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Ice Crystals and their Formations

April 23rd, 2008 by bcote1 · 1 Comment

From the young child trying to figure out why there are no two snowflakes that look exactly the same to the scientist who attempts to answer such questions, ice has been a fascination to anyone who has seen it. The physics behind the delicate shapes has been a great source of study. Such historical figures as Yohannes Kepler and René Descartes have helped to advance this field of research. It is strange to think that such a small thing as a snowflake can bring about such incredible science as was evident in the article “Snow and Ice Crystals” by Yoshinori Furukawa and John S. Wettlaufer from the journal Physics Today (December 2007).

As said before, the study of ice crystals has been around for some time. More recently, however, a nuclear physicist named Ukichiro Nakaya delved into observing thousands of ice crystals. He came to categorize them into different morphologies. He concluded that each crystal was the product of the temperature and the saturation of the atmosphere in which it formed. He found that different temperatures brought about various shapes (i.e. plate versus prism) and that the plate and prism grew into more ornate and complicated crystals at higher saturations, such as the dendrite and needle respectively. Nakaya came up with a diagram that summarized his findings. It can be found through the following link: Nakaya Diagram.

The transition between prism to plate to prism is sharp and abrupt with changing temperatures. This brings about confusion and complication and it still not fully understood. What causes such sudden alteration in shape?

At the bottom of the diagram, there are the basic prism and plate shapes. Their facets are “smooth on all scales down to the molecular level.” Some remain smooth up to 0°C while others become rough at lower temperatures. This change from a smooth to rough surface, as well as the liquid present on the crystal and the equilibrium/nonequilibrium shape of the crystal all affect those transitions from prism to plate in the Nakaya diagram.

It is not uncommon for ice crystals to have a small liquid film around them at temperatures below freezing. The molecules at the surface are not held in place as tightly as the ones in the main bulk of the crystal. Thus they have more mobility. This liquid layer has an effect on the roughening transition for the crystal. The roughening transition is just the point where the snow crystal goes from being faceted to being rough. It differs depending on whether the ice is in contact with water or with water vapor. The roughening transition is also a function of temperature. Below the roughness temperature, the crystal is smooth. If the temperature of the atmosphere is greater than that of the roughening transition, the equilibrium shape of the crystal will be rough. The roughness of the crystal in turn helps determine the shape and growth of the crystal.

One can see that all the factors affecting the prism-plate transition in the Nakaya diagram also rely upon each other. Each facet of the crystal has different properties (in other words, it is anisotropic) and thus can take on different forms. Thus understanding the structure or nature of the surface of the crystal is indispensable in comprehending the shape the crystal will grow to. But in the end, there is still much more to be learned about this subject.

This article was an interesting one to read, but in the end for me it raised more questions than were answered. The authors never really explained the roughening transitions or when/why it occurs. Perhaps it was my misunderstanding (or misreading), but the only thing I got from that was that it was affected by the liquid present on the crystal.

Brianna Coté

Tags: MTR3440

1 response so far ↓

  • 1 sidd // May 15, 2008 at 6:25 am

    Thanks for the link to Nakaya. A long time ago in another life, i did some research on the roughening transition, albeit in a different system. Could you point me to some more work on ice-water and ice-vapor surfaces, especially as they relate to clouds ?


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