Microstructure of shear-thickening concentrated suspensions determined by flow-USANS
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Reversible shear thickening in colloidal suspensions is a consequence of the formation of hydroclusters due to the dominance of short-ranged lubrication hydrodynamic interactions at relatively high shear rates. Here, we develop and demonstrate a new method of flow-ultra small angle neutron scattering to probe the colloidal microstructure under steady flow conditions on length scales suitable to characterize the formation of hydroclusters. Results are presented for a model near hard-sphere colloidal suspension of 260 nm radius (10% polydisperse) sterically stabilized silica particles in poly(ethylene glycol) at shear rates in the shear thinning and shear thickening regime for dilute, moderately concentrated, and concentrated (ordered) suspensions. Hydrocluster formation is observed as correlated, broadly distributed density fluctuations in the suspension with a characteristic length scale of a few particle diameters. An order–disorder transition is observed to be coincident with shear thickening for the most concentrated sample, but the shear-thickened state shows hydrocluster formation. These structural observations are correlated to the behavior of the shear viscosity and discussed within the framework of theory, simulation, and prior experiments.
KeywordsSuspensions Colloids Shear-thickening Microstructure Hydrocluster
Research was sponsored by the US Army Research Office and the US Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-05-2-0006. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research office, the Army Research Laboratory, or the US Government. The US Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon. This work utilized facilities supported in part by the National Science Foundation under Agreement No. DMR-0454672. We acknowledge the support of the National Institute of Standards and Technology, US Department of Commerce, in providing the neutron research facilities used in this work, as well as the assistance of Lionel Porcar and Man-Ho Kim. The authors thank Prof. Peter Schurtenberger (Uni. Fribourg) for helpful discussions about the cluster peak.
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