The constituents of soft matter systems such as colloidal suspensions, emulsions, polymers, and biological tissues undergo microscopic random motion, due to thermal energy. They may also experience drift motion correlated over mesoscopic or macroscopic length scales, \textit{e.g.} in response to an internal or applied stress or during flow. We present a new method for measuring simultaneously both the microscopic motion and the mesoscopic or macroscopic drift. The method is based on the analysis of spatio-temporal cross-correlation functions of speckle patterns taken in an imaging configuration. The method is tested on a translating Brownian suspension and a sheared colloidal glass.

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.

We study the freezing kinetics of colloidal polycrystals made of micelles of Pluronic F108, a thermosensitive copolymer, to which a small amount of silica nanoparticles of size comparable to that of the micelles are added. We use rheology and calorimetry to measure Tc, the crystallization temperature, and find that Tc increases with the heating rate \dot{T} used to crystallize the sample. To rationalize our results, we first use viscosity measurements to establish a linear mapping between temperature T and the effective volume fraction, {\phi}, of the micelles, treated as hard spheres. Next, we reproduce the experimental \dot{T} dependence of the crystallization temperature with numerical calculations based on standard models for the nucleation and growth of hard spheres crystals, classical nucleation theory and the Johnson-Mehl-Avrami-Kolmogorov theory. The models have been adapted to account for the peculiarities of our experiments: the presence of nanoparticles that are expelled in the grain boundaries, and the steady increase of T and hence {\phi} during the experiment. We moreover show that the polycrystal grain size obtained from the calculations is in good agreement with light microscopy data. Finally, we find that the {\phi} dependence of the nucleation rate for the micellar polycrystal is in remarkable quantitative agreement with that found in previous experiments on colloidal hard spheres. These results suggests that deep analogies exist between hard-sphere colloidal crystals and Pluronics micellar crystals, in spite of the difference in particle softness. More generally, our results demonstrate that crystallization processes can be quantitatively probed using standard rheometry.

We use x-ray photon correlation spectroscopy to investigate the structural relaxation process in a metallic glass on the atomic length scale. We report evidence for a dynamical crossover between the supercooled liquid phase and the metastable glassy state, suggesting different origins of the relaxation process across the transition. Furthermore, using different cooling rates, we observe a complex hierarchy of dynamic processes characterized by distinct aging regimes. Strong analogies with the aging dynamics of soft glassy materials, such as gels and concentrated colloidal suspensions, point at stress relaxation as a universal mechanism driving the relaxation dynamics of out-of-equilibrium systems.

We use novel space and time resolved light scatterering methods to investigate the dynamics of colloidal suspensions under gravitational stress. In the first part of my talk, I'll discuss the behavior of colloidal gels, where the volume fraction is moderate (phi~0.05-0.2) and attractive interactions are induced via the depletion mechanism. I'll show that a single parameter, the time dependent compressive strain rate, rules both the macroscopic settling of the gel and its microscopic dynamics [1]. Additionally, gels prepared at various volume fractions and interaction strength exhibit a universal beahvior in that their asymptotic concentration profile can be scaled onto a master curve [2]. In the second part of my talk, I'll discuss nearly neutrally boyant suspensions of colloidal spheres, to which small amounts of fast-settling probe spheres are added. The focus will be on the volume fraction dependence of the suspension viscosity and its relationship with the structural relaxation time. [1] G. Brambilla, S. Buzzaccaro, R. Piazza, L. Berthier, L. Cipelletti, Highly nonlinear dynamics in a slowly sedimenting colloidal gel, Phys. Rev. Lett. 106, 118302 (2011) [2] S. Buzzaccaro, E. Secchi, G. Brambilla, R. Piazza and L. Cipelletti, Equilibrium concentration profiles and sedimentation kinetics of colloidal gels under gravitational stress, J. Phys.: Condens. Matter 24 284103 (2012)

We use confocal microscopy, rheology and light scattering to investigate the structure and the plasticity of a composite material, formed by Pluronics F108, a commercial triblock copolymer to which small amounts of colloidal nanoparticles are added. At sufficiently high polymer concentration and temperature, the block copolymer forms micelles that are regularly arranged on a FCC lattice, while the nanoparticles are preferentially segregated in the grain boundaries between micelle crystallites. We show that both the segregation efficiency and the grain size can be tuned by varying the speed at which the sample is solidified and the nanoparticle amount. We rationalize our findings by extending classical theories for nucleation and growth of crystals to account for the presence of the nanoparticles. We moreover investigate material fatigue and plasticity by measuring microscopic rearrangements in our composite material submitted to thousands of deformation cycles. Light scattering data collected at several scattering vectors and for various deformation amplitudes can be scaled onto a single master curve, exhibiting two regimes. In the first regime, the shear-induced dynamics become increasingly slower with time ("aging"), while a stationary regime where plasticity does not evolve anymore is eventually reached. Remarkably, the cross-over between the aging and the stationary regime depends on the probed length scale, larger length scales reaching the stationary state earlier. We propose a simple physical picture to rationalize these results.

We use confocal microscopy, rheology and light scattering to investigate the structure and the plasticity of a composite material, formed by Pluronics F108, a commercial triblock copolymer to which small amounts of colloidal nanoparticles are added. At sufficiently high polymer concentration and temperature, the block copolymer forms micelles that are regularly arranged on a FCC lattice, while the nanoparticles are preferentially segregated in the grain boundaries between micelle crystallites. We show that both the segregation efficiency and the grain size can be tuned by varying the speed at which the sample is solidified and the nanoparticle amount. We rationalize our findings by extending classical theories for nucleation and growth of crystals to account for the presence of the nanoparticles. We moreover investigate material fatigue and plasticity by measuring microscopic rearrangements in our composite material submitted to thousands of deformation cycles. Light scattering data collected at several scattering vectors and for various deformation amplitudes can be scaled onto a single master curve, exhibiting two regimes. In the first regime, the shear-induced dynamics become increasingly slower with time ("aging"), while a stationary regime where plasticity does not evolve anymore is eventually reached. Remarkably, the cross-over between the aging and the stationary regime depends on the probed length scale, larger length scales reaching the stationary state earlier. We propose a simple physical picture to rationalize these results.