Challenges of public health and sustainable development require replacing in food producs animal proteins by plant proteins. In this optics, it is crucial to understand the structure and kinetic of formation of a film of plant proteins film in order to improve the control of emulsions and foams stabilized by these proteins. In this talk I will present experimental results on the interfacial properties of wheat gluten and sunflower proteins. Thanks to a combination of tensiometry, viscoelasticity and ellipsometry, a consistent and rational physical picture of the dynamics of the interfacial properties is achieved. For gluten proteins, a time-concentration superposition of the data is evidenced whatever the subphase concentration, which reveals that protein adsorption at the interface is dominated by bulk diffusion. We propose a consistent physical picture of the multistep diffusion-controlled irreversible adsorption of the gliadin proteins at an air/water interface [1]. Our data shows clearly three different regimes for the film formation (Figure 1). In the first regime, the film elasticity and surface protein concentration increase concomitantly due to the progressive coverage of the interface by proteins. In the second regime, an increase of the dissipative viscoelastic properties is associated to an anomalous evolution of the optical profile that we attribute to conformational changes of the proteins at the interface induced by surface pressure. In the last stage, at even higher surface pressure, the optical profile is not modified while the elasticity of the interfacial layer dramatically increases presumably due to the film gelation as the result of the formation of intermolecular bonds. Overall all our experimental results indicate that wheat gluten displays behaviour typical of soft proteins due to their structural flexibility. Sunflower proteins, by contrast, can be considered as hard proteins, and as such, do not reorganize once adsorbed at an interface and display a simpler film formation dynamics. In addition the failure at high concentration of the time-concentration superposition of the tensiometry and viscoelastic data strongly suggest an aggregation process.

We study the fatigue of a soft glass submitted to repeated oscillatory shear deformation using a unique instrument that simultaneously probes the mechanical response of the sample and its microscopic dynamics. The soft glass, a dense packing of microgel particles, exhibits at rest a spontaneous ballistic dynamics, characterized by compressed exponential relaxations, in line with rearrangements being due to the relaxation of internal stresses in a soft solid. Oscillatory shear measurements show a characteristic single step yielding process. To better understand the microscopic origin of yielding, we couple dynamic light scattering to shear rheology and track both the reversible non affine dynamics and the decay of higher order correlation echoes, up to 500 cycles. Several regimes are found by increasing the shear amplitude. Unperturbed spontaneous solid-like relaxation is measured in the linear regime. In this regime, the non-affinities are fullyreversible and originate presumably from spatial fluctuations of the sample elastic modulus. By contrast, the non-affinities become partially irreversible in the non-linear regime and stem from plastic rearrangements. At the onset of non-linearity (γ~6%), we find that dynamics accelerates sharply but still exhibit a solid-like compressed exponential relaxation. In the fully fluidized regime (γ>30%), the microscopic dynamics is however qualitatively different and exhibits a stretched exponential decay, characteristic of a supercooled liquid. Interestingly, we find over a relatively broad range of strain amplitude, which macroscopically corresponds to the regime of a prominent loss peak in the rheology data, the coexistence of a fast liquid-like mode and a slower solid-like mode. Overall, our experimental results that combine macroscopic and microscopic data provide a rational scenario for the fatigue yielding of a heterogeneous soft solid.

Interactions and assemblies of wheat proteins

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.

We investigate the dynamics of thin sheets freely expanding in air. The sheets are produced by impacting a drop onto a small solid target or onto a cushion of liquid nitrogen, in order to suppress any dissipation process. To disentangle the role of capillary, viscous and elastic forces in the dynamics of the sheets several materials are used, whose rheological characteristics are tuned over several orders of magnitude: viscous liquids, viscoelastic Maxwell fluids and ultrasoft solids. In this talk, I will present our approaches to rationalize the spreading of sheets for viscous, viscoelastic and elastic materials by taking into account surface tension, elastic deformation and shear and biaxial viscous dissipations.