Strategies for Rapid NMR Rheometry by Magnetic Resonance Imaging Velocimetry

Strategies for NMR-based rheometry are discussed with particular attention given to ease of implementation, robustness, and measurement speed. The techniques are based on NMR velocimetry of Poiseuille flow, and together with measurements of the pressure drop, the velocimetric data may be processed to yield measures of the shear viscosity over the range of shear rates present in the Poiseuille flow field of the test fluid. Methods for NMR velocimetry are briefly reviewed, and three methods all based on the pulsed-field-gradient technique are compared experimentally; they involve (1) direct two-dimensional imaging of the tube cross section, (2) one-dimensional imaging of a Cartesian projection of the tube cross section followed by Abel inversion to obtain radial profiles, and (3) measurement of the bulk velocity spectrum of the tube cross section and conversion to the radial velocity profile. The second and third of these techniques allow the most rapid measurements (potentially less than one minute) and show promise for on-line NMR rheometry.