Study on the supercooling characteristics of freezing soil based on nucleation theory

Abstract

The clarification of freezing in a soil-water system is critical for assessing the formation of a freezing zone and liquid water flow. The supercooling phenomenon of soil pore solutions has been found during the freezing process, but the mechanism remains poorly understood. In this study, we propose a free energy function of soil-water systems based on the Classical Nucleation Theory (CNT). The analytical solution of the critical nucleation problem of saline soil-water system is obtained by combining the initial freezing temperature model and Pitzer activity coefficients model in electrolyte solutions. Then, the freezing-thawing experiments of saline soil with various salt contents were conducted for verifying the analytical solution. The derived boundary nucleation rate is the quantitative solution of the critical condition for the supercooling. The findings suggested that the theory results agreed well with the experiment results. For the salt-free soil-water system, the critical maximum radius of supercooling was 7.15 nm. We compared eight classical ice-water interfacial tension models, and the “Reinhardt & Doye” and “DeMott & Rogers” models showed excellent performance when using the new free energy theoretical framework to predict the crystallization nucleation rate of soil-water systems. A positive correlation between the boundary nucleation rate and soil-water potential is detected. According to the influencing factors, the boundary nucleation rate of soil-water system can be divided into three zones: salt nature control zone (R>100μm), salt-pore mixed control zone (100μm>R>100nm), and pore size control zone (R<100nm).

Plain Language Summary

Unfrozen water refers to the soil moisture that still exists in the form of liquid water in the soil at negative temperatures, which is critical for determining the material properties of frozen soil. The phenomenon of soil water not being able to freeze as soon as the temperature falls below the freezing temperature is referred to as “supercooling”. However, the critical condition of supercooling is still unclear. In this paper, we developed a nucleation model that can be applied to saline soils based on the thermodynamic approach. Subsequently, the critical conditions were verified by freezing-thawing tests of soil samples with different salt contents. We further optimized the model of this paper by selecting the best model for the intermediate variables. Quantitative relationships between the initiation condition of supercooling and soil properties were established. We also delineated pore size intervals for distinguishing differences in supercooling properties at different pore sizes. The proposed theoretical framework is developed from Classical Nucleation Theory, which may provide a theoretical reference for revealing the freezing-thawing mechanism of soil-water systems.