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Ice-crystal growth in supercooled water is one of the most familiar examples of phase-transition dynamics, playing essential roles in various natural phenomena on Earth. Despite its fundamental importance, the microscopic view at the elementary step level remains elusive. Here, using an advanced optical microscope, we find self-organization of elementary steps during ice-crystal growth, called step-bunching instability (SBI), driven by the competition between step dynamics, interfacial stiffness, and latent heat diffusions. We also find that the SBI transiently induces screw dislocations and resulting spiral growth in the late stage of the growth process. Furthermore, quantitative observations with a two-beam interferometer allow us to obtain insights into the relative importance of the various mechanisms of the step-step interactions. Our finding offers a significant clue to understanding the general mechanism of melt growth beyond ice-crystal growth, inseparably involving several broad research fields, including cryobiological, geophysical, and material branches.

Ice-crystal growth in supercooled water is one of the most familiar examples of phase-transition dynamics, playing essential roles in various natural phenomena on Earth. Despite its fundamental importance, the microscopic view at the elementary step level remains elusive. Here, using an advanced optical microscope, we find self-organization of elementary steps during ice-crystal growth, called step-bunching instability (SBI), driven by the competition between step dynamics, interfacial stiffness, and latent heat diffusions. We also find that the SBI transiently induces screw dislocations and resulting spiral growth in the late stage of the growth process. Furthermore, quantitative observations with a two-beam interferometer allow us to obtain insights into the relative importance of the various mechanisms of the step-step interactions. Our finding offers a significant clue to understanding the general mechanism of melt growth beyond ice-crystal growth, inseparably involving several broad research fields, including cryobiological, geophysical, and material branches.

We grew prism faces of ice crystals heteroepitaxially on a {10 (1) over bar0} CdSe wafer. Then, we observed the lateral growth of elementary spiral steps on prism faces in vapor by advanced optical microscopy. On prism faces, we found that outcrops of screw dislocations are mostly located in the interiors of prism faces and that distance L between adjacent spiral steps is uniform, whereas these two features are not observed on basal faces. We measured the velocity Vstep of elementary spiral steps on prism faces under various supersaturation and determined the step kinetic coefficient beta. Then, we revealed that the value of fi on prism faces monotonically decreases with decreasing temperature (T) from -2.6 to -25.0 degrees C. In this T range, the value of beta on prism faces is significantly smaller than that on basal faces (determined previously). We further analyzed the relation between L and driving force for crystallization Delta mu and found that at T similar to -15 degrees C, the step ledge free energy kappa on prism faces reaches a peak (8.5 x 10(-10) J/m), implying the change in surface structures at this temperature.

In situ X-ray scattering measurements of insect body surface lipids were successfully attempted by using a synchrotron X-ray source. The temperature-dependent structural changes of the cuticular hydrocarbons covering the forewing of an American cockroach were able to be followed, which showed that the majority of the hydrocarbons were in a liquid state even far below the critical temperature of water transpiration through the body surface. The results clearly demonstrated that synchrotron radiation X-ray scattering has the potential to obtain the detailed information about the intact lipid structure and physical properties on insect body surfaces.

The surfaces of ice crystals near the melting point are covered with thin liquid water layers, called quasi-liquid layers (QLLs), which play crucial roles in various chemical reactions in nature. So far, there have been many spectroscopic studies of such chemical reactions on ice surfaces, however, revealing the effects of atmospheric gases on ice surfaces remains an experimental challenge. In this study, we chose HNO3 as a model atmospheric gas, and directly observed the ice basal faces by advanced optical microscopy under partial pressure of HNO3 (similar to 10(-4) Pa), relevant to those found in the atmosphere. We found that droplets (HNO3-QLLs) appeared on ice surfaces at temperatures ranging from -0.9 to -0.2 degrees C with an increase in temperature, and that they disappeared at temperatures ranging from -0.6 to -1.3 degrees C with decreasing temperature. We also found that the size of the HNO3-QLLs decreased immediately after we started reducing the temperature. From the changes in size and the liquid-solid phase diagram of the HNO3-H2O binary system, we concluded that the HNO3-QLLs did not consist of pure water, but rather aqueous HNO3 solutions, and that the temperature and HNO3 concentration of the HNO3-QLLs also coincided with those along a liquidus line.

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 degrees 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 degrees 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.

The surfaces of ice crystals near the melting point are covered with thin liquid water layers, called quasi-liquid layers (QLLs), which play crucial roles in various chemical reactions in nature. So far, there have been many spectroscopic studies of such chemical reactions on ice surfaces, however, revealing the effects of atmospheric gases on ice surfaces remains an experimental challenge. In this study, we chose HNO3 as a model atmospheric gas, and directly observed the ice basal faces by advanced optical microscopy under partial pressure of HNO3 (similar to 10(-4) Pa), relevant to those found in the atmosphere. We found that droplets (HNO3-QLLs) appeared on ice surfaces at temperatures ranging from -0.9 to -0.2 degrees C with an increase in temperature, and that they disappeared at temperatures ranging from -0.6 to -1.3 degrees C with decreasing temperature. We also found that the size of the HNO3-QLLs decreased immediately after we started reducing the temperature. From the changes in size and the liquid-solid phase diagram of the HNO3-H2O binary system, we concluded that the HNO3-QLLs did not consist of pure water, but rather aqueous HNO3 solutions, and that the temperature and HNO3 concentration of the HNO3-QLLs also coincided with those along a liquidus line.