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.