In-situ optical microscopy observation of elementary steps on ice crystals grown in vapor and their growth kinetics

Ice is one of the most abundant materials on the earth's surface, and its growth governs various natural phenomena. Hence, the molecular-level understanding of ice crystal surfaces is crucially important. However, it is generally acknowledged that the molecular-level observation of ice crystal surfaces by ordinary microscopy techniques, such as atomic force microscopy and scanning electron microscopy, is very difficult at temperatures near the melting point (0 °C). To overcome such difficulties, we have developed laser confocal microscopy combined with differential interference contrast microscopy (LCM-DIM). We proved that LCM-DIM can visualize individual elementary steps (0.37 nm in thickness) on a basal face by observing two-dimensional nucleation growth. Then we found by LCM-DIM that spiral steps on a basal face exhibit a double-spiral pattern, which can be expected from ice's crystallographic structure. In addition, we revealed that temperature dependence of growth kinetics of elementary spiral steps on a basal face exhibits complicated behaviors, which show the presence of unknown phenomena in the growth kinetics. Furthermore, we proved that surface diffusion of water admolecules on a basal face plays a crucially important role in the lateral growth of elementary steps when the distance between adjacent spiral steps is smaller than 15 µm. These findings will provide a clue for unlocking growth kinetics of ice crystals. In addition, through the use of LCM-DIM much progress has been made in studies on the surface melting of ice and the interaction between ice and atmospheric gasses.