Long-range hydrodynamic interactions enhance colloidal gelation
July 10-14, 2016
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 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.
We employ a minimal model of the hydrodynamic forces between particles, which emphasizes the most important elements of the fluid physics during gelation. 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. Away from the gel boundary, differences in nearest neighbor distribution persist. This result necessitates a fundamental rethinking of how both microscopic and macroscopic models for gelation kinetics in colloids are developed.
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James Swan and Zsigmond Varga, "Long-range hydrodynamic interactions enhance colloidal gelation" in "Colloidal, Macromolecular & Biological Gels: Formulation, Properties & Applications", ECI Symposium Series, (2016). http://dc.engconfintl.org/cmb_gels/8