Events in Physics
Departmental Colloquium L4
Time-evolution scenario for short-range depletion gels subjected to the gravitational stress
Roberto Piazza,
Department of Chemistry, Material Science, and Chemical Engineering “G. Natta” Politecnico di Milano
When the attractive forces induced by the presence of a high molecular weight additive acting as depletant become strong enough, a colloidal suspension undergoes a liquid–liquid (L-L) phase separation that, if the depletion forces are sufficiently short-ranged compared to the size of the colloidal usually gets arrested by the formation of a disordered colloidal solid with a gel-like structure. I shall present a survey of the experimental results on the settling and restructuring dynamics of colloidal gels obtained by Photon Correlation Imaging and Ghost Particle Velocimetry, two novel optical correlation techniques particularly suited to the investigation of the microscopic dynamics of spatially heterogeneous samples. Three distinct regions can be set apart within the L-L coexistence region of the phase diagram. When depletion forces are barely sufficient to drive the system within the metastable region, an initial disordered gel hosts the rapid nucleation of crystallites, which stress the gel structure until it fully collapses, leading to the formation of a macroscopic colloidal crystal. For stronger attractive forces, two distinct scenarios are observed, depending on the particle volume fraction ϕ0 of the original suspension. At low ϕ0, the gel breaks after a short delay time into separate clusters, which rapidly settle until they compact in a denser disordered phase. The latter eventually undergoes a slow compression, which is accounted for by a “poroelastic” model where the microscopic gel dynamics is fully ruled by its macroscopic deformation. Yet, it is the intermediate stage between cluster settling and final compaction which displays very peculiar features, evidenced by anomalous settling profiles which are totally unusual in standard colloidal sedimentation processes. For larger values of ϕ0, gel breaking is conversely suppressed, the structure undergoes a continuous compression, and the microscopic dynamics is characterized by logarithmic time-correlation functions resembling those found for attractive glasses.