Pair interactions between neighboring particles and turning couples on long bodies formed from touching bodies give rise to flow induced microstructures. In Newtonian fluids, pair interactions in a fluidized suspension lead to dispersions with particles arranged in lines across the stream. In viscoelastic fluids, sedimenting particles aggregate into chained bodies parallel to the stream when the flow is slow and normal stresses dominate, and into across the stream arrays again when the flow is supercritical and dominated by inertia. The microstructural arrangements in Newtonian and viscoelastic fluids are maximally different. Simple mathematical arguments are given here which identify the forces and couples that give rise to all of the main observed microstructures. A mechanism for intensifying normal stresses by shear thinning is proposed.
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