(518d) The Importance of Hydrodynamic Interactions during Shear-Induced Clustering in Polymer-Colloid Suspensions

The appearance of shear-induced particle clusters is common to many polymer-colloid mixtures including solutions and melts. Due to the apparent alignment of these clusters along the vorticity direction when observed in the flow-vorticity plane, cluster formation has been hypothesized to arise from the influence of polymer normal stresses on particle-particle interactions. To test this hypothesis, we have performed 3D microstructural measurements under shear flow in non-aggregating polymer-colloid mixtures using flow-small angle neutron scattering (flow-SANS) in the flow-vorticity and flow-gradient planes. As a model system, we use oil-in-water nanoemulsions in the presence of polyethylene glycol, which form non-aggregating viscoelastic networks. These fluids exhibit significant flow-induced anisotropy in the droplet microstructure consistent with cluster formation, which develops during strong shear thinning of the fluid. Specifically, butterfly scattering appears in the flow-vorticity plane at shear rates corresponding to the shear thinning, with projected orientation in the vorticity direction, in agreement with previous studies. However, significant anisotropy also develops in the flow-gradient plane, with orientation along the compressional axis of shear. These results suggest vorticity-aligned aggregates possess anisotropic cross section, and that hydrodynamic interactions in the flow plane, in addition to polymer normal stresses, play an important role in the formation of shear-induced clusters. To demonstrate this, we show collapse of the flow-induced alignment and shear thinning responses over a number of different viscoelastic parameters through a modified Peclet number for the suspended colloids. Overall, this structural information provides a basis to control the rheology and suspension microstructure of non-aggregating polymer-colloid mixtures.