Velocity field measurements of deformational flows with magnetic resonance have become well established, in particular through the use of phase-contrast imaging - the traversal of both k- and q-space (conjugates to positions and displacements, respectively). In combination with (potentially space-resolved) spectroscopy, it forms the basis of rheo-NMR1 .
Wormlike micelles solutions are a prime example of a non-Newtonian sample, exhibiting shear thinning and even shear-banding, in which a sheared fluid partitions into two bands of apparently differing viscosity. This interface between bands is often seen to fluctuate in space and in time; whether this is due to slip and stick of fluid with the wall of the deformation device or other instability requires apparatuses and analysis techniques suited to the geometries, flow rates and materials being used.
We have rethought the traditional rheo-NMR experimental set-up, and designed novel shearing geometries and an NMR compatible rheometer. These are intended to explore the influence of curvature induced stress variations on the development of shear banding within a 10wt% cetylpyridinium chloride/sodium salicylate micellar solution in 0.5M NaCl. To measure the stress response of materials under shear during NMR experiments, an inline torque sensor has been added to our drive shaft.
With this hardware, we have undertaken comparative studies of wormlike micelles solution dynamics. Using PGSE encoding for velocity, we move from "slow" phase-encoded imaging techniques, through RARE2 ("fast", ~1s imaging time, one velocity component) through to multiple-echo EPI ("faster" GERVAIS3 , ~200ms imaging time, three-dimensional velocity). We present data on transient effects (startup, cessation of flow), slip-stick effects (surface interaction with fluid) and the spatio-temporal nature of fluctuations in the shape and position of shear bands. All three scenarios require the high temporal resolution and accurate hardware to varying degrees.