Influence of curvature and surface roughness on shear flow of complex fluids studied with emerging Rheo-NMR hardware

Rheo-NMR [1] has existed as a tool for studying fluids under mechanical deformation for nearly 25 years. In many cases it provides spatially and temporally resolved maps of NMR spectra, intrinsic NMR parameters (e.g. relaxation times) or motion (e.g. diffusion or flow). As a consequence, Rheo-NMR has been established as a complementary technique to conventional rheological measurements. However, the restriction on the size of shear geometries, along with practical concerns for NMR measurements (e.g. signal to noise and NMR safe materials) have lead to Rheo-NMR shear devices which differ from those used in traditional rheology; creating a difference in the physical set-up for experimentation.

In an effort to understand the influence of this difference, new Rheo-NMR shear geometries were built; including a plate-plate device for a Bruker Wide Bore magnet and a series of concentric cylinder shear cells for a Bruker Super Wide Bore (SWB) magnet. The plate-plate device was designed such that the shear surfaces could be mounted on a commercial rheometer and thus slip velocities, as a function of surface roughness, could be estimated from conventional rheometry techniques [2] . A priori knowledge of slip could then be used when designing and interpreting NMR experiments. The SWB system accommodates shear devices with diameters up to 72 mm, allowing the fluid response to cell curvature to be systematically explored for the first time with NMR techniques. The design of the hardware used in this study was based upon previous work [3] which has expanded the range of shear profiles available (including oscillatory shear) and includes the possibility to simultaneously measure torque (i.e. shear stress) during NMR experiments.

Initially, using these new devices, results from NMR velocimetry experiments of Newtonian behaving fluids were used to verify the hardware and methods. Next, wall slip of a wormlike micellar (WLM) solution (10% cetylpyridinium chloride and sodium salicylate in NaCl brine) was explored as a function of surface roughness using the plate-plate geometry where, for the range of roughnesses investigated, differences in slip were only observed between a 'smooth' machined surface and roughened walls. Finally, shear banding of the WLM solution was measured in a series of 10 concentric cylinder shear cells, where the shear rate at which the onset of shear banding was observed had a dependence on cell curvature and the stress variation across the fluid domain.

These measurements highlight the importance of shear cell design and construction for all rheometry techniques, including but not limited to Rheo-NMR.