## 3 Sample Environments
### Shear and Rheology
Some developments were recently reported on the implementation of the technically more challenging case, where one wants to probe the 1-2 plane [42; 48] by directing the neutron beam through the gap between the two cylinders that make up the Couette cell. With a small enough beam, it is possible to probe the different flow profiles as a function of the position within the gap [48]. Such spatial mapping is not commonly associated with neutron scattering since the neutron beams tend to be rather large and can only be reduced in size by sacrificing intensity. For stable flow rheological applications, this is a problem only limited by the available beam time and neutron flux, and not by any technical aspects. From an experimental point of view, it should be remarked that it is often necessary to obtain scattering data along all three shear planes [49]. Unfortunately, this requires the use of two different Couette cells capable of covering the same temperature and shear range. Designs for a cell that can accommodate experiments in all three shear directions are available, but rare [42], and still require substantial reconfiguration when changing between geometries. In addition to the above-mentioned shear flow geometries, there is the less popular but useful plate-plate geometry [50] used in a range of SANS experimentation, such as studies on the self-assembly of nanofibers composed of fibronectin mimetic peptide-amphiphiles [51]. The design for a cone-plate geometry instrument for use in reflectometry is discussed in the framework of test experiments involving polystyrene/deuterated polystyrene, with the flat bottom plate modified to be transparent to neutrons [52]. A modified commercial instrument, for example, was used for the study of asphaltenes in the reflectometry configuration [53]. However, placing the neutron beam in the gap between the cone and plate, like what has been successfully done with X-rays, requires that very tight neutron beam collimation be used, which impacts data rate and quality. An interesting development is the use of Grazing Incidence SANS or GIANS. The feasibility of this technique in combination with a cone-plate rheometer was demonstrated in experiments on bolabamphiphilic arginine-coated peptide nanotubes [54]. However, data collection took about 5 h per sample, indicating that this method still requires further development. The more difficult problem of how one goes about studying elongational flow appears to still be out of reach of real time neutron scattering experiments. The methodology of flash freezing a material that has been subjected to elongational/extensional flow off-line and then using the frozen samples to carry out a static measurement was reported in 1990 [55], and the method was still regarded as recently as 2016 as being the best method to study dendritic polymer blends with linear chains [56]. Here, the main issue remains that the time-resolution of the physical process does not match the time-resolution that can be reasonably achieved on SANS beamlines. Whilst the examples being discussed above fall under the 'drag flow' category, there are also configurations that allow 'pressure flows' to be studied [57]. Capillary- or Poiseuille-flow and slit-flow are examples of pressure flow. This type of flow is important to understand the fundamental behavior of chain molecules in conditions relevant for industrial processing. For instance, in injection molding, the predominant physical processes are Poiseuille flow and thermal quenches. Not only is flow in an unrestricted path relevant, but studies of the behavior of the material as it flows around obstructions and restrictions are also required. Such information provides firstly, insights into material properties, and secondly, these results can be used to validate computational models as to how entangled polymers behave under elongational flow or when encountering a change in flow geometry. In other words, such physical results provide predictions on how to process materials [58; 59].