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Due to the abundance of ice on earth, the phase transition of ice plays crucially important roles in various phenomena in nature. Hence, the molecular-level understanding of ice crystal surfaces holds the key to unlocking the secrets of a number of fields. In this study we demonstrate, by laser confocal microscopy combined with differential interference contrast microscopy, that elementary steps (the growing ends of ubiquitous molecular layers with the minimum height) of ice crystals and their dynamic behavior can be visualized directly at air-ice interfaces. We observed the appearance and lateral growth of two-dimensional islands on ice crystal surfaces. When the steps of neighboring two-dimensional islands coalesced, the contrast of the steps always disappeared completely. We were able to discount the occurrence of steps too small to detect directly because we never observed the associated phenomena that would indicate their presence. In addition, classical two-dimensional nucleation theory does not support the appearance of multilayered two-dimensional islands. Hence, we concluded that two-dimensional islands with elementary height (0.37 and 0.39 nm on basal and prism faces, respectively) were visualized by our optical microscopy. On basal and prism faces, we also observed the spiral growth steps generated by screw dislocations. The distance between adjacent spiral steps on a prism face was about 1∕20 of that on a basal face. Hence, the step ledge energy of a prism face was 1∕20 of that on a basal face, in accord with the known lowertemperature roughening transition of the prism face.in situ observation | monomolecular steps | two-dimensional nucleation growth | spiral growth I ce is one of the most abundant materials on earth, and its phase transition governs a wide variety of phenomena, such as weather, environment-related issues, life in a cryosphere, and cosmic evolution, etc. Hence the molecular-level understanding of ice crystal surfaces is crucially important. For example, ice crystal surfaces play a key role in heterogeneous physical/chemical reactions, such as the degradation of ozone and organic compounds adsorbed on ice crystal surfaces by UV light irradiation (1-4), the suppression of the growth of ice in living things by antifreeze proteins adsorbed on ice crystal surfaces (5-8), etc., as well as in the growth and sublimation/melting of ice crystals.A crystal bounded by flat crystal faces grows layer by layer (9, 10), utilizing laterally growing molecular layers that have the minimum height determined by the crystal structure. Hence, growing ends of such molecular layers, so-called "elementary steps," which ubiquitously exist on a crystal surface, play a key role during the physical/chemical reactions and the growth and sublimation/melting of ice crystals. Therefore to clarify such phenomena at the molecular level, first one has to observe elementary steps on ice crystal surfaces.Many optical microscopy studies have been carried out to date to observe the surface morphology of ice crystals, such ...

Ice plays crucially important roles in various phenomena because of its abundance on Earth. However, revealing the dynamic behavior of quasi-liquid layers (QLLs), which governs the surface properties of ice crystals at temperatures near the melting point, remains an experimental challenge. Here we show that two types of QLL phases appear that exhibit different morphologies and dynamics. We directly visualized the two types of QLLs on ice crystal surfaces by advanced optical microscopy, which can visualize the individual 0.37-nm-thick elementary steps on ice crystal surfaces. We found that they had different stabilities and different interactions with ice crystal surfaces. The two immiscible QLL phases appeared heterogeneously, moved around, and coalesced dynamically on ice crystal surfaces. This picture of surface melting is quite different from the conventional picture in which one QLL phase appears uniformly on ice crystal surfaces.in situ observation | laser confocal microscopy | differential interference contrast microscopy

Since the pioneering prediction of surface melting by Michael Faraday, it has been widely accepted that thin water layers, called quasi-liquid layers (QLLs), homogeneously and completely wet ice surfaces. Contrary to this conventional wisdom, here we both theoretically and experimentally demonstrate that QLLs have more than two wetting states and that there is a first-order wetting transition between them. Furthermore, we find that QLLs are born not only under supersaturated conditions, as recently reported, but also at undersaturation, but QLLs are absent at equilibrium. This means that QLLs are a metastable transient state formed through vapor growth and sublimation of ice, casting a serious doubt on the conventional understanding presupposing the spontaneous formation of QLLs in ice-vapor equilibrium. We propose a simple but general physical model that consistently explains these aspects of surface melting and QLLs. Our model shows that a unique interfacial potential solely controls both the wetting and thermodynamic behavior of QLLs.surface melting | quasi-liquid layer | advanced optical microscopy | pseudo-partial wetting | wetting transition I n general, surfaces and interfaces yield unique phase transitions absent in the bulk (1-5). Surface melting (or premelting) of ice (3, 4) is one typical and classical example that has been known since the first prediction by Michael Faraday in 1842 (6). He hypothesized that thin water layers, now called quasi-liquid layers (QLLs), wet ice crystal surfaces even at a temperature below the melting point. Since then, this phenomenon has attracted considerable attention not only because of its importance in the fundamental understanding of melting (a solid-toliquid transition) itself but also as a link to a diverse set of natural phenomena in subzero environments: making snowballs, slippage on ice surfaces, frost heave, recrystallization and coarsening of ice grains, morphological change of snow crystals, electrification of thunderclouds, and ozone-depleting reactions (3, 4, 7). Furthermore, it is now recognized that surface melting is not specific to ice but rather is universally seen in a wide range of crystalline surfaces such as metals, semiconductors, ceramics, rare gases, and organic and colloidal systems (8-12). Its underlying physics is therefore also inseparable from material science and technology.Although the origin of surface melting, including the nature of QLLs themselves, is still far from completely understood and a matter of active debate (13-18), it is at least phenomenologically believed that surface melting is driven by the reduction of the surface free energy by the presence of intervening liquid between the solid and gas phases (3,4,13,19). More sophisticated approaches have also been proposed in terms of surface phase transitions (1,3,4,20). In contrast to such theoretical speculations, however, the direct observation and the accurate characterization of QLLs by experiments are still highly challenging because of their thinness, assumed to be less t...

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