We use time-resolved dynamic light scattering to investigate the slow dynamics of a colloidal gel. The final decay of the average intensity autocorrelation function is well, with τ f ∼ q −1 and p decreasing from 1.5 to 1 with increasing q. We show that the dynamics is not due to a continuous ballistic process, as proposed in previous works, but rather to rare, intermittent rearrangements. We quantify the dynamical fluctuations resulting from intermittency by means of the variance χ(τ, q) of the instantaneous autocorrelation function, the analogous of the dynamical susceptibility χ4 studied in glass formers. The amplitude of χ is found to grow linearly with q. We propose a simple -yet general-model of intermittent dynamics that accounts for the q dependence of both the average correlation functions and χ.c EDP Sciences

We explore the different local symmetries in colloidal glasses beyond the standard pair correlation analysis. Using our newly developed X-ray cross correlation analysis (XCCA) concept together with brilliant coherent X-ray sources, we have been able to access and classify the otherwise hidden local order within disorder. The emerging local symmetries are coupled to distinct momentum transfer (Q) values, which do not coincide with the maxima of the amorphous structure factor. Four-, 6-, 10-and, most prevalently, 5-fold symmetries are observed. The observation of dynamical evolution of these symmetries forms a connection to dynamical heterogeneities in glasses, which is far beyond conventional diffraction analysis. The XCCA concept opens up a fascinating view into the world of disorder and will definitely allow, with the advent of free electron X-ray lasers, an accurate and systematic experimental characterization of the structure of the liquid and glass states.coherent X-ray diffraction ͉ higher-order correlations ͉ structure D isordered matter, such as glasses and liquids, does not exhibit translational symmetry and in turn is able to accommodate different local symmetries in the same system, among them the icosahedral local order, which belongs to the forbidden motifs in periodic structures. This mysterious and so far experimentally inaccessible localized order within disorder has been fascinating scientists for many decades (1-5), because it is held responsible for the undercooling of liquids and the existence of the glass state. Similarly, nonperiodic materials have always attracted the attention of materials scientists, because they do carry-through these structural degrees of freedom-a unique potential to display novel smart functions (6-8).The microscopic understanding of the structure and properties of crystals has advanced rapidly during the last decades. The translational invariance of the crystalline state allowed the introduction of the Brillouin Zone concept, thus enabling an elegant and powerful theoretical description of the thermal, electronic and magnetic properties. At the same time, crystal diffraction has continuously been developed to such a fine art that even complex biological structures can be solved today with atomic resolution (when forced to form a crystal). In severe contrast to this, the local microscopic structure of disordered matter has remained a challenge and a mystery (1-3). Our lack of knowledge on the local order within disorder constrains the development of a better understanding of the properties of liquids and glasses (9). In turn, the open question of how the structure of the liquid and amorphous states can be accessed experimentally has become one of the holy grails in condensed matter science (10).The fundamental limits of conventional (X-ray, neutron, electron) diffraction from disordered materials are accountable for this situation, because such techniques only allow to extract the pair distribution function g(r) ϭ n 0 Ϫ2 ͗(0)(r)͘ of the single particle density (r) ϭ ͚...

Time resolved correlation (TRC) is a recently introduced light scattering technique that allows one to detect and quantify dynamic heterogeneities. The technique is based on the analysis of the temporal evolution of the speckle pattern generated by the light scattered by a sample, which is quantified by cI(t, tau), the degree of correlation between speckle images recorded at time t and t + tau. Heterogeneous dynamics results in significant fluctuations of cI(t,tau) with time t. We describe how to optimize TRC measurements and how to detect and avoid possible artifacts. The statistical properties of the fluctuations of cI are analyzed by studying their variance, probability distribution function, and time autocorrelation function. We show that these quantities are affected by a noise contribution due to the finite number N of detected speckles. We propose and demonstrate a method to correct for the noise contribution, based on a N--> infinity extrapolation scheme. Examples from both homogeneous and heterogeneous dynamics are provided. Connections with recent numerical and analytical works on heterogeneous glassy dynamics are briefly discussed.

We introduce a new dynamic light scattering method, termed photon correlation imaging, which enables us to resolve the dynamics of soft matter in space and time. We demonstrate photon correlation imaging by investigating the slow dynamics of a quasi-two-dimensional coarsening foam made of highly packed, deformable bubbles and a rigid gel network formed by dilute, attractive colloidal particles. We find the dynamics of both systems to be determined by intermittent rearrangement events. For the foam, the rearrangements extend over a few bubbles, but a small dynamical correlation is observed up to macroscopic length scales. For the gel, dynamical correlations extend up to the system size. These results indicate that dynamical correlations can be extremely long-ranged in jammed systems and point to the key role of mechanical properties in determining their nature.

