When monitoring the crystallisation process at a set temperature, the cell was first cooled down and kept constant at the target temperature until the system was stable. Afterwards, the MgSO 4 solution was injected into the flow cell with a syringe via a tube, and the outlets on both sides of the flow cell were sealed with parafilm. The sample holder including the cell was promptly placed at the centre of the measurement chamber and terahertz spectra and images were acquired. The time from injecting the solution to the start of the measurement was minimised to no more than 30 s, in case of triggering undesired nucleation. The temperature was kept as constant as possible during the whole crystallisation process, until crystals formed across the cell in the view of the optical probe and the terahertz spectra did not exhibit further changes. The experiment was either terminated at this point, or the behaviour of the system during slow heating to room temperature was studied. In the latter case, the temperature was increased by 0.2 • C min -1 up to 25 • C in the flow cell. This was found to be an ideal heating rate to introduce a constant temperature change to the cell. For all measurements, three thermocouples monitored the temperature at various posi-tions: in the water bath, inside the metal sample holder, and at the inlet of the cell, and one data point was acquired per second. The optical probe used for image acquisition was set to acquire one photograph every two seconds. After each crystallisation measurement, the liquid cell was thoroughly cleaned to remove grown crystals, contaminations, or seeds which could influence subsequent measurements. The cleaning solution was prepared from commercial EDTA solution (pH = 8; Fisher Scientific, Loughborough, UK) and NaOH solution (Reagecon Diagnostics, Shannon, Ireland) to adjust the pH to 10 in which MgSO 4 exhibits a higher solubility.