## 3 Sample Environments
### Shear and Rheology  
The flow of materials is a research area of interest to both the fundamental as well as the applied/industrial research communities, and one that lends itself to neutron scattering experiments. The cell walls of conventional Couette cells commonly used in neutron scattering experiments can be easily penetrated by neutrons and the amount of material required is small, which keeps deuteration costs to a minimum. Couette cells have a long history of commercial development and as such, are a mature technology able to keep a stable flow over the entire time required for the acquisition of data with good signal to noise ratio; one of the first mentions of the use of a Couette cell in a scattering experiment was in 1984 [38]. Rheology, however, is a complicated subject in which the observed components of a flow profile depend on the configuration of the cell or in which direction one probes the flow profile within the cell. In Figure 5, the different flow planes are defined and further discussions about the terminology can be found in an extensive review [39]. For the directions where one can probe the desired flow field whilst keeping the Couette cell axis vertical, i.e., the 1-3 and 2-3, radial and tangential directions, respectively, it is feasible to perform quantitative rheological experiments with neutron scattering [40; 41]. However, a cell for the 1-2 direction, where one has to align the neutron beam in the gap parallel between the two Couette cylinders, has proven more difficult to design [42]. With the Couette cylinder axis horizontal, the material must be confined, otherwise gravity will force low viscosity materials out of the cell. For the conventional Couette geometry where the cylinder axis is vertical, one can rely on commercially available rheometers. Since the topic of shear and rheology is rather popular, there have been numerous in-depth reviews on the subject [39]. These reviews include oversights of other available shear geometries that have been used on X-ray and neutron beamlines. Measuring the axis torque allows quantitative measurements of shear  
Figure 5: The different configurations in which a Couette type shear cell can be used. The different flow fields are indicated with \(\
u\) indicating the flow direction, \(\
abla\)v the flow gradients, and \(\omega\) the vorticity direction (**a**). In neutron scattering experiments, the 1–3 (**b**) and 2–3 (**c**) directions can be probed with a cell having a vertical rotation axis. On the other hand, the 1–2 (**d**) direction requires a cell with a horizontal rotation axis. Figure provided by ]. [PERSON] [43]. stress simultaneously with macroscopic strain gradients imposed by the frequency of rotation and microscopic deformation observed in the SANS data [42]. As mentioned, for most neutron scattering experiments there are commercially available instruments, where the replacement of the commercial Couette cylinders by cylinders made out of neutron transparent materials like quartz and titanium can be straightforward. The literature contains a wide range of subjects that have been studied using Rheo-SANS methods. They range from investigations into the shear banding mechanism in a fluid comprised of cetyltrimethylammonium bromide wormlike micelles [44], to detailed studies of polymer chain conformations composed of polyethylene-polypropylene blends with Si particles nanocomposites [45], to the shear-induced formation of rod-like entities in metallo-supramolecular gels based on a multitopic cyclam bis-terpyridine [46]. The Couette geometry can also be used to probe material relaxation properties by using an oscillatory flow protocol, which is mentioned in the section on time-resolution. The 1-3 and 2-3 Couette geometries are now fairly standard for neutron scattering experiments. Progress in instrumentation in the coming years will most likely be incremental and most developments will focus on the analysis of neutron-rheology data sets [47].