Couette and parallel plate viscometers are two commonly used flow geometries to characterize shear viscosity of concentrated suspensions. In Couette flow, it is well documented that prolonged shearing causes a decrease in the apparent viscosity of concentrated suspensions due to shear-induced particle migration from the annulus region to the stagnant region under the bob. In this study, the technique of nuclear magnetic resonance imaging (NMRI) was used to measure the evolution of suspension concentration profiles in Couette and parallel-plate flow devices upon shearing. Neutrally buoyant suspensions of nearly monodisperse, non-Brownian spherical particles at a volume fraction of 0.5 in a Newtonian fluid were used. The same flow cells and suspensions were also used in a rheometer to measure the changes in shear stress under identical experimental conditions such that a direct comparison can be made between the stress and concentration data. For Couette flow, the NMRI data correlated very well with the stress measurements and directly confirmed the Leighton–Acrivos [J. Fluid. Mech. 181, 415 (1987)] shear-induced migration theory. In torsional flow between parallel plates, no detectable change was found in particle concentration in the radial direction, but some decrease in the apparent viscosity was observed. These results provide some important clues for developing and evaluating more general descriptions of particle migration for nonrectilinear shear flows.

Solid-state 13C, 15N, and 29si NMR spectra were obtained on silatrane and a series of derivatives. The isotropic 13C chemical shifts are largely insensitive to substituent-induced structural changes. In contrast, the isotropic '5N and 29Si chemical shifts and chemical shift powder patterns are quite sensitive to substituent-induced structural changes. As the silicon-nitrogen internuclear distance (rS&N) decreases, the isotropic 15N chemical shift increases and the isotropic 29si chemical shift decreases. The magnitude of the 15N chemical shift anisotropy (CSA) decreases and the "Si CSA increases as rS+N decreases. The change in 61, and the CSA for 15N is primarily due to the change in SI, which increases as rSi-N decreases. For 29Si, all three principal elements change as rS&N is varied. Variations in the 15N chemical shift tensor elements are ascribed to changes in the N--Si transannular interaction. For the 29Si chemical shift tensor, the observed changes are ascribed to a combination of effects due to changes in the transannular interaction and direct substituent effects. A simple molecular orbital treatment is useful in understanding the relationship between the transannular effect and the 15N chemical shift tensor and in supporting the use of the 15N chemical shift as an indicator of the transannular interaction.

No abstract

No abstract

No abstract