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Orientation and microstructure in sheared Brownian suspensions of anisotropic dicolloidal particles

Orientation and microstructure are investigated in sheared Brownian suspensions of hard dicolloidal particles, with the dicolloids modeled as two fused spheres of varying radii and center to center separations. Two different particle shapes named homonuclear (aspect ratio 1.1) and fused-dumbbells (aspect ratio 1.5) were considered. Hydrodynamic interactions between the particles were computed with a modified lubrication model called Fast Lubrication Dynamics. Studies were conducted for a wide range of volume fractions between $0.3 \leq ϕ\leq 0.5$ and Pèclet numbers between $0 \leq Pe \leq 1000$. The microstructure was found to be disordered at all volume fractions, though signatures of weak string like ordering were evident particularly in $ϕ=0.5$ homonuclear suspensions at intermediate to high shear rates ($Pe$ in the range 10-100). Complex orientation behavior was observed as a function of shape, shear rates, and volume fractions. At very low shear rates, random orientation distribution was observed in all cases. At the highest shear rates, orientation distribution in suspensions of homonuclear particles exhibited a shift towards an alignment with the vorticity axis at all volume fractions, while in suspensions of fused-dumbbells it exhibited a shift away from the vorticity axis at low volume fractions and a negligible shift at higher volume fractions. The orientation behavior is further characterized by examining the orientation distribution in the velocity--gradient plane -- in this case an increased particle alignment with the velocity axis is generally observed with increasing volume fractions, but not universally with increasing shear rates. Mechanistic origins for the complex orientation behavior as a function of shear rate, volume fraction, and particle shape is described.