[M-IS12] Interface- and nano-phenomena on crystal growth and dissolution

[MIS12-04] Low- and High Density Unknown Waters at Ice/Water Interfaces under High Pressure

*Hiromasa Niinomi¹, Akira Kouchi², Tetsuya Hama³, Hiroki Nada⁴, Tomoya Yamazaki², Yuki Kimura² (1.Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2.Institute of Low Temperature Science, Hokkaido University, 3.Komaba Institute for Science, The University of Tokyo, 4.National Institute of Advanced Industrial Science and Technology (AIST))

Keywords:water, liquid polymorphism, ice, melt/crystal interface, low-density liquid, high-density liquid

Water is an abundant liquid on the Earth, and its property plays a crucial role in governing the environment on the Earth. Moreover, our lives are built on mysterious properties of water unlike normal liquids, which is often missed due to its commonness, such as non-linear dependence of thermodynamic response function on temperature. It is, therefore, indisputable that the elucidation of the origin of the property of water is highly desirable not only for Earth science but also for a broad range of scientific fields. However, the origin of the mysterious properties of water still remains unclear. The most prevalent scenario to explain water’s property, so-called second critical point scenario, hypothesizes the existence of two kinds of liquid polymorphism of water –high density liquid (HDL) and low density liquid (LDL)–. The non-linear dependence of thermodynamic response function is elegantly explained by the existence of liquid-liquid critical point (LLCP) in which water separates into LDL and HDL through liquid-liquid phase separation (LLPS) under low temperature and high pressure condition [1,2]. However, because theoretically-predicted LLCP lies in an experimentally inaccessible ‘no-man’s land’, where deep supercooling of liquid water required to reach the LLCP is obstructed by fast crystallization kinetics beyond experimentally accessible timescales [3], direct observation of LLPS of water to the two kinds of liquid polymorphisms as the definitive proof of the hypothesis is still lacking. Amid this situation, we have previously reported that layers and droplets of a high-density unknown water (HDUW) with clear interface appeared macroscopically at interfaces between water and high pressure ices grown or melted in water on (de)pressurization by sapphire anvil cell under experimentally-accessible conditions [4].
In this presentation, we show by in situ optical microscopy that layers and droplets of a low-density unknown water (LDUW) appear macroscopically at the interface between water and ice I_h grown or melted in water on (de)pressurization under accessible conditions. Moreover, we report that the ratios of interfacial tension to viscosity, so-called characteristic velocities, of LDUW and HDUW were successfully determined by analyzing dynamics of LDUW and HDUW. Our discovery of the pair of LDUW and HDUW in analogous with LDL and HDL should provide insights on the mystery of liquid polymorphism of water.

[1] P. H. Poole et al., Nature 1992, 360, 324-328.
[2] D. A. Fuentevilla et al., Phys. Rev. Lett. 2006, 97, 195702.
[3] K. H. Kim et al. Science 2017, 358, 1589-1593.
[4] H. Niinomi et al. J. Phys. Chem. Lett., 2020, 11, 6779-6784.