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.

The complete understanding of the mechanism driving colloid-polyelectrolyte complexation still represents a fundamental problem of great interest in soft matter. Polyelectrolyte adsorption onto oppositely charged surfaces represents the core of this problem and a number of theoretical studies, using different approaches, have been published on this subject.During the past few decades, colloid-multivalent ion complexation has been investigated by using either model systems, such as solid hard colloids, soft colloids of biological interests, or hydrophilic globular proteins6. In all cases two distinct but intimately related phenomena accompany and drive the self-assembly, i.e. charge-inversion and reentrant condensation.Although these phenomena have been observed in a variety of polyelectrolyte-colloid mixtures in different conditions, in all previously reported works the charge density on the colloid surface was fixed, or, at least, it could not be changed without changing the ionic strength or the pH of the suspending medium. Thermoresponsive microgels, whose synthesis is initiated by charged groups and is equivalent to an end-group functionalization, are characterized by a thermodynamic volume phase transition (VPT) that gives the opportunity to tune finely the adsorption of polyelectrolytes simply by changing temperature. Indeed, by controlling the particle volume, the VPT affects dramatically the microgel charge density and hence the polyelectrolyte adsorption.We investigated the complexation of thermoresponsive anionic poly(Nisopropylacrylamide) (PNiPAM) microgels and cationic -polylysine (epsilon-PLL) chains. By combining electrophoresis, light scattering, transmission electron microscopy (TEM) and dielectric spectroscopy (DS) we studied the adsorption of epsilon-PLL onto the microgel networks and its effect on the stability of the suspensions. We show that the volume phase transition (VPT) of the microgels triggers a large polyion adsorption. Two interesting phenomena with unique features occur: a temperature-dependent microgel overcharging and a complex reentrant condensation. The latter may occur at fixed polyion concentration, when temperature is raised above the VPT of microgels, or by increasing the number density of polycations at fixed temperature. TEM and DS measurements unambiguously show that short PLL chains adsorb onto microgels and act as electrostatic glue above the VPT. By performing thermal cycles, we further show that polyion-induced clustering is a quasi-reversible process: within the time of our experiments large clusters form above the VPT and partially re-dissolve as the mixtures are cooled down.Finally we give a proof that the observed phenomenology is purely electrostatic in nature: an increase of the ionic strength gives rise to the polyion desorption from the microgel outer shell. By showing that the VPT of thermoresponsive ionic microgels can be employed to trigger polyion adsorption and tune reentrant microgel condensation, our work lays the foundation for a groundbreaking strategy to tune electroadsorption ruled by temperature and that can be employed in a variety of fields spanning wastewater and soil remediation, nanoencapsulation of small charged nanoparticles, and selective drug delivery.

By combining rheology, light scattering, small angle X-ray scattering and simulations we explore the glassy dynamics of soft colloids using microgels and charged particles interacting by steric and screened Coulomb interactions, respectively. In the supercooled regime, the structural relaxation time (SRT) of both systems grows steeply with volume fraction, reminiscent of the behavior of colloidal hard sphere systems [1]. This suggests that softness has no impact on the growth of the SRT on approaching the glass transition, as confirmed by computer simulations. We show that the onset of a finite yield-stress, occurring within the supercooled regime, systematically corresponds to a minimum of the stretching exponent characterizing the decay of the two-time intensity autocorrelation function.Softness becomes relevant only at very large packing fractions, when the system falls out of equilibrium. In this non-equilibrium regime, the SRT depends surprisingly weakly on packing fraction and time correlation functions exhibit compressed exponential decays consistent with internal stress-driven relaxation. The transition to this novel regime coincides with the onset of an anomalous decrease of local order with increasing density, reminiscent of the reentrant glass transition predicted theoretically in ultrasoft systems [2,3], and corresponds to a weakening of the volume fraction dependence of the plateau modulus of the suspensions. We propose that these peculiar dynamics result from a competition between the non-equilibrium aging dynamics of the glassy state and the tendency of soft systems to refluidize at high packing fractions.

In our work we report the measurement of non-equilibrium interfacial tension of polymer and hard sphere suspensions in contact with their own solvent. By visualizing fingering instability (VF) in radial Hele-Shaw geometry, appearing when the solvent displaces suspensions of colloids or polymers, we measure interfacial tensions in function of the volume fraction of the suspended objects, showing that the internal degrees of freedom of the particles drive the low volume fraction behavior (Figure 1). Our results support the existence of a positive tension between miscible fluids, confirm the quadratic scaling predicted by Korteweg [4] for long linear and crosslinked polymers and show a positive rapidly growing tension for hard sphere suspensions up to maximum packing, whose description necessitates a theoretical framework going beyond the classic square gradient model. We rationalize our findings assuming the suspension/solvent interface in local thermodynamic equilibrium, computing explicitly the square gradient contribution to the interfacial tension for polymer/solvent and simple molecular liquid mixtures and proposing a phenomenological model capturing the compositional dependence of the interfacial tension for large concentration gradients. Finally we include and analyze data reported in literature and obtained via spinning drop tensiometry that validate the model and we propose the analysis of fluid dynamic instability as a new tool to probe interfacial stresses.