The physics and chemistry of ice

This themed issue compiles a selection of invited talks given at the 14th International Conference on the Physics and Chemistry of Ice (PCI) hosted by the Paul Scherrer Institute (Switzerland) in January 2018. PCI is an international symposium series dating back 55 years and is devoted to research on all aspects of ice. Its 14^th edition reflects the ongoing interest in ice research. Apart from its environmental relevance on Earth and in Space and its importance in everyday life, ice is an excellent model system for phase transitions, interface and deformation processes that are studied in materials in general, but with distinct advantages of availability, accessibility, non-toxicity, design and transparency.

This themed issue covers the variety of topics central to this conference series, addressing the scientific disciplines of glaciology, snow physics, ice nucleation, biology, astrophysics and atmospheric science. The range of scales that these topics cover spans from molecular interactions to regional phenomena. It is striking how similar scales and research questions are tackled in different disciplines. Bringing these disciplines together to exchange knowledge on our predictive understanding and on experimental techniques is the sole purpose of this conference series.

For example, in biology and oceanography, the precise mechanism of how cold-blooded animals survive in sub-zero environments is still open for debate. In their contributions, Furukawa et al. [1] and Braslavsky & Chasnitsky [2] seek a molecular level understanding of ice nucleation of relevance to biological systems, reporting on the interplay and adsorption of antifreeze protein and antifreeze glycoproteins with ice surfaces. Furukawa describes experiments on the free growth of ice crystals in supercooled water both on the ground and under the microgravity condition without any influence of convection, which has been realized in the International Space Station, discussing the anisotropic effect of antifreeze glycoproteins [3–5]. Braslavsky describes investigations of the dynamic nature of the proteins and ice interaction, following the rich literature on this topic [6–13]. This dynamic description highlights that binding of proteins to ice is irreversible and that the freezing temperature depression is sensitive to the time allowed for the proteins to accumulate on ice surfaces. Ice nucleation research is also booming in atmospheric science and much of the attention is on the need to improve the description of aerosol and cloud processes. In their contribution, Kusalik et al. [14] point out that the ordering processes associated with the nucleation and growth of ice crystals have proven difficult to study directly with experiments, in part due to the stochastic nature of the underlying molecular processes. Molecular simulations provide an excellent method to study crystal nucleation and growth of ice at a molecular level, because they are able to directly probe the microscopic environment. In this issue, the Kusalik paper describes simulation strategies focusing on the parametrization of the systems' molecular order and on the potential energies of crystallization.

The interaction with impurities plays a crucial role in the phase changes and in the structure of ice and remains a controversial topic. Bove & Ranieri [15] review experimental and molecular dynamic simulation work on salty ices and on ice clathrates under planetary conditions and discuss the stability, dynamics and properties of these filled ice structures [16–19]. The behaviour of ice, either pure or ion- and gas-filled, is of paramount importance for the description of the interior of ice bodies in our universe and gives deeper insights on both quantum effects and hydrophobic/hydrophilic interactions in ice. Wettlaufer [20] highlights conditions where interfacial effects significantly contribute to, or even dominate, phase transitions in the absence and presence of impurities. Interestingly, these effects might affect both the thermodynamics and the kinetics of a system. The recognition of the inherent relevance of the ice interfacial region not only for phase transitions, but also for chemistry and exchange processes, was further reflected by a number of contributions to the conference probing chemistry of or structural features at the interface with high spatial selectivity [21–29]. A common focus of these studies is the surface pre-melting, or quasi liquid layer (QLL) and the debates on its chemical and physical uniformity, its on-set temperature and its extent into the interfacial layer continue.

Amann-Winkler et al. [30] focus on transitions of ice in the absence of impurities reviewing connections between two distinct amorphous states of ice with different density (high- and low-density amorphous ice, HDA and LDA) to the occurrence of two distinct liquid phases with their specific density [31–35]. The glass transition in both amorphous states, LDA and HDA, was investigated using different experimental techniques and discussions on this have provoked controversy in recent years'. Their recent results using X-ray diffraction as well as X-ray correlation spectroscopy support the previous findings of HDA undergoing a glass–liquid transition at ambient pressure around 110 K and are consistent with the hypothesis of a liquid–liquid transition between HDL and LDL.

In most natural conditions, ice and snow deform plastically, mostly by the movement of linear crystalline defects under stress: the dislocations. Weiss [36] provides here a review of the dynamics of dislocations in ice, in comparison with the case of other crystalline materials such as metals. While dislocations are generally supposed to move regularly under an imposed mechanical load, Weiss & Grasso [37] first used acoustic emission to show that dislocations in ice were moving intermittently, in avalanches. In this field again, ice has proven to be a model material, owing to its strong viscoplastic anisotropy, with a dislocation dynamic characterized by this ‘wild behaviour’. Weiss compares this behaviour with more isotropic materials (Copper, Aluminium) within which a ‘mild’ dislocation dynamic (continuous movement) dominates but still includes a few avalanches, questioning the stability of the dislocation structure.

Baker [38] continues the discussion on the structure focusing on techniques used to characterize the microstructures of snow, firn (multi-year snow) and ice on a larger scale. These techniques, some of which also reveal the location of impurities in the samples, include: transmission electron microscopy, synchrotron-based X-ray topography, cold-stage scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, electron channelling patterns and electron backscatter diffraction (EBSD); cold-stage confocal scanning optical microscopy coupled with Raman spectroscopy; and micro X-ray computed tomography. As an illustration, in terms of characterizing the deformation behaviour, X-ray diffraction was a pioneering technique in revealing the nature of individual dislocations [39] and cryo-EBSD can now reveal the complexity of dislocation structures, including the role of non-basal dislocations [40].

Baker's contribution deals with both the understanding of the microstructure, its heterogeneity, and how they evolve with time, as this is key to understanding both their mechanical properties and physical properties of snow and ice on Earth. The contribution by Wells et al. [41] goes one step further in scale and deals with the fundamentals of the multiphase behaviour of sea ice, with a particular focus on continuum phase-averaged models of sea ice thermodynamics, and the generation of convective flows through the porous ice. The paper discusses the nonlinear dynamics and the implications for the macroscopic ice structure, brine rejection and brine channel formation in growing sea ice. The growth of sea ice and the resulting brine rejection have key implications for climate and polar ecosystems.

Last but not least, Warren [42] describes large-scale phenomena of great importance to Earth. The radiative properties of ice and ice-containing media such as snow, sea ice, and clouds, can be applied to energy budgets, satellite-remote sensing and radio-echo sounding of glaciers and ice sheets. Warren reviews the use of radiative transfer models for clouds to snow and discusses how absorption of visible and near-ultraviolet radiation by snow is dominated by traces of impurities such as soot and dust.

This themed issue gives evidence for the interest in ice research spanning disciplines from the molecular to the global scale. We thank the 112 participants for their contributions and for making the PCI-2018 such a stimulating event. We are pleased about the good balance between senior and young scientists in attendance, and in particular the globally balanced participation with a high number of contributions from outside Europe, including 26 participants from Asia, 14 from North America, 3 from Israel, 2 from Russia and 1 from Australia. Much is to be learned and we look forward to the next PCI conference in Sapporo in 2022.

Data accessibility

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Competing interests

We declare we have no competing interests.

Funding

We thank SPECS; the Swiss Snow, Ice and Permafrost Society; the city of Zürich; and the canton of Zürich for supporting this conference.