In supercooled molecular fluids or concentrated colloids and grains, the dynamics slow down markedly with no distinct structural changes as the glass 1 or the jamming 2 transition is approached. There is now ample evidence that structural relaxation in glassy systems can only occur through correlated rearrangements of particle 'blobs' of size ξ (refs 3-7), leading to dynamics that are heterogeneous both in time and in space. On approaching these transitions, ξ grows in glass-formers 6-8 , colloids 3,4,9 and driven granular materials 10 alike, strengthening the analogies between the glass and the jamming transitions and providing a possible explanation for the slowing down of the dynamics. However, little is known yet on the behaviour of dynamical heterogeneity very close to dynamical arrest. Here, we measure in colloids the maximum of a 'dynamical susceptibility', χ * , that quantifies the temporal fluctuations of the dynamics, the growth of which is usually associated with that of ξ (ref. 11). We find that χ * initially increases with particle volume fraction, but drops markedly very close to jamming. We show that this behaviour results from the competition between the growth of ξ and the reduced particle displacements associated with rearrangements in very dense suspensions, unveiling a richer-than-expected scenario.Dynamical heterogeneity is a key ingredient in many of the most advanced attempts to understand and rationalize the glass and the jamming transitions. The recent observation of a critical-like growth of temporal and spatial dynamical fluctuations in a two-dimensional athermal system approaching jamming 10 , similar to that hypothesized for glass-formers 12 , has raised hope that the glass and the jamming transitions may be unified, calling at the same time for further, tighter experimental verifications. Here, we investigate temporal dynamical heterogeneity in a three-dimensional thermal system, concentrated colloidal suspensions close to the maximum packing fraction. Temporal and spatial dynamical heterogeneity are usually closely related: the former can be quantified by a 'four-point dynamical susceptibility' χ 4 (the variance of a time-resolved correlation function describing the system relaxation), the amplitude of which is proportional to ξ 3 (refs 11,13,14). Surprisingly, we find that very close to jamming, temporal and spatial dynamical heterogeneity decouple: whereas ξ continuously grows with volume fraction, the amplitude of temporal fluctuations drops sharply close to the maximum packing fraction. These findings challenge current scenarios where the slowing down of the dynamics on approaching jamming is accompanied by enhanced temporal fluctuations of the dynamics.The dynamics of colloidal hard spheres slows down markedly close to ϕ = ϕ g ≈ 0.58; beyond ϕ g , ultraslow relaxations on short length scales are still observed 3 , until dynamics freeze at the maximum (random) packing fraction, ϕ max . We study concentrated suspensions of polyvinyl chloride xenospheres 15 suspended in dioctyl p...

We present a new method to extract the intermediate scattering function from series of coherent diffraction patterns taken with 2D detectors. Our approach is based on analyzing speckle patterns in terms of photon statistics. We show that the information obtained is equivalent to the conventional technique of calculating the intensity autocorrelation function. Our approach represents a route for correlation spectroscopy on ultrafast timescales at X-ray free-electron laser sources.

The supramolecular organization of wheat gluten proteins is largely unknown due to the intrinsic complexity of this family of proteins and their insolubility in water. We fractionate gluten in a water/ethanol mixture (50/50 v/v) and obtain a protein extract which is depleted in gliadin, the monomeric part of wheat gluten proteins, and enriched in glutenin, the polymeric part of wheat gluten proteins. We investigate the structure of the proteins in the solvent used for extraction over a wide range of concentration, by combining X-ray scattering and multiangle static and dynamic light scattering. Our data show that, in the ethanol/water mixture, the proteins display features characteristic of flexible polymer chains in a good solvent. In the dilute regime, the proteins form very loose structures of characteristic size 150 nm, with an internal dynamics which is quantitatively similar to that of branched polymer coils. In more concentrated regimes, data highlight a hierarchical structure with one characteristic length scale of the order of a few nm, which displays the scaling with concentration expected for a semidilute polymer in good solvent, and a fractal arrangement at a much larger length scale. This structure is strikingly similar to that of polymeric gels, thus providing some factual knowledge to rationalize the viscoelastic properties of wheat gluten proteins and their assemblies.

We report on an x-ray photon correlation spectroscopy experiment investigating the surface structure and dynamics of colloidal particles suspended in a supercooled viscous liquid. The static structure factor in the direction parallel and perpendicular to the surface reveals a more disordered structure at the surface as compared to the bulk. The particles display heterogeneous ballistic dynamics parallel to the surface. The particle dynamics in the direction perpendicular to the surface is much slower and does not show the hallmarks of ballistic motion.