## 3 Sample Environments
### Stop Flow/Chemistry On-Line  
To be able to follow structure formation due to chemical reactions requires reaction cells that are resistant to the chemistry being carried out and also capable of attaining the desired temperature and pressure range. In the case of low viscosity solutions, the reaction products can sediment out and therefore change the composition of the sample as'seen' by the probe beam [74]. This can be overcome by the use of a tumbling cell, which rotates the sample solution around the beam axis, preventing sedimentation. These cells are routinely used in all neutron scattering facilities. As an alternative, a cell of sufficient thickness with a mechanical stirrer can also be used. As with any equipment that contains moving parts, this leads to a more complicated temperature control system. One design that contains an array of four tumbling cells, makes use of air cooling/heating, is capable of a limited temperature range of 10-50 \({}^{\circ}\)C, and is suitable for slow reaction kinetics without thermal variations [20]. By placing the equipment on a translation stage and triggering the chemical reactions at different times, it is possible to alternate between the samples and map out the kinetics over extended time scales, also making effective use of the allocated beamtime. Figure 8: An experimental X-ray/neutron scattering set-up incorporating optical digital image correlation. This combination of experiments can increase considerably the information content of experiments and is relatively simple to implement. Figure made available by authors [71]. For faster experiments, sedimentation is less of a factor and here one can consider the use of conventional stop-flow cells. Due to the required beam size, the sample volume will be rather large, and thus there are constraints on the achievable mixing- and dead-times, but workable solutions do exist. A review on the use of stop flow cells in SAXS and SANS experiments was written by [PERSON] [75], who pioneered such experiments on SANS beamlines. A practical example of such experiments is the formation of microgels due to the precipitation and subsequent polymerization of N-Isopropylacrylamide [76]. Here the early stage of gel formation was the point of interest, which is often the most difficult part since structural changes tend to develop faster than in later stages. A commercially available stop-flow cell was used at a fixed temperature. Data were collected over a period of 20 min with a time framing rate of 5 s/frame. However, in order to obtain sufficient quality data, the experiments were repeated three to four times and the results were averaged. The data yielded the total particle volume and the number density of particles. A similar home-built device was used to report on cationic liposomes complexes with DNA. However, instead of temperature, a pH jump was induced by the stopped flow to trigger the reaction [77]. By using continuous flow methods, a larger variety of experiments is feasible even though much larger quantities of materials are required. In some cases, it is even feasible to use a recirculating flow and gradually change the conditions without incurring the penalty of requiring large volumes of sample. This is especially relevant when dealing with deuterated materials. A purposely designed sample environment meeting these requirements was reported. See Figure 9 [78]. For test experiments, aqueous solutions of polyoxyethylene alkylether carboxylic acids, a class of surfactants with strong pH-responsive properties, were used. During the experiments, the pH was changed by titration. As the authors remarked, these time-resolved experiments, down to 1 s/frame, were made feasible in recent years by more efficient detectors and increased neutron flux thanks to improvements in neutron guide technology.