Figure (a) shows the absorption at 0.5 THz directly extracted from the terahertz spectra acquired throughout the crystallisation process, which was at the lower bound of the frequency window and no crystalline feature was expected. With the information of frequency, temperature, and absorption, the corresponding concentration of the amorphous phase was determined as shown in Figure (b) using the calibration curve in Figure . The concentration first decreased gradually due to the water depletion from the two ends of the flow cell. In the second region, the concentration increased drastically as crystals started to grow which resulted in relatively denser liquid in the area that was probed with THz-TDS. Afterwards, the concentration reduced again and slowly reached the same equilibrium point as in the beginning, which was assumed as the stable concentration at that temperature. In this way, the temperature-dependent absorption was converted to information that was independent of temperature.