Ice crystal surfaces melt at temperatures below 0°C, and then quasi-liquid layers (QLLs) are formed. However, revealing the dynamic behavior of QLLs, which dominates the surface properties of ice crystals at temperatures near the melting point, remains an experimental challenge. To observe ice crystal surfaces at the molecular level, we and Olympus Engeneering Co. Ltd. recently developed a laser confocal microscope combined with a differential interference contrast microscope (LCM-DM), which can directly visualize individual 0.37-nm-thick elementary steps on ice crystal surfaces [1, 2]. By utilizing LCM-DIM, we found the presence of two types of QLL phases that exhibit different morphologies and dynamics on ice basal faces [3]. In this study, we demonstrate the similarities and differences in the generation mechanisms of two types of QLL phases, as schematically shown in the figure [4]. 

We directly visualized the appearance of round liquid-like droplets (α-QLLs) and thin liquid-like layers (β-QLLs) on ice basal faces by LCM-DIM. We found that α-QLLs always appear at outcrops of dislocations, and that β-QLLs emerge from crystal surfaces where many microdefects are embedded. These results clearly demonstrate the similar function that strain induces the appearance of both types of QLLs. We also found that β-QLLs are spontaneously formed at interfaces between basal faces and α-QLLs when the diameter of the α-QLLs becomes larger than several tens of µm. This result arose from the different structures of α- and β-QLLs: the β-QLLs probably have a structure intermediate between those of basal faces and α-QLLs, resulting in a reduction of the total interfacial free energy. In the conference, we will also briefly demonstrate the thermodynamic stabilities of the α- and β-QLL phases. 

References 

1) G. Sazaki, et al., J. Crystal Growth, 262, 536-542 (2004). 

2) G. Sazaki, et al. Proc. Nat. Acad. Sci. USA., 107, 19702-19707 (2010).  

3) G. Sazaki, et al. Proc. Nat. Acad. Sci. USA., 109, 1052-1055 (2012).  

4) G. Sazaki, et al. Crystal Growth & Design, 13, 1761-1766 (2013). 

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