Molecular insights into the unfrozen water flow in mica pores

Unfrozen water migration within frozen soils plays a vital role in cold-region engineering. However, the underlying microscopic mechanisms that govern unfrozen water flow are not well understood. In this study, non-equilibrium molecular dynamics simulations were employed to investigate the flow behavior of unfrozen water confined between mica and ice surfaces. A three-phase mineral-water-ice (MWI) model and a mineral-water-mineral (MWM) model were developed for comparison to explore the effects of interfacial properties, temperature, and external forces on water flow. By applying different external forces to the MWI model, the hindering effect of the increasing flow velocity on pore water freezing was revealed. Temperature is identified as a dominant factor influencing flow properties, with decreasing temperatures enhancing viscosity and reducing water mobility. Besides, the comparative analyses between the two models demonstrate that the ice exhibits a stronger inhibitory effect on water flow compared to the mica surface, leading to increased viscosity and reduced hydraulic conductivity. Hydrogen bond network analysis reveals that the MWI model forms a greater number of hydrogen bonds than the MWM model, with a higher proportion of shorter bond lengths between water molecules, indicating a more stable hydrogen-bonding structure and stronger resistance to water flow imposed by the ice interface. This study quantitatively analyzes the hydrodynamic behavior of confined water under the simultaneous influence of ice and mineral surfaces, providing a comprehensive atomic-scale understanding of unfrozen water flow in frozen soils and highlighting the necessity of considering the effects of ice in macroscopic seepage predictions.