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.

We investigate the fracture nucleation and propagation of reversible double transient networks, constituted of water solutions of entangled surfactant wormlike micelles reversibly linked by various amounts of telechelic polymers thus producing transient double
 networks when the micelles are sufficiently long and entangled. Two different geometries of fracture are considered: (i) For a filament stretching geometry, 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 nonlinear 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 nonlinear viscoelasticity [1].(ii) For a Hele-Shaw cell geometry based on the injection of a low viscosity fluid into the viscoelastic material confined between two plates, we show 
that cracks nucleate when the sample deformation rate involved is
 comparable to the inverse of the shortest relaxation time scale of the networks. For a double network, significant rearrangements of the micelles occur as a crack nucleates and propagates. We show that birefringence develops at the crack tip over a finite length, ξ, which corresponds to the length scale over which micelle alignment occurs. We find that ξ is larger for slower cracks, suggesting an increase of ductility.

We investigate freely expanding sheets formed by ultrasoft gel beads, and liquid and viscoelastic drops, produced by the impact of the bead or drop on a silicon wafer covered with a thin layer of liquid nitrogen that suppresses viscous dissipation thanks to an inverse Leidenfrost effect. Our experiments show a unified behavior for the impact dynamics that holds for solids, liquids, and viscoelastic fluids and that we rationalize by properly taking into account elastocapillary effects. In this framework, the classical impact dynamics of solids and liquids, as far as viscous dissipation is negligible, appears as the asymptotic limits of a universal theoretical description. A novel material-dependent characteristic velocity that includes both capillary and bulk elasticity emerges from this unified description of the physics of impact.

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.

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.