We investigate freely expanding liquid sheets made of either simple Newtonian fluids or solutions of high molecular water-soluble polymer chains. A sheet is produced by the impact of a drop on a quartz plate covered with a thin layer of liquid nitrogen that suppresses shear viscous dissipation thanks to an inverse Leidenfrost effect. The sheet expands radially until reaching a maximum diameter and subsequently recedes. Experiments indicate the presence of two expansion regimes: the capillary regime, where the maximum expansion is controlled by surface tension forces and does not depend on the viscosity, and the viscous regime, where the expansion is reduced with increasing viscosity. In the viscous regime, the sheet expansion for polymeric samples is strongly enhanced as compared to that of Newtonian samples with comparable zero-shear viscosity. We show that data for Newtonian and non-Newtonian fluids collapse on a unique master curve where the maximum expansion factor is plotted against the relevant effective \textit{biaxial extensional} Ohnesorge number that depends on fluid density, surface tension and the biaxial extensional viscosity. For Newtonian fluids, this biaxial extensional viscosity is six times the shear viscosity. By contrast, for the non-Newtonian fluids, a characteristic \textit{Weissenberg number}-dependent biaxial extensional viscosity is identified, which is in quantitative agreement with experimental and theoretical results reported in the literature for biaxial extensional flows of polymeric liquids.

Wheat gluten, one of the most complex viscoelastic protein networks in nature, is unique to get the specific texture of bread. Due to its complex protein composition, its insolubility in most solvents and the very high molar mass of half of the proteins (glutenin, the other half being gliadin), the architecture of the network is still not well understood. In this work, we have investigated model gluten protein extracts with contrasted compositions in glutenin and gliadin solubilized in a mild chaotropic solvent: ethanol/water (50/50 v/v). The samples display a liquid-liquid phase separation with an upper critical solution temperature that depends on the protein composition. The phase diagrams are consistent with the presence of supramolecular assemblies of proteins. To confirm the presence of these assemblies and fully characterize the objects dispersed in ethanol/water, we have used an asymmetrical flow field-flow fractionation (AsFlFFF) setup coupled with differential refractive index, multi-angle light scattering and dynamic light scattering detections to probe very dilute protein suspensions. We have identified three classes of objects, with distinctive molar mass, characteristic size and conformation: protein monomers, polymeric structures, and very loose protein assemblies with molar mass larger than 2.10 6 g/mol. A molecular characterization of the species by size exclusion chromatography in a denaturing solvent shows that polymers and assemblies are mainly composed of glutenin and ω-gliadin. The high content of ω-gliadin, devoid of cysteines, indicates the importance of non-covalent interactions involved in protein assemblies and might play a major role in gluten rheology

The mechanical properties of soft matter are of great importance in countless applications, in addition of being an active field of academic research. Given the relative ease with which soft materials can be deformed, their non-linear behavior is of particular relevance. Large loads eventually result in material failure. In this Perspective article, we discuss recent work aiming at detecting precursors of failure by scrutinizing the microscopic structure and dynamics of soft systems under various conditions of loading. In particular, we show that the microscopic dynamics is a powerful indicator of the ultimate fate of soft materials, capable of unveiling precursors of failure up to thousands of seconds before any macroscopic sign of weakening.