Pulsed field-gradient spin echo N.M.R. studies of flow in fluids

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Abstract
An analysis is presented of the effects of various flow patterns on spin echo shapes and amplitudes in the presence of both static and pulsed gradients in the applied magnetic field. Two approaches are used, one based on the Bloch equations, the other on a molecular average picture. In the case of flow patterns with velocity gradients the effects of self-diffusion across these on the N.M.R. experiments is pointed out and shown to be negligible in most cases of interest. Extension of a multipulse sequence for the study of flow to include pulsed field gradients is outlined. Experiments are described which largely confirm the theoretical predictions (except in the case of echo shapes) for plug flow and laminar flow in a circular pipe. The problem of loss of labelled nuclei from the receiver coil during the experiment is treated in detail for the case of laminar flow. The method is applied to a brief study of the flow properties of agar gel as a function of agar concentration. It is shown by using both phase sensitive and diode detectors that the flow pattern changes from laminar at low agar concentration to a sheared plug flow at higher concentrations.