Rapid Measurement of Transient Velocity Evolution using GERVAIS
by Davies, C.J., Sederman, A.J., Pipe, C.J., McKinley, G.H., Gladden, L.F. and Johns, M.L.
Rapid velocity measurements using GERVAIS (Gradient Echo Rapid Velocity
and Acceleration Imaging Sequence), an EPI (Echo Planar Imaging) based
technique capable of measuring velocity over an observation time of
several milliseconds, are performed on a wide-gap Couette Rheo-NMR cell
for the first time. A variable delay time between a control signal to
initiate a transition in flow and the start of the measurement sequence
is incorporated to allow investigation of the transient evolution of
the velocity field following a step change in rotation rate. Both the
commencement and the cessation of imposed shear stress are
investigated for (i) a shear banding micellar solution of CPyCl
(cetylpyridiniumchloride)/NaSal (sodium salicylate) in brine and
(ii) a low molecular weight PDMS (polydimethylsiloxane) oil.
With respect to the micellar solution, an elastic shear wave is
seen to propagate across the cell following the commencement of
shear stress whilst an oscillatory ‘re-coil’ is observed
following the cessation of shear stress; neither of these
phenomena were observed for the PDMS oil which exhibited a purely
viscous response as expected for an incompressible Newtonian fluid.
This technique has potential applications across a wide range of
transient rheological investigations, particularly with respect to
optically opaque materials.