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Changing foam stability with salt

Anniina Salonen Laboratoire de Physique des Solides, Université Paris Sud, Orsay

Jeudi 8 mars 2018 - 14h00 - Amphi Urbain


Foams are used in food products, for cleaning purposes, to extract petrol and in many other industrial processes and products. This makes the control of their properties and stability such important questions. Aqueous foams are collections of gas bubbles in water and they are intrinsically unstable, which means that they will always eventually disappear. The lifetime of the foams depends on the efficiency of the used stabilisers in slowing down the different mechanisms of ageing.
We are most familiar with surfactant-stabilised foams. In general such foams are rather unstable and disappear within hours. However, we have shown that the presence of salt can change foam stability dramatically. The addition of salt can lead to the precipitation of surfactant, where the surfactant crystals can behave as particles at interfaces (as seen in the photograph). The stability of these foams depends on the properties of the precipitate, which in turn can be controlled by both the concentration, and the type of salt used. Therefore we can create rather versatile superstable foams, which destabilize upon command, making simple surfactant foams surprisingly smart.

The three possible states of polyelectrolyte complex coacervates : light and SANS signatures of the soluble, insoluble and macroscopic phase complexes.

Jean-Paul Chapel Centre de Recherche Paul Pascal (CRPP), Pessac

Jeudi 22 mars 2018 - 14h00 Amphi Schützenberger


The complexation of oppositely charged polyelectrolytes (PEs) through electrostatic interaction is a ubiquitous associative process found in both natural and synthetic systems. Depending on the strength of the interaction, at charge stoichiometry the complexation proceeds either through a liquid-solid or a liquid-liquid phase separation leading to the formation of a solid precipitate/aggregate or a liquid complex coacervate, respectively. The aggregates are out-of-equilibrium structures very sensitive to the formulation pathway whereas the coacervate phase is in equilibrium with the polymer-poor phase (supernatant). Beyond the mixing method, the resulting morphologies are very sensitive to the individual PE features (weak/strong, charge density…) and external parameters like the nature and concentration of salt, temperature and pH of solution. We present in this work some experimental evidences of the local structures of the three different association states found in the PAA/PDADMAC polylectolyte coacervating system, i.e., soluble and dispersed PECs, together with the macroscopic coacervate phase. The distinctive SANS signatures of the various phases will help to disambiguate various morphologies found in a PE complex coacervate system. In particular, we show the unambiguous presence of the very controversial soluble complexes between PEs with a large chain length asymmetry. Indeed, with just few short guest chains, the long host chain holds the characteristic of a charged PE, that is, its water solubility in a thermodynamic sense. With more short chains, the hydrophobic segments start to associate and microphase separate generating dispersed PECs. The core-shell structure evolves into compact sphere as the mixing charge ratio Z approach 1. Soluble PECs are absent for more symmetric systems or in the presence of salt where only dispersed PECs are obtained. At stoichiometry (Z=1) complex coacervation occurs. This dense phase can be regarded as a network of random mixed polyion chains with a mesh size much smaller than the Rg of PE chains. An additional scattering maximum is found in our system at high q arising from the relatively stiff PDADMAC cylinders (non-electrostatic persistence length 3 nm), randomly distributed in the concentrated network as anticipated by the “jammed state” proposed by the Rawiso’s group.

A first experimental step to understand the physics of shear-thickening suspensions.

Cécile Clavaud IUSTI, Marseille

Jeudi 29 mars 2018 - 14 h00 - Amphi Urbain


Shear-thickening is an intriguing rheological behavior occurring in some suspensions. It consists in a brutal increase in the viscosity above a critical shear rate. The most famous example of such a behavior is that of a suspension of corn starch in water. A recent microscopic model explains this phenomenon as a frictional transition which arises from the presence of a repulsive force between the grains. In this talk, I will present experiments where we test the hypotheses of this model. In particular, we use a model system of silica beads in saline solutions, in which we can control the range of the repulsion. These experiments evidence the frictional transition, which we also link it to the shear-thickening behavior of the suspension.

Technologie Additives : une nouvelle façon de penser la fabrication de pièces céramiques

Thierry Chartier Science des Procédés Céramiques et des Traitements de Surface, European Ceramic Center, Limoges

Jeudi 22 février 2018

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Some open problems in adhesion (of rough surfaces).

Michele Ciavarella Polyetchnico di Bari, Italie

Mardi 6 février 2018

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Self-assembled nanostructures of block copolymer : the impact of macromolecular architecture

Oleg Borisov Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et les Matériaux, Pau

Jeudi 25 janvier 2018

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