Hokkaido University physicists have been examining why ice is wet, a fact that is at odds with the physical properties of other solids. When a block of ice is held at temperatures below zero, it has water on the surface and this water does not freeze (convert from a liquid to a solid). This strange phenomenon was noted by the ‘father of electricity’, the physicist Michael Faraday over 150 years ago.
To unravel the mystery, the Japanese scientists used an advanced optical microscope, which they put together with the help of the Olympus Corporation. Using the special optical instrument, the researchers were able to study how thin water layers (termed quasi-liquid layers) appear and then disappear on the surface of the ice. This liquid effect was seen to occur at different temperatures and under different pressures.
The collected data showed how thin water layers do not homogeneously and completely wet the surface of ice; thus the ice was not wet from the same liquid for a prolonged period of time. The quasi-liquid layers are not stable and regularly vaporize.
A second observation is that the quasi-liquid layers only form when the ice is growing, that is as the bulk of the ice solidifies, and that this occurs under supersaturated or unsaturated vapor conditions. This means the quasi-liquid layers are a transient state formed through vapor production. Should a true steady state be achieved, which requires somewhat idealized conditions not found in nature, then the ice would not be wet.
In a research statement, the lead scientist, Dr. Ken-ichiro Murata explains: “Our results contradict the conventional understanding that supports quasi-liquid layer formation at equilibrium. However, comparing the energy states between wet surfaces and dry surfaces, it is a corollary consequence that quasi-liquid layers cannot be maintained at equilibrium.”
It is hoped that the fining offers a framework that can be used to interpret surface melting on other crystalline surfaces. The research is published in the journal Proceedings of the National Academy of Sciences, in a paper headed “Thermodynamic origin of surface melting on ice crystals.”
