Molecular dynamics simulation of film water thickness and properties at different interfaces in partially saturated frozen soil systems.

The film water, with an exceptional capacity to maintain a premelting, liquid-like state even under subzero conditions, provides a potential dynamic conduit for the movement of water in frozen soils. However, the distinctive structural and dynamic characteristics of film water have not been comprehensively elucidated. In this study, molecular dynamics (MD) simulations were conducted to examine the freezing of a system containing ice, water, silica, and gas. The simulations revealed that as the temperature approaches the melting point, the air-water interface tends to possess a thicker layer of unfrozen water, characterized by a higher diffusion coefficient and lower viscosity. In contrast, the film water near the silica-water interface tends to be thinner and remains relatively unaffected by temperature, with only one twentieth of the diffusion coefficient and nearly 20 times the viscosity observed at the air-water interface. These distinct characteristics resulted from the varying interactions between water molecules and their immediate surroundings. Consequently, the film water in proximity of the silica can be assumed to be relatively immobile compared to that of air-water interface. These findings have implications for the study of unsaturated frozen soil systems, in particular, the importance of considering the film water at the air-water interface in the modeling framework.