Perspective: Surface freezing in water: A nexus of experiments and simulations

Abstract

Surface freezing is a phenomenon in which crystallization is enhanced at a vapor-liquid interface. In some systems, such as n-alkanes, this enhancement is dramatic, and results in the formation of a crystalline layer at the free interface even at temperatures slightly above the equilibrium bulk freezing temperature. There are, however, systems in which the enhancement is purely kinetic, and only involves faster nucleation at or near the interface. The first, thermodynamic, type of surface freezing is easier to confirm in experiments, requiring only the verification of the existence of crystalline order at the interface. The second, kinetic, type of surface freezing is far more difficult to prove experimentally. One material that is suspected of undergoing the second type of surface freezing is liquid water. Despite strong indications that the freezing of liquid water is kinetically enhanced at vapor-liquid interfaces, the findings are far from conclusive, and the topic remains controversial. In this perspective, we present a simple thermodynamic framework to understand conceptually and distinguish these two types of surface freezing. We then briefly survey fifteen years of experimental and computational work aimed at elucidating the surface freezing conundrum in water.

Studying rare events using forward-flux sampling: Recent breakthroughs and future outlook

“…An alternative thermodynamic explanation was recently provided by Lupi et al based on the importance of the entropy of cubic and hexagonal stacks in small nuclei. 249 Role of Vapor-Liquid Interfaces in Homogeneous Nucleation: With these rate calculations at hand, several researchers turned their attention to exploring a confounding-and controversial-phenomenon in atmospheric physics known as surface freezing, 250 or enhancement of homogeneous ice nucleation close to a vaporliquid interface. Surface freezing was first hypothesized by Tabazadeh et al 251 to explain apparent discrepancies among rates measured for microdroplets of different sizes, a hypothesis that was later tested by several researchers.…”

“…Wetting is a process that can occur in a system of two coexisting phases 1 and 2, which are in contact with an external surface s. According to classical thermodynamics, the equilibrium behavior of such a system will depend on a dimensionless parameter called wettability, 250 which is given by the Young equation:…”

Forward flux sampling calculation of homogeneous nucleation rates from aqueous NaCl solutions

“…4 In other words, the existing experimental techniques lack the spatiotemporal resolution necessary for detecting critical nuclei. 5 While molecular simulations do not suffer from such lack of resolution, nucleation, as a rare event, usually occurs at timescales considerably longer than microseconds, for the typical system size (N ∼ 10 3 ) used in simulations. This fact, combined with the lack of thermal averaging provided by sampling single trajectory, places the direct calculation of nucleation rates beyond the reach of conventional molecular dynamics simulations.…”

Nanosecond Freezing of Water at High Pressures: Nucleation and Growth near the Metastability Limit

“…First-order phase transitions and their kinetics remain a fascinating topic with applications to virtually every major industry and to frontier fields like astronomy and inertial confinement fusion [1]. Despite its significance, many aspects of this topic are still largely unexplored, even for a fundamentally important substance like water at ambient pressure [2][3][4]. At this low pressure, it is difficult to deeply undercool liquid water [5,6], and so the driving force for freezing is rather limited in magnitude.…”