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.