Quasi-liquid Layers in Grooves of Grain Boundaries and on Grain Surfaces of Polycrystalline Ice Thin Films

In nature, a large proportion of ice is present in a polycrystalline state. Thus, understanding the formation of quasi-liquid layers (QLLs) on/in polycrystalline ice is indispensable for understanding a wide variety of natural phenomena. In this study, we observed surfaces of polycrystalline ice thin films using our advanced optical microscope. We focused our attention on the macroscopic fluidity of objects observed on polycrystalline ice surfaces as evidence for the presence of QLLs. Systematic observations under various temperatures and water vapor pressures showed that, with increasing temperature, QLLs first appeared preferentially in grooves of grain boundaries and continued to exist at −1.9 ± 0.4 °C, irrespective of the water vapor pressure (even in immediate vicinities of the vapor–ice equilibrium curve). From this result, we concluded that the QLLs were formed by melting of grain boundaries to relax lattice mismatches. With a further increase of temperature, droplet-type QLLs appeared on grain surfaces at −0.7 ± 0.2 °C. However, as time elapsed, the droplet-type QLLs on the grain surfaces spontaneously disappeared within 5 ± 3 min even though temperature and water vapor pressure were kept constant. Such appearance and subsequent disappearance of the droplet-type QLLs on the grain surfaces were observed even under relatively highly supersaturated and undersaturated conditions.