Quasi-water layer sandwiched between hexagonal ice and wall and its influences on the ice tensile stress

The presence of a quasi-water/premelting layer at the interface between wall and ice when the temperature (T) is below the melting point was extensively observed in experiments. In this work, molecular dynamics simulations are performed to analyze the underlying physics of the quasi-water layer and the effects of the layer on the ice tensile stress. The results indicate that each molecule and its four nearest neighbours in the quasi-water layer representing an equilibrium structure gradually form a tetrahedral ice-like arrangement from an unstructured liquid-like structure along the direction away from the wall. The average density of the quasi-water layer is equal to or higher than the bulk density of water at T ≥ 240 K or T ≤ 240 K respectively, and reaches 1.155 g cm^-3 at T = 210 K, suggesting a structural correlation with the high-density liquid phase of water. Depending on the temperature and wall wettability, the thickness of the quasi-water layer (H_q) ranges from ∼2 Å to ∼25 Å. For prescribed hydrophilic walls, H_q increases monotonically with temperature, and is almost proportional to(T_m - T)^-1/3, where T_m is the melting temperature of ice. H_q keeps an almost constant value (2 Å) as the temperature increases and rises sharply after passing a threshold temperature of T ≈ 250 K. In the joint effects of the wall wettability and quasi-water layer's thickness, the ice tensile stress decreasing monotonically at a larger temperature shows an upward trend and then reduces to almost a constant value as the wall changes from a hydrophobic to a hydrophilic one. The results reveal the potential development of anti-icing/de-icing techniques by heating the wall or modifying its surface to increase H_q.