Ice sweats before it melts

Water, which covers a great part of the earth’s surface, has chemical and physical qualities that put it in a class apart from other substances. And the conditions, particularly temperatures, on earth allow water to display its special features in all its forms, as a vapour, liquid and solid. M Alejandra Sánchez, Tanja Kling, Tatsuya Ishiyama, Marc-Jan van Zadel, Patrick J Bisson, Markus Mezger, Mara N Jochum, Jenée D Cyran, Wilbert J Smit, Huib J Bakker, Mary Jane Shultz, Akihiro Morita, Davide Donadio, Yuki Nagata, Mischa Bonn, and Ellen HG Backus — a team working in Germany, the Netherlands, Japan and the USA — have reported, in the journal, Proceedings of the Academy of Sciences, the discovery of a nano layer-level build-up of a form of water on the surface of low temperature ice long before it warms and turns, in bulk, to liquid water. As water, in the form of ice, is abundant on the earth in glaciers, deep ice formations, rivers, lakes, seas, and even as snow and hail, the insight enriches our understanding of the planet’s important geological processes. The most striking and commonly known feature of water is the way its components arrange themselves when it approaches the temperature of freezing. Water reduces in volume, like most other materials, with lowering temperature and energy till it reaches four oC — from that temperature till it freezes, water begins to expand, or grow less dense. And after it freezes at zero oC, it shows a series of crystalline forms as it cools to lower temperatures. But what is less well known, perhaps, is that even after it freezes, there is a thin layer of liquid water, which is below its freezing temperature on the surface of ice. English scientist Michael Faraday discovered the existence of this layer of water on the surface of ice in 1859. A ready explanation for the ease with which things slide on water is in pressure lowering its melting point. The ice-skater’s blade is thus seen as pressing down on slabs of ice and creating a layer of melted water below the blade, and the skater can be considered to floating, rather than sliding. But the fact is that even without the application of pressure, there is a layer of liquid water covering the surface of ice. Its presence is a prominent example of a change of phase, from ice to water, that happens even before the ice melts, the authors say in the paper. This socalled quasi liquid layer, which wets the crystalline ice phase at its interface with air, has received special attention during the last decade. But from where the thickness of this layer arises and how it changes with temperature — as ice warms from cooler temperatures — have remained uncertain, the authors say. It has been found that the water film begins to form at 73 oC below freezing and grows, gradually and continuously, from a thickness of two nanometres to more than 45 nm by the time it warms to two oC below freezing. The paper says that the theoretical models, on the other hand, have suggested that the growth should be in steps, as successive layers of water molecules detach from bulk ice. The great thinness of the layer, however, has not allowed a detailed study of its structure or its progress as the temperature changes. The authors now report that the step-wise increase in thickness has been confirmed in their study, with a special method to view the nanometre-thin layer. The method of investigation used was by allowing infrared laser photons to interact with water molecules at the surface of ice. In bulk ice or water, scattering centres of incident photons are oriented equally in all directions, whereas at the surface, as in the QLL, the symmetry is broken. The so-called sum frequency generation spectroscopy method relies on a pair of frequencies of lasers, one fixed and the other variable, combining and adding when the variable laser resonates with structures on the surface being investigated. The interaction of interest was with the hydrogen-oxygen bond in water, which gets liberated when water changes phase from ice to liquid. Using crystals of ice that had been specially grown and treated to present a uniform surface, the thickness of the water layer was studied as the ice was warmed from 38 oC below freezing. The results reported are that the QLL, which appears be present even below that temperature, shows little variation but suddenly grows thicker at about 16 oC below freezing. This indicates that there is a discrete addition of a layer at that temperature. Further, the spectra of these water molecules on the surface of ice are different from those of liquid water that have been supercooled to the same low temperature. This indicates that the structure of the QLL differs from that of liquid water. An area of immediate application is in the study of sliding of glaciers over a bed of ice. As stated earlier, water is a material that is in present on the surface of the earth in bulk and in all its phases. Water is also perhaps the most powerful carrier in the circulation of heat and temperature gradients. Understanding its surprising modes of behaviour under different conditions would help manage challenges of heat flows in a warming world.

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