(423h) The Role of Hydrodynamic Interactions in Colloidal Dispersionswith Short-Ranged Attraction and Long-Ranged Repulsion

Colloidal gels are formed during arrested phase separation. Sub-micron, mutually attractive particles aggregate to form a system-spanning network with high interfacial area, far from equilibrium. Models for microstructural evolution during colloidal gelation have often struggled to match experimental results with long standing questions regarding the role of hydrodynamic interactions. In the present work, we demonstrate simulations of gelation in a dispersion of colloids interacting pair-wise via a short-ranged attraction and long-ranged repulsion with and without hydrodynamic interactions between the suspended particles. The disparities between these simulations are striking and mirror the experimental-theoretical mismatch in the literature. The hydrodynamic simulations agree with experimental observations, however.

We explore a simple model of the competing transport processes in gelation that anticipates these disparities, and conclude that hydrodynamic forces are essential. Near the gel boundary, there exists a competition between compaction of individual aggregates, which suppresses gelation and coagulation of aggregates, which enhances it. The time scale for compaction is mildly slowed by hydrodynamic interactions, while the time scale for coagulation is greatly accelerated by collective motion of particles within an aggregate. This enhancement to coagulation leads to a shift in the gel boundary to lower strengths of attraction and lower particle concentrations when compared to models that neglect hydrodynamic interactions.When aggregation is reaction limited, as is the case for particles interacting via a short-ranged attraction and a long-ranged repulsion, the diffusive dynamics of particle clusters are key to establishing the correct aggregation rate. Long-ranged hydrodynamic interactions between particles result in aggregates that diffuse anisotropically, which promote the growth of percolated networks as opposed to condensed domains.

This result necessitates a fundamental rethinking of how both microscopic and macroscopic models for gelation kinetics in colloids are developed. Only simulations that correctly account for hydrodynamic interactions will provide a realistic description of the stresses in a deforming network and correctly reproduce phenomena observed in experiments such as vorticity alignment under shear flow. We have now developed such a minimal model that captures aggregation kinetics accurately.