Ice sweats before it melts
(appeared in Dec 2016)

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More details of the peculiar ways of water have risen to the surface, says S.Ananthanarayanan.

Water, which covers a great part of the surface of the earth has chemical and physical qualities that set it in a class apart from other substances. And the conditions, particularly temperatures, met with on the earth allow water to display its special features in all its forms, as a vapour, a liquid and a 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 H. G. Backus, a team working in Germany, the Netherlands, Japan and the USA report 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 ice warms and turns, in bulk, to liquid water. As water in form of ice is abundant on the earth, in glaciers and deep ice formations on the earth, and also in rivers lakes and the sea, and even as snow and hail, the insight enriches our understanding of important geological processes of our planet.

The most striking and commonly known feature of water is the way its components arrange themselves as liquid water approaches the temperature of freezing. While water reduces in volume, like most other materials, with lowering temperature and energy till it reaches 4°Centigrade, from that temperature, and till it freezes, water begins to expand, or grow less dense. And after it freezes, at 0°Centigrade, 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 the freezing temperature of water, on the surface of the ice!

It was the English scientist, Michael Faraday who discovered, in 1859, the existence of this layer of water that covers the surface of ice. A ready explanation for the ease with which things slide on water relies on pressure lowering the melting point of water. The ice-skater’s blade is thus seen as pressing down on the slabs of ice and creating a layer of melted water below the blade, and the skater can be considered to float, rather than slide! But the fact is that even without the application of pressure, there is a layer of liquid water covering the surface of ice. This 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 so-quasi liquid layer (QLL) which wets the crystalline ice phase at its interface with air has received special attention during the last decade, but how the thickness of this layer arises and changes with temperature, as ice warms from cooler temperatures has remained uncertain, the authors say.

It has been found that the water film begins to form at 73 °Centigrade below freezing and grows, gradually and continuously, from a thickness of 2 nanometers to more than 45 nanometers by the time it warms to 2 °Centigrade below freezing. The theoretical models, on the other hand, the paper says, 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, just nanometers, however, has not allowed detailed study of its structure, or it progress as the temperature changes, so far. The authors of the paper in PNAS now report that the step-wise increase in thickness has been confirmed in their study, with a special method to view the nano-meter-thin QLL

The method of investigation used was by allowing infra-red 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 nano-meter-thin layer of water on ice, the symmetry is broken. The so-called sum frequency generation spectroscopy (SGF) 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°Centigrade below freezing. The results reported are that the QLL, which appears be present even below the temperature of 38°Centigrade below freezing, shows little variation with temperature, but suddenly grows thicker at about 16°Centigrade below freezing. This indicates that there is a discrete, addition of a layer at this temperature. The spectra of these water molecules on the surface of ice, furthermore, are different from those of liquid water that has been super-cooled to the same low temperature. This indicates that the structure of the QLL differs from that of liquid water, which has significance in understanding the surface of ice.

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 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 on the earth. Understanding its surprising modes of behaviour under different conditions would help manage challenges of heat flows in a warming world.


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