(533h) Rheology and Microstructure of Concentrated, Near Hard-Sphere Colloidal Dispersions Under Steady Shear and Large Amplitude Oscillatory Shear in All Three Planes of Shear

We present the first experimental measurement of the complete 3-D microstructure of a shearing, near hard-sphere colloidal dispersion that is made possible by the development of novel instrumentation implemented on the small angle neutron scattering (SANS) beamlines of the ILL and the NIST Center for Neutron Research.  This work is motivated by the intriguing rheology of Shear Thickening Fluids (STFs), which are concentrated colloidal suspensions that exhibit increases in viscosity under high shear rates of deformation. STFs aptly respond to an imposed impact stress and therefore, these novel materials have the potential to be implemented in any number of impact protection applications. STFs have been critically studied for their steady shear properties [N.J. Wagner and J. F. Brady, Physics Today. 2009. 62 (10): 27-32]. However, little is understood about the nonlinear dynamic properties of STFs. To address this, we utilize large amplitude oscillatory shear (LAOS) to investigate the dynamic rheological properties of a model STF suspension [Kalman, D.P. and N.J. Wagner, Rheologica Acta. 2009, 48: 897-908]. We develop a new flow-SANS sample environment for time and spatially resolved measurements in the plane of flow (1-2 plane) and combine this with rheo-SANS in the radial direction (1-3 plane) and tangential direction (2-3 plane) to elucidate the connection between the microstructure and the measured shear rheology. For the first time, the microstructure of a colloidal suspension undergoing steady shear is measured in all three planes of shear. The flow-induced microstructure under steady and oscillatory shear flow is compared to the rheological behavior via stress-SANS laws that separate the thermodynamic and hydrodynamic components of the stress that drive shear thinning, shear thickening, and first normal stress differences.  A new non-equilibrium microstructure is discovered during a LAOS that proves to be important for understanding the rheology of concentrated colloidal suspensions. This first, complete measurement of rheological and microstructural properties is invaluable for testing constitutive models for colloidal suspension rheology.