NUCLEAR MAGNETIC RESONANCE STUDIES OF FLOWING LIQUIDS (NMR IMAGING, VISCOSITY)
Abstract
A new method for measuring viscosity in NMR samples is presented. It is based on the detection of the transition from the azimuthal Couette Flow into the counterrotating vortex cells characterizing Taylor Vortex Flow. The critical Taylor number at which this transition is observed is dependent on the viscosity of the solution and the tube geometry. By determining the critical frequency at which the flow transition is observed, the corresponding kinematic viscosity can be calculated. The method has been used successfully to measure the viscosity of some pure liquids and liquid mixtures with viscosities ranging from 5 to 20 cSt with differences within 2% of those measured by conventional methods. Other ranges were examined experimentally and discrepancies were interpreted. NMR images of flow in concentric cylinders are presented for the first time. Magnetic field gradients provide the spatial resolution to characterize the flows. Distinctive NMR patterns have been observed for Couette Flow, Taylor Vortex Flow (TVF) and Wavy Flow. The Z-gradient pattern for Couette Flow is featureless and proportional to the gradient. The X-gradient consists of a series of sidebands whose position could not be accounted for by spinner frequency alone but could be simulated using Williams and Gutowsky's spinning sideband theory. The Z-gradient pattern of TVF consists of a series of spinning sidebands superimposed on a broad background signal. The sidebands are the result of coherent motion perpendicular to the linear gradient generated by the vortex cells. The potential of NMR as a technique capable of characterizing flow in concentric cylinders at the molecular level is presented for the first time. Preliminary experiments to explore the effect of molecular motion on liquid properties and characterize TVF further are presented. They involve measurement of relaxation times T(,1) of molecules undergoing coherent vortex motion; probing molecular motion within vortex cells using a selective excitation sequence and detecting coherence in motion using a modified spin echo sequence.
Degree
Ph.D.
Subject Area
Analytical chemistry
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