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Center for Nonlinear Studies |
We study via theoretical and computational modeling the evolving structure and time-dependent rheological properties of an aging colloidal gel, with a focus on understanding the non-equilibrium forces that drive late-age coarsening. Colloidal suspensions span a rich range of states—from dispersed to arrested, and from liquid-like to solid-like behavior. In a colloidal suspension where particles experience attractive forces, the particle attractions can lead to phase separation—analogous to the phase transition of steam to liquid water—into particle-rich and particle-poor regions separated by a single interface. But this separation is sometimes interrupted before full separation occurs: at certain particle concentrations and interparticle potentials, the same attractions between particles that promote phase separation also inhibit it, leading to kinetic arrest of the phase separation and the subsequent formation of a space-spanning network—a gel. When attractions between particles are on the order of just a few kT, e.g. as arises in the presence of a polymer depletant, the kinetic arrest of the phase separation can lead to a non-fractal bi-continuous morphology, a so-called ‘reversible’ colloidal gel. In such gels, thermal kicks from the solvent are strong enough to dislodge particle bonds, which then reform, allowing aging and restructuring of the gel over long times. Because particle diffusion is dramatically slowed by inter-particle attractions, however, such have difficulty reaching equilibrium, because the thermal rearrangements required to do so are weak and difficult. Prior studies left open the question of how the particle-rich regions are structured—liquid-like, glassy, or crystalline—whether restructuring takes place via bulk diffusion, surface migration, or coalescence of large structures, and what underlying mechanisms provide the driving force for coarsening. We show that the strands are disordered and nominally glassy, that macroscopic gel coarsening is driven by migration of particles along the network surface, and we connect macroscopic rheology to the underlying driving force.
Host: Ivan Christov