Kinetic Pathways to Gelation and Effects of Flow-Induced Structuring in Depletion Gels

The kinetic pathways to gelation and the effects of flow-induced
restructuring are studied here in depletion flocculated gels with
short-ranged attractions, both experimentally and using computer simulations.
In the experiments, we first carefully diffuse a screening organic
salt to destabilize colloid–polymer mixtures and form a gel.
We hence avoid flow history effects, typical of traditional mixing
protocols. The initial gelation phases are then accessible and observed
by time-resolved confocal microscopy. These insights show that quiescent
gelation reduces heterogeneity and strand size with increasing attraction
strength, with deeper quenches leading to earlier arrest. These findings
are consistent with the simulations which include long-range hydrodynamic
interactions. We then compare these results with gels formed by high-rate
preshear followed by cessation of colloid–polymer–salt
mixtures. The obtained microstructures do not seem in this case to
depend on depletant concentration. Indeed, confocal images reveal
that shear flow significantly impacts gel structure, from fluidization
at high shear rates to dense heterogeneous aggregates formation at
lower rates. We especially show how the heterogeneity is controlled
by the strength of the flow relative to the attraction forces between
the colloids. This study highlights the subtleties behind the preparation
protocols of colloidal gels. In particular, it shows that differences
in kinetic aggregation pathways can overshadow attraction effects,
such as those caused by varying flow conditions during mixing at different
attraction strengths. These insights provide a framework for understanding
gelation kinetics and optimizing structural reproducibility in colloidal
gel experiments.