Two types of quasi-liquid layers on ice crystals are formed kinetically

Abstract

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...

Thermodynamic origin of surface melting on ice crystals

“…Surprisingly, this work revealed that, contrary to the common belief that QLLs completely and homogeneously wet ice surfaces, they are spatiotemporally heterogeneous and are absent in the equilibrium conditions (22,(24)(25)(26). Furthermore, we have observed that QLLs exhibit more than one wetting morphology: droplet type, thin-layer type, and their coexistence (sunny-side-up type) at supersaturation (22,(24)(25)(26). This finding fundamentally requires us to recast the conventional understanding based on spatiotemporally averaged equilibrium theories and experiments (e.g., scattering, spectroscopy, and ellipsometry), because of ignorance of the counterintuitive nature of QLLs.…”

“…For ΔF < 0, ice surfaces lower the total free energy by ΔF by getting wet, which drives the spontaneous formation of QLLs and permits the equilibrium surface melting. However, we previously found the absence of QLLs at and near the ice-vapor equilibrium (26). As shown in Movie S4, a droplettype (partially wet) QLL gradually disappears on an ice surface at the ice-vapor equilibrium (T = −1.5 °C and p = 540 Pa) for sufficiently long time to observe the whole kinetics directly.…”

“…A series of our recent studies (22,(24)(25)(26) have assumed the presence of two different liquid phases (α-and β-QLL phases) to explain a variety of wetting morphologies of the QLLs without characterizing the order parameter distinguishing these two liquid phases. Hereafter we will show that such a nontrivial assumption is not necessary in the context of the standard wetting theory (27).…”

Melting the ice one layer at a time

“…The study of Sánchez et al (4) does not provide insight into what happens above 257 K, so it is unclear whether melting continues in a bilayer-by-bilayer manner beyond the single step postulated at 257 K. However, much closer to the bulk melting temperature a complementary series of high-resolution optical interferometry measurements has provided insight (16,17).…”

Templated Synthesis for Nanoarchitectured Porous Materials

“…For example, Kunitake and co-workers introduced hydrophilic oligoethylene glycol bis-acrylate molecules in the interlayer spaces of a synthetic multilayered double-alkyl cationic lipid templates (Figure 3). 53,54 Crosslinking polymerization within the template nanospaces upon the irradiation of an ultrahigh pressure Hg lamp in the presence of a photoinitiator resulted in highly oriented hybrids. Extracting the template components by immersing the hybrid film in methanol led to the formation of self-standing transparent films within which 2 nm thick 2D polymer nanosheets are stacked together.…”