorporation but shows differences after a long immersion time in water (H2O or D2O), indicating that the hydration is a slow process. This work shows that the use of deuterium is an interesting alternative for the synthesis of AAOs with well-controlled and specific morphologies and, from a fundamental point of view, can bring new general understanding about AAOs formation

Giant slip length at a supercooled liquid-solid interface

International audienceThe effect of temperature on friction and slip at the liquid-solid interface has attracted attention over the last 20 years, both numerically and experimentally. However, the role of temperature on slip close to the glass transition has been less explored. Here we use molecular dynamics to simulate a bidisperse atomic fluid, which can remain liquid below its melting point (supercooled state), to study the effect of temperature on friction and slip length between the liquid and a smooth apolar wall in a broad range of temperatures. At high temperatures, an Arrhenius law fits well the temperature dependence of viscosity, friction, and slip length. In contrast, when the fluid is supercooled, the viscosity becomes super-Arrhenian, while interfacial friction can remain Arrhenian or even drastically decrease when lowering the temperature, resulting in a massive increase of the slip length. We rationalize the observed superlubricity by the surface crystallization of the fluid, and the incommensurability between the structures of the fluid interfacial layer and of the wall. This study calls for experimental investigation of the slip length of supercooled liquids on low surface energy solids