## 3 Sample Environments
### Pressure  
Pressure is the thermodynamic parameter that is relatively easy to change homogeneously. For the larger sample volumes routinely used in neutron scattering, this allows the reduction/elimination of large pressure gradients over the sample, allowing for a better controlled experiment producing more robust results. Most high-pressure experiments use hydrostatic pressure, which is not directional. An example of uni-axial pressure exerted on a sample in combination with a SANS experiment can be found in soil- or geo-mechanics [29]. The pore size distribution in bentonite clay was investigated as a function of pressure whilst simultaneously loading the cell with dry CO\({}_{2}\) and water in order to elucidate the role that intercalation of these two components play in the complicated pore-pressure diagram. Pressures up to 10 MPa at ambient temperature were applied. An example where hydrostatic pressure was applied was in research to determine if CO\({}_{2}\) could be stored in porous shales. Here, CO\({}_{2}\) was not only the object of study but also the neutron contrast variation medium [30]. Pressures of 25-40 bar were achieved over a temperature range of 20-60 \({}^{\circ}\)C. This type of study can be relevant to soft matter where, for example, the porosity of a material must be characterized to understand the impact of processing conditions. Not only does the static characterization of pores attract attention, but also the combination of hydrostatic and uniaxial pressure is relevant for understanding flow in porous materials [31]. In the fracking of oil-containing shales, water is used as the pressurizing agent. In the case of neutron scattering experiments, this allows one to exploit the difference in scattering length between light and heavy water, in addition to the natural air-matrix scattering length difference. Uniaxial stresses, \(\sigma_{\text{ax}}\), in combination with the hydrostatic pressure inside the pores, p, allows to generate an effective stress \(\sigma_{\text{e}}=\sigma_{\text{ax}}\)-p of 600 Bar. An example of the effects of hydrostatic pressure is surfactant solutions of SDS in D\({}_{2}\)O-forming microemulsions, as shown in Figure 4. Here, a static pressure was applied and the effect of pressure is the continuous transformation of spherical particles to elongated micelles [32]. The pressure cell used was capable of a maximum pressure of 10 kbar and a maximum temperature of 80 \({}^{\circ}\)C.  
Figure 3: (**a**) Left hand panel, SANS curves obtained during a temperature jump on a micellar solution of P(S-b-NIPAM-b-S), from 29 to 35 \({}^{\circ}\)C, where the first structural changes were observed 7 s after initiation of the experiment. (**b**) The right-hand panel shows the structural models determined from the data, where micelles collapse and subsequently aggregate into clusters (reproduced with permission from [28], [PERSON], 2012). A detailed design for a temperature-controlled high-pressure cell developed for biological applications, specifically protein denaturation studies, was described by [PERSON] et al. [33]. The sample cell was capable of a low temperature of \(-18\)\({}^{\circ}\)C with a pressure of 2 kbar. Because of the cell's large thermal mass, which is required for mechanical strength at these elevated pressures, it required 1 hour for sample equilibration. When working at sub-zero temperatures, one stands the risk of ice crystal formation. Down to about \(-20\)\({}^{\circ}\)C, ice formation can be inhibited by anti-freeze agents, but this will alter the biologically relevant environment of the proteins. By using pressure, the freezing point can be lowered, and the use of anti-freeze agents is avoided. There are several other sample cell designs available [34]. Pressure jumps can perturb a sample over the whole volume exposed to the neutron beam without any pressure gradients and can be relatively simply and accurately repeated.