Large deformations of soft elastic beads spinning at high angular velocity in a denser background fluid are investigated theoretically, numerically, and experimentally using millimeter-size polyacry-lamide hydrogel particles introduced in a spinning drop tensiometer. We determine the equilibrium shapes of the beads from the competition between the centrifugal force and the restoring elastic and surface forces. Considering the beads as neo-Hookean up to large deformations, we show that their elastic modulus and surface energy constant can be simultaneously deduced from their equilibrium shape. Also, our results provide further support to the scenario in which surface energy and surface tension coincide for amorphous polymer gels.

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.

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 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.

La possibilité pour un fluide viscoélastique de se fracturer a été mise en évidence pour la première fois dans les années 60 [1]] mais reste beaucoup moins documentée [2] que l’étude de la fracturation des matériaux solides Les hydrogels transitoires représentent une classe de matériaux viscoélastiques particulièrement bien adaptée pour l’investigation de fractures dans les fluides viscoélastiques, car leurs temps de relaxation et leur modules élastiques sont facilement modulables. Nous illustrons quelques aspects de la nucléation et de la propagation de fractures dans des réseaux transitoires modèles constitués de micelles se tensio-actifs pontées par des polymères téléchéliques en solution aqueuse. Deux géométrie seront considérées : cellule de Hele-Shaw [3] et rhéomètrie extensionnelle [4].[1] JF Hutton. Nature 200 (1963) 646
[2] C. Ligoure and S.Mora, Rheologica Acta, 52 (2013) 91-114 [3] , Foyart G., Ligoure C., Mora S., Ramos L. ACS Macro Letters, 5 (2016) 1080-1083 [4] , Arora S., Shabbir A., Hassager O., Ligoure C., Ramos L., JOR 61 (2017).1267-1275