Material failure is ubiquitous on length scales ranging from a few nanometers, as in fracture of atomic or molecular systems, up to geological scales, as in earthquakes. The detection of any precursors that may point to incipient failure is the Holy Grail in many disciplines, from material science to engineering and geology. Material failure is particularly relevant to soft matter systems (colloids, emulsions, polymers etc.), which are an ideal benchmark to investigate how mechanical stress impacts condensed matter. Indeed, soft matter is very susceptible to even modest mechanical loads and most soft materials can be conveniently investigated by powerful optical methods such as microscopy or light scattering.We have built up a shear-cell that allows us to couple rheology to light scattering measurements on soft solids. In this talk I will focus on a fractal colloidal gel produced by in-situ aggregation of silica nanoparticles in water and show how reversible and irreversible (microscopic and macroscopic) deformations, build up when the gel is submitted to a shear stress. Thanks to our set-up, we are able to elucidate the physical processes at play at the onset of yielding.

Liquid sheets that freely expand in air can be produced by the impact of a small drop of liquid on a cylindrical target of size comparable to that of the drop. In this talk I will illustrate with two examples how the imaging with a fast camera of the dynamics of the sheets thus formed can shed light on fundamental and industrial issues. On the one hand, we quantify the interplay between elasticity and viscosity and the effects of a strong and fast deformation on viscoelastic droplets. On the other hand, we rationalize the role of dilute emulsions in the destabilization mechanism of liquid sheets and its consequence for anti-drift phenomena in agricultural sprays.

Liquid sheets that freely expand in air can be produced by the impact of a small drop of liquid on a cylindrical target of size comparable to that of the drop. In this talk I will illustrate with two examples how the imaging with a fast camera of the dynamics of the sheets thus formed can shed light on fundamental and industrial issues. On the one hand, we quantify the interplay between elasticity and viscosity and the effects of a strong and fast deformation on viscoelastic droplets. On the other hand, we rationalize the role of dilute emulsions in the destabilization mechanism of liquid sheets and its consequence for anti-drift phenomena in agricultural sprays.

Structuring wheat gluten proteins into gels with tunable mechanical properties would provide more versatility for the production of plant protein-rich food products. Gluten, a strongly elastic protein material insoluble in water, is hardly processable. We use a novel fractionation procedure allowing the isolation from gluten of a water/ethanol soluble protein blend, enriched in glutenin polymers at an unprecedented high ratio (50%). We investigate here the viscoelasticity of suspensions of the protein blend in a water/ethanol (50/50 v/v) solvent, and show that, over a wide range of concentrations, they undergo a spontaneous gelation driven by hydrogen bonding. We successfully rationalize our data using percolation models and relate the viscoelasticity of the gels to their fractal dimension measured by scattering techniques. The gluten gels display self-healing properties and their elastic plateaus cover several decades, from 0.01 to 10,000 Pa. In particular very soft gels as compared to standard hydrated gluten can be produced.

The destabilization of free liquid sheets is of great practical importance for aerosol dispersions. The disintegration of a sheet through the formation of holes was first mentioned by Dombrowski in the 50's and has been later reported for different types of complex fluids, including surfactant solutions, surfactant-stabilized air bubbles, and dilute emulsions. However, despite its relative ubiquity, the physical mechanisms at the origin of the perforation process have not been conclusively elucidated so far. We study the destabilization of a freely expanding sheet resulting from the impact of a single drop of fluid onto a small target, and show that dilute oil-in-water emulsion-based sheets disintegrate through the nucleation and growth of holes. We show that the velocity, v, and thickness, h, fields of the sheet are not perturbed by the presence of holes, and follow the scaling expected for a plain inviscid fluid, v∼r/t and h ∼ 1/rt, with t the time elapsed since the drop impact and r the radial position. In addition the velocity Vc of the opening of holes is constant and quantitatively follows a Taylor-Culick law, V_c=√(2γ/(ρh)), with ρ the density, and γ the surface tension of the emulsion. We demonstrate that each perforation event is preceded by a pre-hole that thins out the sheet and widens with time. The pre-hole dynamics follows a powerlaw evolution, with an exponent ¾, theoretically predicted for a liquid spreading on another liquid of higher surface tension due to Marangoni stresses. The surface tension gradient stress is counterbalanced by a viscous stress that drags the subsurface fluid, whose flow causes a local thinning of the film leading ultimately to its rupture. Quantitative comparisons between the spreading dynamics of pre-holes and that of a drop of the emulsion oil phase deposited on the emulsion aqueous phase confirm this physical picture.

The destabilization of free liquid sheets is of great practical importance for aerosol dispersions. The disintegration of a sheet through the formation of holes was first mentioned by Dombrowski in the 50's and has been later reported for different types of complex fluids, including surfactant solutions, surfactant-stabilized air bubbles, and dilute emulsions. However, despite its relative ubiquity, the physical mechanisms at the origin of the perforation process have not been conclusively elucidated so far. We study the destabilization of a freely expanding sheet resulting from the impact of a single drop of fluid onto a small target, and show that dilute oil-in-water emulsion-based sheets disintegrate through the nucleation and growth of holes. We show that the velocity, v, and thickness, h, fields of the sheet are not perturbed by the presence of holes, and follow the scaling expected for a plain inviscid fluid, v∼r/t and h ∼ 1/rt, with t the time elapsed since the drop impact and r the radial position. In addition the velocity Vc of the opening of holes is constant and quantitatively follows a Taylor-Culick law, V_c=√(2γ/(ρh)), with ρ the density, and γ the surface tension of the emulsion. We demonstrate that each perforation event is preceded by a pre-hole that thins out the sheet and widens with time. The pre-hole dynamics follows a powerlaw evolution, with an exponent ¾, theoretically predicted for a liquid spreading on another liquid of higher surface tension due to Marangoni stresses. The surface tension gradient stress is counterbalanced by a viscous stress that drags the subsurface fluid, whose flow causes a local thinning of the film leading ultimately to its rupture. Quantitative comparisons between the spreading dynamics of pre-holes and that of a drop of the emulsion oil phase deposited on the emulsion aqueous phase confirm this physical picture.

SANS study of a gluten protein gel: Why are SAXS and SANS profiles diffrents?