Abstract
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.