Dynamic measurement of bulk porosity during freezing in NaCl solutions using broadband terahertz time-domain spectroscopy

Sea ice is a porous medium composed of brine and ice, with the volume fraction of brine, namely the porosity, being an important physical parameter. However, its non-contact and dynamic measurement remains highly challenging due to the complexity of the multiphase environment. Broadband terahertz wave exhibits relatively high transmittance through ice compared to infrared radiation, making it highly promising for quantitative and dynamic observations. Apart from single-phase measurements, no experimental work has yet demonstrated the quantitative resolution of solid–liquid phase transitions using terahertz spectroscopy. In this work, we achieve dynamic measurements of the brine volume fraction at the focal point of the terahertz beam with salinities ranging from 0.9 wt% to 2.1 wt% under fixed temperature conditions, during the bottom-up freezing process of brine over a period of three hours. The time-domain signals are valid when the brine volume fraction is below 60%, while the upper detection limit determined from the dynamic range within 0.5-1THz in the frequency domain is around 40%, with the optimal measurement range being below 25%. Compared with the ideal saturation curves calculated from the measured temperature and the mass conservation of salt and water, the accuracy of our measurement results and the trend with salinity variations are validated. The demonstration of quantitative and dynamic measurement of brine freezing indicates the feasibility and potential of broadband terahertz absorption spectroscopy for real-time analysis in solid–liquid multiphase environments and porous media, such as sea ice.