Ice surfaces just below 0 °C can host two distinct quasi-liquid phases — droplet-like α-QLLs and thinner, better-wetting β-QLLs — that optical microscopy reported as not equilibrium surface-melting films but metastable phases formed kinetically from supersaturated vapor, a no-QLL-at-equilibrium conclusion that later literature has partially moderated. The source work (Asakawa et al., PNAS 2016) left three linked issues open: the elementary mass-transfer kinetics between vapor, the two QLL phases, and the ice substrate have never been measured separately; the molecular-level structure of α- and β-QLLs — whether they are structurally distinct liquids or two wetting morphologies of one liquid — and the apparent contradiction that the less stable β phase wets ice better, are unresolved; and the ice–vapor interfacial free energy needed to place the QLL phases in a wetting phase diagram has not been accurately evaluated. A later-2016 companion paper and a 2022 follow-up established a phenomenological wetting thermodynamics of the two QLL morphologies, and a 2019 PRL measured ice growth inside QLLs (one of the three mass-transfer channels) on basal faces. The separate measurement of vapor-to-QLL and α-to-β mass transfer, the molecular-level structural identification of the two phases, and a direct interfacial-free-energy evaluation from observed two-dimensional nucleation rates remain open.

Background

Surface premelting is the phenomenon in which the outermost molecular layers of a crystal become disordered and liquid-like below the bulk melting point. For ice, a proposal dating to the mid-nineteenth century holds that a thin liquid-like film — the quasi-liquid layer (QLL) — covers the surface near 0 °C, and QLLs are invoked to explain ice friction, gas uptake by snow and clouds, thundercloud electrification, and snow-crystal growth habits (reviews: Slater and Michaelides 2019; Nagata et al. 2019). The conventional theory of Kuroda and Lacmann (1982) treated the QLL as a thermodynamic surface-melting phase that completely wets the ice surface at ice–vapor equilibrium, using interfacial free energies estimated from the density of dangling (unsaturated) hydrogen bonds on the ice surface. Laser confocal microscopy combined with differential interference contrast microscopy (LCM-DIM) — an optical technique capable of resolving individual elementary steps of molecular height (0.37 nm) on ice — overturned parts of this picture in the 2010s. Sazaki et al. (PNAS 2012) discovered that QLLs on ice appear as two coexisting morphologies with different optical contrast, later termed α- and β-QLLs: α-QLLs are droplet-like (round) and β-QLLs are flatter, thin-layer-like, with β appearing at higher supersaturation (water-vapor pressure above the ice–vapor equilibrium value). Asakawa et al. (PNAS 2016) then showed on the basal (0001) faces (the hexagonal ice crystal’s top face, perpendicular to the crystal’s hexagonal symmetry axis) that both QLL types appear only above a critical supersaturation and vanish near equilibrium: they are kinetically formed metastable condensates, not equilibrium melted layers. This posed three linked open issues. First, meaningful kinetics requires separately measuring mass transfer of water molecules along three coupled channels: (1) vapor to QLLs, (2) QLLs to the ice surface (how ice grows through its own QLL), and (3) between α- and β-QLLs; in 2016 the authors stated they had no practical idea how to measure any of them. Second, the structure of the two QLL phases was unknown: β-QLLs are less stable (they disappear first when supersaturation is lowered) yet are much flatter, i.e. apparently wet ice better than α-QLLs — a stability–wettability contradiction that molecular-scale structure should resolve; the authors began developing Raman spectroscopy combined with LCM-DIM for this. Third, the conventional theory's failure was traced to an overestimated ice–vapor interfacial free energy γiv\gamma_{iv} (the excess free energy per unit area of a bare ice–vapor interface, which controls whether liquid films or droplets lower the surface free energy); the authors proposed measuring γiv\gamma_{iv} accurately from directly observed two-dimensional (2D) nucleation rates — the rate at which monomolecular islands nucleate on a crystal face, whose exponential supersaturation dependence encodes the step free energy and interfacial energetics. A companion study (Murata et al., PNAS 2016) showed with two-beam interferometry and wetting theory that QLLs exhibit several wetting states, including pseudo-partial wetting (a droplet sitting on its own thin liquid film) and a first-order wetting transition between them, and are absent at equilibrium — a phenomenological framework that still requires the missing kinetic and structural data, and an accurate γiv\gamma_{iv}, to be predictive.

Problem Statement

Establish quantitatively how the two kinetically formed quasi-liquid phases on ice surfaces are produced, exchange mass, and differ physically. Concretely, for ice crystal surfaces in controlled water vapor near 0 °C: (i) measure separately the elementary mass-transfer kinetics of water molecules along the three coupled channels identified by Asakawa et al. (2016) — vapor to QLLs, QLLs to the ice crystal (molecular incorporation during growth/sublimation), and between the α- and β-QLL phases — so that each channel's kinetic coefficients (rates as functions of temperature and supersaturation) are individually determined rather than inferred from net growth; (ii) determine the molecular-level structure of α- and β-QLLs (e.g. by in situ vibrational spectroscopy or equivalent probes combined with optical microscopy), settle whether the two observed QLL phases are structurally distinct liquids or two wetting morphologies of one liquid, and thereby resolve the stability–wettability contradiction: why the less stable β-QLLs form flatter, better-wetting layers on ice than the more stable α-QLLs; and (iii) evaluate the ice–vapor interfacial free energy accurately and independently — for example from directly observed two-dimensional nucleation rates on ice faces — to replace the overestimated dangling-bond estimate in the Kuroda–Lacmann theory and to fix the equilibrium and nonequilibrium wetting conditions under which each QLL phase can exist. Progress on any channel or sub-question (e.g. the basal-face QLL–ice growth kinetics measured in 2019) counts toward, but does not complete, the objective, which requires the full set of separated kinetics plus structural and interfacial-energetic identification of the two QLL phases.

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