Virtually all real-life crystalline materials have defects. In particular, most metals and ceramics are aggregates of crystalline grains. Grain-boundaries (GBs), the two-dimensional lattice defects that separate the different grains of a crystal, control the mechanical properties of polycrystalline materials. Although GB motion is known to play important roles in plastic (i.e. irreversible) deformation, the microscopic origin of the plasticity of polycrystalline materials is still largely unknown, because of the limitations of available experimental tools to record, during deformation, the dynamics of the process with a nanometer resolution. To overcome these limitations, we use a colloidal analog of atomic polycrystals obtained by doping a copolymer micellar crystal with nanoparticles. Nanoparticles act as impurities, and as such, they segregate in the grain boundaries of the colloidal polycrystal, allowing their visualization by light and confocal microscopy and by scattering techniques. In addition, the microstructure of the polycrystal can be tuned by varying the nanoparticle volume fraction and the crystallization rate. To investigate the plasticity of colloidal polycrystals, we perform multispeckle time-resolved dynamic light scattering measurements on the samples submitted to cyclic shear deformations using a novel light scattering apparatus specifically designed to access the dynamics of the network of GBs. Plasticity is quantified by analyzing the correlation of the scattered intensity measured after a given number of deformation cycles. We demonstrate that the shear-induced GB dynamics at the origin of plasticity is aging in a length-scale dependence manner. We extract a characteristic length above which the GB dynamics is stationary, which is found to increase with the shear amplitude. Our data suggest a hierarchical organization of the GB network under shear and provides a novel framework to understand the plasticity of polycrystals and of disordered materials in general.

Fracture processes are ubiquitous in soft materials, even in complex fluids, subjected to stresses. To investigate these processes in a simple geometry, we use a model self-assembled transient gel and study the instability patterns obtained in a radial Hele-Shaw cell when a low viscosity oil pushes the more viscous transient gel. Thanks to an analysis of the morphology of the patterns, we find a discontinuous transition between the standard Saffman-Taylor fingering instability and a fracturing instability as the oil injection rate increases. Our data suggest that the flow properties of the gel ahead of the finger tip controls the transition towards fracturing. By analyzing the displacement field of the gel in the vicinity of the fingers and cracks, we show that in the fingering regime, the oil gently pushes the gel, whereas in the fracturing regime, the crack tears apart the gel, resulting in a strong drop of the gel velocity ahead of the crack tip as compared to the tip velocity. We find a unique behavior for the whole displacement field of a gel around a crack, which is drastically different from that around a finger, and reveals the solid-like behavior of the gel at short time. Our experiments and analysis provide quantitative yet simple tools to unambiguously discriminate a finger from a crack in a visco-elastic material.

Crack initiation and propagation have been previously studied in our group for model complex fluids using pendant drop experiments [1]. We will present here results obtained in a radial Hele-Shaw cell obtained when a low viscosity oil pushes a more viscous complex fluid, resulting in instabilities of the oil/gel interface. We use as complex fluid, we use a soft transient network gel made of an oil in water microemulsion reversibly link by telechelic polymers. We find a discontinuous transition between the standard Saffman-Taylor fingering instability and a fracturing instability as the oil injection rate increases. We show that the transition towards fracturing is driven by the rheological properties of the gel ahead of the finger tip. By analyzing the displacement field of the gel in the vicinity of the finger or crack, we show that in the fingering regime, the oil gently pushes the gel, whereas in the fracturing regime, the crack tears apart the gel, resulting in a strong drop of the gel velocity ahead of the crack tip as compared to the tip velocity. We find a universal behavior for the whole displacement field of a gel around a crack, which is drastically different from that around a finger, and reveals the solid-like behavior of the gel at short time. We will also present preliminary results obtained with another class of transient gels with tunable morphology for which rheology data suggest a brittle to ductile transition [4]. Thanks to the anisotropy of these gels, birefringence measurements can be performed , allowing one to relate the crack properties to the structural properties of the complex fluid in the vicinity of the cracks. [1] H. Tabuteau, S. Mora, M. Ciccotti, C. Hui, C. Ligoure, Soft Matter 7, 9474 (2011) [2] C. Ligoure, S. Mora, Rheologica Acta 52, Issue 2, 91-114 (2013) [3] H. Zhao, Maher JV., Phys.Rev. E 47, 4278 (1993) [4] L. Ramos, A. Laperrousaz, P. Dieudonné, C. Ligoure, Phys.Rev.Lett. 107, 148302 (2011)

Wheat gluten proteins are insoluble in water and are among the most complex protein networks in nature. In spite of their extensively use in particular for bread making, their viscoelastic properties and their structuration are still largely unknown. Here we report the visco-elastic behavior of model systems composed of gluten proteins near the liquid-solid transition. We build model systems by dispersing in ethanol-water mixtures two major protein groups, gliadin and glutenin, that we have purified from gluten. We find a slow evolution over time scales of the order of days of the linear frequency dependence complex modulus of the samples. Interestingly, we find that all data acquired at different protein concentrations and different times after sample preparation can be scaled onto a master curve, showing a cross-over from a soft solid at low frequency to a visco-elastic fluid at high frequency. We are currently using scattering techniques to unveil the peculiar structure of the protein network that would lead to the self-similar visco-elasticity.

Most solids like metals and ceramics are polycrystals, i.e. aggregates of crystalline grains separated by 2D defects, the grain boundaries (GBs). Although GB sliding is believed to be involved in the irreversible deformation (plasticity) of polycrystalline solids, the microscopic mechanisms at play are still poorly understood, partly because common techniques for atomic polycrystals visualization preclude any information on their dynamics under stress. We propose a colloidal analog of atomic polycrystals obtained by doping a copolymer micellar crystal with nanoparticles (NPs) and for which the grain size is tuned by changing the crystallization rate. During the crystallization NPs are partially expelled from the lattice and segregated into the GBs allowing one to probe their structure with confocal microscopy (Fig.1) or light scattering (Fig.2). Moreover, the NPs structure into the GBs is probed with neutron scattering (Fig.2). In order to investigate the plasticity of the colloidal polycrystals, dynamic light scattering measurements on the sample submitted to cyclic deformation are performed using a novel light scattering apparatus specifically designed to access the dynamics of the GBs. Plasticity is quantified by analyzing the correlation of the scattered intensity measured after a given number of deformation cycles. We demonstrate that the shear-induced GB dynamics at the origin of plasticity is aging in a manner that depends on the probed length-scale. We extract a characteristic length above which the GB dynamics is stationary. This characteristic length is found to increase with the shear amplitude. Our data suggest a hierarchical organization of the GB network under shear and provides a novel framework to understand the plasticity of polycrystals.

not available

not available