Simulations of electrorheological and particle mixture suspensions: Agglomerate and layer structures

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Abstract
Brownian dynamics (BD) simulations have been carried out of the rheology of two model weakly aggregated suspensions-model electrorheological (ER) fluids with anisotropic dipole interactions and depletion flocculated (DF) systems with isotropic well potential interactions. At rest, both systems form an infinite gel-like aggregate network. Under shear, they shear thin and undergo shear-induced agglomeration to structures more compact than the gels at rest. The present work reports simulations of much larger model systems (N=500) than considered previously and makes a comparison between the two systems under shear. We investigate a range of volume fractions in the ER case and elucidate further the microscopic mechanisms of flow in the ER and DF cases. In the case of ER fluids at 10% and 30% volume fraction, layer structures form normal to the vorticity axis at low shear. These are not found at 50% volume fraction. The layer transition is associated with some hysteresis in the Stokesian (large particle/high shear rate) regime. An intermediate liquid phase exists between the high shear rate hexagonal string phase and the layered phase. The ER model appears to have a yield stress followed by power law shear thinning. In the case of the isotropic depletion system, an additional structural transition occurs. At low shear rates, globular shaped agglomerates are formed, but on increasing the shear rate, a transition occurs to a new structure layered normal to the vorticity axis. This is associated with a change in the slope of the viscosity with shear rate, also seen in the experimental system.