We investigate by scattering techniques the structure of water-based soft composite materials comprising a crystal made of Pluronic block-copolymer micelles arranged in a face-centered cubic lattice and a small amount (at most 2% by volume) of silica nanoparticles, of size comparable to that of the micelles. The copolymer is thermosensitive: it is hydrophilic and fully dissolved in water at low temperature (T ~ 0{\deg}C), and self-assembles into micelles at room temperature, where the block-copolymer is amphiphilic. We use contrast matching small-angle neuron scattering experiments to probe independently the structure of the nanoparticles and that of the polymer. We find that the nanoparticles do not perturb the crystalline order. In addition, a structure peak is measured for the silica nanoparticles dispersed in the polycrystalline samples. This implies that the samples are spatially heterogeneous and comprise, without macroscopic phase separation, silica-poor and silica-rich regions. We show that the nanoparticle concentration in the silica-rich regions is about tenfold the average concentration. These regions are grain boundaries between crystallites, where nanoparticles concentrate, as shown by static light scattering and by light microscopy imaging of the samples. We show that the temperature rate at which the sample is prepared strongly influence the segregation of the nanoparticles in the grain-boundaries.

We study the crystallization of a colloidal model system in presence of secondary nanoparticles acting as impurities. Using confocal microscopy, we show that the nanoparticles segregate in the grain boundaries of the colloidal polycrystal. We demonstrate that the texture of the polycrystal can be tuned by varying independently the nanoparticle volume fraction and the crystallization rate, and quantify our findings using standard models for the nucleation and growth of crystalline materials. Remarkably, we find that the efficiency of the segregation of the nanoparticles in the grain-boundaries is determined solely by the typical size of the crystalline grains.

Les techniques classiques de diffusion de la lumière sont des méthodes bien établies pour sonder la structure et les propriétés dynamiques d'un grand nombre de systèmes de la matière molle (suspensions, polymères, émulsions, phases de tensioactifs...). Une limite importante de ces méthodes est la nécessité de travailler dans le régime de diffusion simple, ce qui restreint leur emploi aux produits essentiellement transparents. Dans cet exposé, je discuterai la Diffusing Wave Spectroscopy (DWS), une extension de la diffusion de la lumière aux produits turbides. La puissance et la versatilité de cette méthode seront démontrées en s'appuyant sur des exemples d'applications, de la mesure de taille des particules, à l'étude de la stabilité des mousses, à la caractérisation des propriétés mécaniques (rhéologiques) de systèmes de la matière molle.

Composite materials, comprising nanoparticles (NPs) dispersed in a matrix, are of great interest, since the NPs can enhance the matrix properties or impart new functionalities and because the matrix can act as a template that structures the particles at the nanometer scale. However, controlling the spatial distribution of NPs remains a challenging task, as it usually depends crucially on the particles surface chemistry. We present here a general approach to confine NPs in colloidal materials in a controlled fashion. We design nanocomposite material obtained by dispersing small quantities of NPs (at most 2%) in a colloidal crystalline matrix composed of thermosensitive copolymer micelles. The volume fraction of the micelles increases with temperature T, until crystallization occurs due to entropic reasons. Hence our system allows crystallization to be induced at the desired rate simply by varying T. By analogy with atomic crystals, we expect the NPs, which act as impurities, to be partially expulsed from the growing lattice and to segregate in the grain boundaries. We use scattering techniques and confocal microscopy to probe the microscopic and mesoscopic structures of the composite materials. We show that the NPs do not perturb the crystalline structure of the micelles but that different crystallization rates lead to different NPs rearrangements. We show that the NPs segregate in the grain-boundaries of the copolymer polycrystal. We demonstrate that the texture of the polycrystal can be tuned by varying independently the nanoparticle volume fraction and the crystallization rate, and quantify our findings using classical nucleation theory. Remarkably, we find that the efficiency of the segregation of the NPs in the grain-boundaries is determined solely by the typical size of the crystalline grains.

Actin networks are an essential component of living cells, to which they impart the required mechnaical properties. Here, we study in-vitro cross-linked actin networks with a combination of rheology, microscopy and a novel space-resolved dynamic light scattering method. Our measurements suggest that the aging of the network is due to the progressive release of internal stresses, made possible by unbinding - rebinding events within the network.