I'll review recent experiments on the slow dynamics of colloidal hard spheres, a model system for the glass transition, and other soft materials, where particle interactions and/or high particle concentration lead to jamming. I'll describe new scattering methods to investigate not only the average dynamics, but also their spatial and temporal fluctuations. In particular, I'll show that while in colloidal hard spheres spatial correlations of the dynamics increase only modestly on approaching the glass transition, in other jammed materials the slow dynamics are correlated over macroscopic length scales.

We use optical microscopy to investigate the yielding mechanism of concentrated emulsions submitted to an oscillatory strain. While rheological measurements suggest a smooth transition from linear elasticity to plastic behavior as the amplitude of the imposed strain is increased, direct imaging of the sheared emulsions reveals that at a microscopic level yielding occurs abruptly. Indeed, we find that a well-defined critical strain c separates a regime where no irreversible droplet rearrangements occur during oscillatory strain from a plastic regime where almost all droplets undergo irreversible displacements. Above the volume fraction at which droplets achieve random close packing, the amplitude of the critical strain c increases linearly with droplet volume fraction. Surprisingly, we find that yielding is associated with very small droplet displacements, with almost no structural rearrangement. These rearrangement events are very heterogeneous both in space and time, with bursts of plastic activity separating long quiescent periods. The typical size of the rearranged regions is modest at random close packing and increases with increasing droplet concentration.

We use a combination of original light scattering techniques and particles with unique optical properties to investigate the behavior of suspensions of attractive colloids under gravitational stress, following over time the concentration profile, the velocity profile, and the microscopic dynamics. During the compression regime, the sedimentation velocity grows nearly linearly with height, implying that the gel settling may be fully described by a (time-dependent) strain rate. We find that the microscopic dynamics exhibit remarkable scaling properties when time is normalized by strain rate, showing that the gel microscopic restructuring is dominated by its macroscopic deformation.

Crosslinked and bundled actin filaments form networks that are essential for the mechanical properties of living cells. Reconstituted actin networks have been extensively studied not only as a model system for the cytoskeleton, but also to understand the interplay between microscopic structure and macroscopic viscoelastic properties of network-forming soft materials. These constitute a broad class of materials with countless applications in science and industry. So far, it has been widely assumed that reconstituted actin networks represent equilibrium structures. Here, we show that fully polymerized actin/fascin bundle networks exhibit surprising age-dependent changes in their viscoelastic properties and spontaneous dynamics, a feature strongly reminiscent of out-of-equilibrium, or glassy, soft materials. Using a combination of rheology, confocal microscopy and space-resolved dynamic light scattering, we demonstrate that actin networks build up stress during their formation and then slowly relax towards equilibrium owing to the unbinding dynamics of the crosslinking molecules.