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Emanuela Del Gado: „Static and dynamic heterogeneities in colloidal gels”
Polymer Physics, ETH Zurich, Switzerland

The nature of slow dynamics and structural arrest in attractive colloidal suspensions is of fundamental relevance in a number of technological applications (from food processing to paints or designing of responsive materials), but also to the connection that these phenomena show with the glassy dynamics observed in molecular liquids. In particular, one of the most debated point is that, due to structuring, the slow dynamics of attractive colloidal suspensions is often coupled to gelling phenomena: Although it is now accepted that the role of gel formation in the slow dynamics is non trivial and strongly dependent on concentration, this role is still far from being fully elucidated.
I will show how dynamical heterogeneities (DH), studied in a model colloidal suspension via Molecular Dynamics, clearly indicate the presence of two different mechanisms for structural arrest, respectively at low and highvolume fractions. In diluted attractive colloids, DH are in fact dominated by the clusters of long living bonds as in irreversible gelation, whereas they are controlled by crowding in dense systems.
I will then focus on understanding the role of the network structure, which is essential to the rheological response of the dilute systems, in the relaxation dynamics. This is done by using a model where directional interactions promote the formation of a large-scale disordered structure. Thanks to a detailed analysis of the structure formation and of relaxation processes over different length scales, I will show that the formation of a persistent network sets in special glassy dynamics, characterized by the coexistence, on different length scales, of very different relaxation processes: on short length scales fast collective motions dominates the relaxation, whereas on large length scales the overall relaxation of the network structure produces slow, stretched exponential decays of time correlations.