Present research subjects

1) Mechanisms and kinetics of the growth of snow/ice crystals in vapor and melt phases

 

1-1) Changes in morphologies of snow crystals

Snow crystals grown in supersaturated water vapor change their morphologies with temperature and water vapor pressure, as shown in Fig. 1. With decreasing temperature, a snow crystal (a snow flake) shows a hexagonal plate shape (from 0 to -4 °C), a hexagonal lod shape (from -4 to -10 °C), a hexagonal plate shape (from -10 to -22 °C), and a hexagonal lod shape (< -22 °C). Fig. 1 is called the "Nakaya diagram": Prof. U. Nakaya was a pioneer who opened up physics of snow and ice, and a founder of Institute of Low Temperature Science, Hokkaido University. Although studies on snow crystals have long history, mechanisms of the morphological changes in snow crystals are still unclear. We are tackling this long-year mystery.


Fig. 1. Changes in morphologies of snow crystals. Furukawa and Wettlaufer (2007) Phys. Today 60,70-71.

1-2) Growth mechanisms of ice crystals in supercooled melt


In supercooled melt (water at a temperature lower than 0 °C), no one could so far study growth mechanisms of ice crystals at the molecular level. In our laboratory, we have been debeloping various "cutting-edge optical microscopy techniques" that can visualize subnanometer-height molecular (atomic) layers on flat crystal surfaces. Fig. 2 demonstrates that molecular layers of 0.37 nm in height (the size of one water molecule) are growing in lateral directions beneath a quasi-liquid layer (a liquid thin layer that can appear even below 0°C on ice crystal surfaces). We have been finding novel physical phenomena. We expect so in the future as well.


Fig. 2. Molecular layers (elementary steps) growing in lateral directions beneath a quasi-liquid layer. Murata et al.(2019) Phys. Rev. Lett. 122, 026102.

1-3) Mechanisms of action of anti-freeze protein

Many ectothermic (heterothermic) animals living in the cryosphere, such as fish, insects, bacteria, etc., need to prevent frozen death under subzero temperature during winter. Special proteins, so called "anti-freeze proteins (AFP)", play a main role in preventing the crystallization of ice in their bodies. We are trying to reveal working mechanisms of AFP using various optical microscopy and interferometry techniques.

2) Quasi-liquid layers (thin liquid water layers) that appear on ice crystal surfaces below 0 °C.

 

2-1) Formation mechanisms of quasi-liquid layers

Ice crystals are fully melted at 0°C. However, even below 0 °C, surfaces of ice crystals are slightly melted and covered with thin liquid water layers, so called quasi-liquid layers (QLLs). This phenomenon is called "surface melting". QLLs play crucially important roles in a wide variety of phenomena, such as the preparation of a snow ball, slipperiness of ice, frost heave, regelation, conservation of tissues and foods, electrification of thunderclounds, etc. However, it was not until the early 2010s that QLLs on ice crystal surfaces could be visualized directly. Fig. 3 demonstrates that two types of QLLs, which show different morphologies (droplets and thin layers), appear on ice crystal surfaces. We are further studying the formation mechanisms and properties of QLLs on ice crystal surfaces.


Fig. 3. Two types of QLLs, droplets and thin layers (marked by white and red arrowheads, respectively), appeared on an ice crystal surface. Black arrowheads and black arrows show molecular layers (elementary steps) and their growth directions, respectively. Sazaki et al. (2012) Proc. Nat. Acad. Sci. USA, 109, 1052-1055.

2-2) Surface melting of polycrystalline ice

In nature, a large proportion of ice is present in a polycrystalline state. Hence, it is essential to understand the behavior of QLLs on/in polycrystalline ice. Polycrystalline ice has many grain boundaries (interfaces between adjacent ice single crystals). In addition, individual grains are randomly oriented. Hence, we are interested in the behavior of QLLs in grain boundaries and on grain surfaces.

3) Ice crystals and environmental issues

 

3-1) Effects of acidic gasses

Acidic gasses, such as hydrogen chloride and nitric acid, in atmosphere promote various heterogeneous chemical reactions on ice crystal surfaces. The formation of an ozone hole (the degradation of ozone) is one of such chemical reactions that give a significant impact on the earth environment. Hence, interactions between ice surfaces and acidic gasses hold a key for various environmental issues in the cryosphere. Fig. 4 demonstrates that droplets of a HCl aqueous solution are formed on ice crystal surfaces, and that the droplets are embedded in the ice crystal during the growth.


Fig. 4. Incorporation of droplets of a HCl aqueous solution into the bulk of an ice crystal. Nagashima et al. (2018) Crystal Growth & Design 18, 4117-4122.

3-2) Chemical reactions on ice crystal surfaces


We are also planning to directly monitor heterogeneous chemical reactions on ice crystal surfaces, in order to make the impact of the study of 3-1) more significant.