How do Quasi-Liquid Layers Emerge from Ice Crystal Surfaces?
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. Here, we demonstrate the similarities and differences in the generation mechanisms of two types of QLL phases, which show different morphologies and dynamics. We directly visualized the appearance of round liquidlike droplets (α-QLLs) and thin liquidlike layers (?-QLLs) on ice basal faces by advanced optical microscopy, which can allow visualization of the individual elementary steps on basal faces. We found that α-QLLs always appear at outcrops of dislocations, and that ?-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 ?-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 micrometers. This result arose from the different structures of α- and ?-QLLs: the ?-QLLs probably have a structure intermediate between those of basal faces and α-QLLs, resulting in a reduction of the total interfacial free energy.