(242c) Transient Microstructure, Rheology of Shear-Thickening Colloidal Suspensions By Time-Resolved Flow-SANS and Relation to Nanotribology

The shear thickening of dense colloidal suspensions is an active area of research aimed to understand the highly nonlinear increase of viscosity under various conditions and how this relates to the nanotribology of the particles comprising the suspension. Efforts in models and experiments have focused on connecting microstructure to the underlying micromechanical mechanism in steady shear rheology, whereas recent theoretical and experimental work shows the ability of transient rheology to distinguish between micromechanical mechanisms (Physics of Fluids. 2021;33(3)). Further, relating this rheology to the nanotribology, such as measured by methods such as lateral force microscopy, provides understanding that can be helpful for formulation of dense suspensions for specific applications (JOR, 2020;64(2):267-82.). While recent studies have focused on the steady-shear microstructure in the shear thickened state, the corresponding measurement of transient microstructure is lacking. Advances in time-resolved flow-small angle neutron scattering (SANS) enable resolving microstructure transients on the scale of 10 milliseconds. In this work, we present measurements of the transient microstructure of a concentrated colloidal suspension of coated silica particles in index matching fluid under shear reversal and shear cessation using time-resolved Flow-SANS in a 1-2 shear cell geometry. To aid in the understanding of the flow reversal experiments, we also present complementary measurements of shear cessation for monitoring transient microstructure recovery by Brownian motion during stress relaxation from the shear thinned and shear thickened states. Rheological measurements of the shear viscosity and first and second normal stress differences along with lateral force microscopy measurements establish this suspension shear thickens due to enhanced lubrication hydrodynamics. Flow reversal experiments demonstrate the persistence of hydroclusters under the entire observation time of shear reversal protocol, indicating the dominance of hydrodynamic interactions for this colloidal suspension of coated particles. These results provide structure and rheology experimental results that can be used to test simulations and theory. Connections and differences between nanotribological measurements and shear rheology are discussed within the Stribeck curve for lubrication.