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...

Surfaces of ice are covered with thin liquid water layers, called quasi-liquid layers (QLLs), even below their melting point (0°C), which govern a wide variety of phenomena in nature. We recently found that two types of QLL phases appear that exhibit different morphologies (droplets and thin layers) [Sazaki G. et al. (2012) Proc Natl Acad Sci USA 109(4):1052−1055]. However, revealing the thermodynamic stabilities of QLLs remains a longstanding elusive problem. Here we show that both types of QLLs are metastable phases that appear only if the water vapor pressure is higher than a certain critical supersaturation. We directly visualized the QLLs on ice crystal surfaces by advanced optical microscopy, which can detect 0.37-nm-thick elementary steps on ice crystal surfaces. At a certain fixed temperature, as the water vapor pressure decreased, thin-layer QLLs first disappeared, and then droplet QLLs vanished next, although elementary steps of ice crystals were still growing. These results clearly demonstrate that both types of QLLs are kinetically formed, not by the melting of ice surfaces, but by the deposition of supersaturated water vapor on ice surfaces. To our knowledge, this is the first experimental evidence that supersaturation of water vapor plays a crucially important role in the formation of QLLs. molecular-level observation | advanced optical microscopy | metastable phase | supersaturation I ce is one of the most abundant materials on Earth, and its surfaces are covered with thin liquid water layers even below their melting point (0°C) (1-4). Such thin liquid water layers are called "quasi-liquid layers" (QLLs). Because QLLs govern the surface properties of ice just below the melting point, it is well acknowledged that surface melting of ice governs a wide variety of phenomena, such as electrification of thunderclouds (4, 5), regelation (4, 6), frost heave (4, 7), conservation of foods, ice skating (1, 8), preparation of a snowman (1), and growth of ice crystals (2, 4). Therefore, it is essential to understand the surface melting of ice crystals at the molecular level.After Michael Faraday proposed the existence of QLLs in 1842 (1), many studies experimentally confirmed the formation of QLLs by various methods (Table S1). All such studies revealed that the thickness of QLLs significantly increases with increasing temperature. However, such the studies used spectroscopy and scattering methods, which can obtain only temporally and spatially averaged information, or optical microscopy, which does not have sufficient spatial resolution. Hence, the nature of surface melting has not been fully unlocked. To further understand the dynamic behavior of QLLs, we need to perform real-time and real-space observations of ice crystal surfaces at the molecular level.Recently, we and Olympus Engineering Co., Ltd., have developed one such technique, namely, laser confocal microscopy combined with differential interference contrast microscopy (LCM-DIM) (9), which can directly visualize the 0.37-nm-thick elementary steps on...

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We measured velocities V step of lateral displacement of individual elementary steps on an ice basal face, for the first time, by advanced optical microscopy, under various bulk water vapor pressure P H2O ∞. Distances L between adjacent spiral steps exhibited considerable variation under constant P H2O ∞. Hence, we analyzed V step as functions of L and P H2O ∞. Then we found that (1) under a constant P H2O ∞, V step decreased with decreasing distances L when L ≤ 15 μm and that V step remained constant when L ≥ 15 μm. We named V step of L ≥ 15 μm (isolated steps) V step int and analyzed dependencies of V step int on P H2O ∞. Then we found that (2) the slope of the V step int vs P H2O ∞ plot gradually decreased with increasing P H2O ∞. We proposed a model that took into account both the volume diffusion of water vapor molecules and the surface diffusion of water admolecules on a terrace. Our model could explain result (1) mainly by the competition of adjacent spiral steps for water admolecules diffusing on a terrace but could not explain the result (2) satisfactorily.

Three-dimensional colloidal crystals made of ferromagnetic particles, such as magnetite (Fe(3)O(4)), cannot be synthesized in principle because of the strong attractive magnetic interaction. However, we discovered colloidal crystals composed of polyhedral magnetite nanocrystallites of uniform size in the range of a few hundred nanometers in the Tagish Lake meteorite. Those colloidal crystals were formed 4.6 billion years ago and thus are much older than natural colloidal crystals on earth, such as opals, which formed about 100 million years ago. We found that the size of each individual magnetite particle determines its morphology, which in turn plays an important role in deciding the packing structure of the colloidal crystals. We also hypothesize that each particle has a flux-closed magnetic domain structure, which reduces the interparticle magnetic force significantly.