Je présenterai le comportement de gels soumis à des contraintes mécaniques extrêmes. Deux types de gels (transitoires auto-assemblés et permanent réticulés) et deux configurations expérimentales imposant de grandes déformations extensionnelles seront explorés. D’une part, je présenterai la déformation biaxiale de nappes libres produites par impact d’une goutte ou perle sur une surface solide dans des conditions de dissipation minimisée. Ces expériences montrent un comportement unifié de la dynamique d’impact pour les solides, liquides et fluides viscoélastiques que nous rationalisons en prenant en compte les effets élasto-capillaires : une nouvelle vitesse caractéristique qui inclut à la fois l’élasticité en volume et l’élasticité de surface émerge de cette description unifiée de l’impact. D’autre part, je présenterai étude de fracture de gels transitoires produite sous déformation uniaxiale (étirement de filament). L’originalité de ce travail est de coupler suivi par imagerie rapide de la propagation d’une fracture et rhéométrie extensionnelle pour analyser et quantifier la nature des mécanismes de fracture.

Cilia are elastic hairlike protuberances of the cell membrane found in various unicellular organisms and in several tissues of most living organisms. In some tissues such as the airway tissues of the lung, the coordinated beating of cilia induce a fluid flow of crucial importance as it allows the continuous cleaning of our bronchia, known as mucociliary clearance. While most of the models addressing the question of collective dynamics and metachronal wave consider homogeneous carpets of cilia, experimental observations rather show that cilia clusters are heterogeneously distributed over the tissue surface. The purpose of this paper is to investigate the role of spatial heterogeneity on the coherent beating of cilia using a very simple one dimensional model for cilia known as the rower model. We systematically study systems consisting of a few rowers to hundreds of rowers and we investigate the conditions for the emergence of collective beating. When considering a small number of rowers, a phase drift occurs, hence a bifurcation in beating frequency is observed as the distance between rowers clusters is changed. In the case of many rowers, a distribution of frequencies is observed. We found in particular the pattern of the patchy structure that shows the best robustness in collective beating behavior, as the density of cilia is varied over a wide range.

Hybrid organic-inorganic perovskites have been intensively studied as promising materials for photovoltaic applications, taking advantage of their high efficiency and low cost processing. Although much recent attention has also been devoted towards unraveling their microscopic optoelectronic properties, the origin of their softness, by comparison to classic semiconductors, is currently still lacking a comprehensive understanding and systematic experimental studies. Here we investigate by coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, low frequency acoustic phonons in four different hybrid perovskite single crystals: MAPbBr3, FAPbBr3, MAPbI3 and α-FAPbI3. We report a very small shear C44 elastic constant for all the compounds and a considerable elastic anisotropy. The extremely low bulk modulus and negative C12 in α-FAPbI3 substantiates its very unstable nature and in FAPbBr3, a tendency towards an incipient ferroelastic transition, is interpreted as further evidence of the influence of plasticity in hybrid perovskites. We observe a systematic lower sound group velocity in the technologically important iodide-based compounds compared to the bromide-based ones. The findings suggest that low thermal conductivity and hot phonon bottleneck phenomena are expected to be enhanced by low elastic stiffness, particularly in the case of the ultrasoft α-FAPbI3.

With the demography growth, there is a huge pressure on protein demand, and the development of plant based proteins is required for a future sustainable food production. Plant proteins are efficient to stabilize interfaces in foams or emulsions, and the understanding of physical mechanisms at the origin of their interfacial behavior is important to develop new products. We investigate the adsorption of wheat grains (gliadin) and sunflower seeds (helianthinin) proteins, at air-water and oil-water interfaces, respectively. A combination of tensiometry, dilatational viscoelasticity and ellipsometry measurements is used to determine the adsorption mechanisms, and characterize the structure and properties of the interfacial protein films formed with different bulk protein concentrations. We demonstrate that a diffusion-controlled adsorption occurs at low bulk protein concentration for helianthinin whereas this mechanism occurs whatever the bulk concentration for gliadins. Surface pressure-induced film relaxation through conformation changes of proteins at the air-water interface is identified for gliadin whereas surface aggregation is observed at high helianthinin concentration. Overall, our experimental results highlight that structural flexibility of proteins appears as a key factor for their interfacial activity.

La possibilité pour un fluide viscoélastique de se fracturer a été mise en évidence pour la première fois dans les années 60 [1]] mais reste beaucoup moins documentée [2] que l’étude de la fracturation des matériaux solides Les hydrogels transitoires représentent une classe de matériaux viscoélastiques particulièrement bien adaptée pour l’investigation de fractures dans les fluides viscoélastiques, car leurs temps de relaxation et leur modules élastiques sont facilement modulables. Nous illustrons quelques aspects de la nucléation et de la propagation de fractures dans des réseaux transitoires modèles constitués de micelles se tensio-actifs pontées par des polymères téléchéliques en solution aqueuse. Deux géométrie seront considérées : cellule de Hele-Shaw [3] et rhéomètrie extensionnelle [4].[1] JF Hutton. Nature 200 (1963) 646
[2] C. Ligoure and S.Mora, Rheologica Acta, 52 (2013) 91-114 [3] , Foyart G., Ligoure C., Mora S., Ramos L. ACS Macro Letters, 5 (2016) 1080-1083 [4] , Arora S., Shabbir A., Hassager O., Ligoure C., Ramos L., JOR 61 (2017).1267-1275

The origin of the unique rheological properties of wheat gluten, the water-insoluble protein fraction of wheat grain, is crucial in bread-making processes and questions scientists since the 18th century. Gluten is a mixture of two types of proteins, monomeric gliadins and polymeric glutenins, formed by subunits linked via disulfide bonds. To better understand the respective role of gliadins (Gli) and glutenins (Glu) in the supramolecular structure of gluten and its link to rheology, we have developed a model gluten system using a food-grade solvent, which allows quantitative comparisons with theoretical models for synthetic polymers [1]. Thanks to an experimental protocol based on a liquid-liquid phase separation, we can extract, from industrial gluten, gluten protein fractions with contrasted mass ratios Glu/Gli (from 0.04 to 2). Stable gluten protein suspensions in a water/ethanol mixture solvent over a wide range of concentrations can be produced. Using scattering techniques, we demonstrate that blends of Gli and Glu exhibit features characteristic of branched flexible polymer chains in a good solvent, and form in a dilute regime large Glu-rich protein assemblies [2]. In the semi-dilute regime, we evidence the existence of large zones (of typical size 60 nm) rich in hydrogen bonds [3]. We show that hydrogen bonds are responsible for the spontaneous sample gelation, whose concentration dependence can be quantitatively accounted using percolation theories developed for polymer gels [4]. Overall, our experimental results illustrate the relevance of model gluten gels to unravel the unique and complex behavior of gluten. [1] Banc, Dahesh, Wolf, Morel, Ramos. Journal of Cereal Science 75, 175 (2017). [2] Dahesh, Banc, Duri, Morel, Ramos. J Phys Chem B 118, 11065 (2014). [3] Banc, Charbonneau, Dahesh, Appavou, Fu, Morel, Ramos. Soft Matter 12, 5340 (2016). [4] Dahesh, Banc, Duri, Morel, Ramos. Food Hydrocolloids, 52, 1 (2016).