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 [1,2]. 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. [1] G. Brambilla et al., Phys. Rev. Lett. 106, 118302 (2011). [2] "Gels settle down", in Physics, Spotlighting exceptional research http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.118302

The structure and rheology of polycrystals is relevant in particular for metals, where post mortem studies are usually performed. In colloidal crystalline systems, the accessible size range makes in-situ investigations possible, under flow and at rest. In this project, termed ‘colloidal metallurgy’, a polycrystalline material is formed spontaneously at high concentrations (34%wt) of spherical Pluronics micelles (F108) in water. The crystalline grains have a local FCC structure (lattice constant 30 nm), and are separated from neighbouring grains of different orientation by grain boundaries (GB), cf. Fig 1, where fluorescent nanoparticles of size comparable to the lattice parameter have been added to visualize the GBs. We have determined the size distribution of grains for different crystallization speeds and nanoparticle contents (typically below 1%). In particular, a universal relationship between grains size and thermal history, and between grain size and amount of nanoparticles has been found, in agreement with theoretical considerations based on classical nucleation theory and constitutional under-cooling. The microscopic structure of silica-containing polycrystals has been studied by contrast-variation small angle neutron scattering, highlighting either the particles or the polymer micelles. The local micellar structure is found to be unaffected by the nanoparticles, which are shown to segregate predominantly in dense zones, presumably GBs. Very small-angle light scattering is currently used to characterize the mesostructure, and the influence of deformation and flow is investigated in a confocal rheo set-up. An alternative view of our findings is that we present here a general approach to confine nanoparticles in colloidal materials in a controlled fashion, independent of the details of the surface chemistry.

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.

Composite materials, comprising nanoparticles dispersed in a matrix, are of great interest, since the nanoparticles can enhance the matrix properties or impart new functionalities and because the matrix can act as a template that structures the particles at the nanoscopic level. However, controlling the spatial distribution of nanoparticles remains a challenging task, as it usually depends crucially on the particles surface chemistry. We present here a general approach to confine nanoparticles in colloidal materials in a controlled fashion, independent of the details of the surface chemistry. We use a nanocomposite material obtained by dispersing small quantities of nanoparticles (at most 2%) in a colloidal crystalline matrix composed of thermosensitive micelles. The volume fraction of the micelles increases with temperature T, until crystallization occurs due to entropic reasons, as in hard sphere colloidal systems. Hence our system allows crystallization to be induced at the desired rate simply by varying T. By analogy with atomic crystals, we expect the nanoparticles, which act as impurities, to be partially expulsed from the growing lattice and to segregate in the grain boundaries. We use small angle neutron scattering to probe the microscopic structure of the composite materials. We show that nanoparticles do not perturb the crystalline structure of the micelles and that different crystallization rates lead to cubic crystals with the same lattice parameter (30 nm) but different nanoparticles rearrangements. In particular, we find that slow crystallization rates induce the formation of regions, up to about tenfold more concentrated in nanoparticles, without macroscopic phase-separation; these regions are the grain boundaries between crystallites. Consistent and complementary results are obtained by confocal microscopy, where the network of grain- boundaries enriched in nanoparticles can be visualized.

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