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Icicle Ripples

November 11, 2013

I'm a native of Upstate New York, which can get quite cold and snowy in the winter months. My undergraduate and graduate education was in upstate locales, and one winter season at Syracuse University, we were subjected to more than 140 inches of snowfall. Surprisingly, this amount is not that far from the mean. My wife still recalls the time when one of the students was rendered immobile after stepping off the campus bus into a snow bank. Two football players exited the bus to pull her out.

The scientifically-minded undergraduates would relax in those winter months by building devices to make artificial icicles. Addition of food coloring to the water gave some very artistic renderings, which, like performance art, would run its course, and disappear in the spring. As you can imagine, some of these icicles were huge.

Icicles on speed limit sign

Unsafe at any speed.

The property of ice with which most people are familiar is its slipperiness. This is caused by a thin layer of water on its surface. The origin of this water has been debated for more than a century with no proper resolution.

(FEMA image by Michael Raphael, via Wikimedia Commons.)


Ice was even a topic of our seminars. I can remember a materials scientist, naturally from Canada, who gave a talk about his research on the extrusion of ice. Just as the purpose of medical research on animals is usually not to cure the animals, the research on ice extrusion was not because extruded ice is useful. It was just a way to use the transparency of ice to make extrusion research easier to do.

Once again the Canadians have stepped forward to push back the frontiers of ice research, this time in an investigation of why some icicles have ripples on their surface. Physicists from the University of Toronto have found that icicles grown from pure water do not exhibit surface ripples and the addition of non-ionic surfactants does not produce surface ripples. However, even small ion concentrations of ionic impurities dissolved in water will induce ripples, and the growth rate of ripples increases very weakly with ionic concentration.[1-3]

The Toronto study was done by experimental physicist, Stephen Morris, and Antony Szu-Han Chen, a Ph.D. candidate. [2] One theory of ripple formation in icicles is that they are caused by the modification of surface tension as the result of the water film that flows along the surface during growth. This theory was directly tested by the introduction of surfactants.[2] Says Antony Chen, who was lead author of the research paper on this study, "Nobody has systematically investigated what causes the ripples, so we began growing them in the lab."

They grew 67 icicles, varying the factors that might have an influence, such as ambient temperature, flow rate of water, the motion of the air surrounding it, and the composition of the source water.[2-3] Their growth apparatus, placed in a refrigerated box, rotated the growing icicles so they would grow evenly on all sides. Their outlines were digitally imaged, and computer analyzed to reveal the icicle cross-section.[2]

The clue to the importance of water purity came from the observation that Toronto tap water produced ripply icicles, while distilled water didn't. The tap water is reasonably pure, but the threshold for rippling is only 20 milligrams of salt per liter.[2] The growth rate of the ripples followed a very weak, roughly logarithmic dependence on ion concentration.[2-3] Analysis of icicles harvested from nature confirmed the rippling dependence on salt concentration.[2]

Icicles ripples (University of Toronto)

A picture is worth a thousand words. Left images (a) are for distilled water, and right images (b) are for distilled water with the addition of 0.008 wt-% NaCl. Red data are the detected edge, and green data are the ripple positions for the left edges. (Images by Antony Szu-Han Chen and Stephen W. Morris from fig. 2 of ref. 1, licensed under a Creative Commons License.)


One interesting feature of icicle growth is that the rippling has a characteristic wavelength, no matter the size of the icicle or where it grows.[3] Aside from the surface tension effect in growth, which these experiments discredited, there's another theory involving heat transfer. Water is presumed to freeze faster at the peaks of the ripples, since heat is removed most effectively there.[3] This results in a positive feedback mechanism in which a ripple promotes more growth at the ripple.

Theory needs to catch up with experiment. Says Stephen Morris,
"Our results have provided strong empirical evidence, but as of yet we don't have a theoretical explanation as to why the impurities have this effect. Neither do we have a theory for why the ripples have a universal wavelength – this still remains a central mystery."[3]
Although much research, such as this, is driven by curiosity, there might also be some important applications. Ice from freezing rain can cause serious problems for power lines, ships, bridges and airplanes.[3] This research was funded by the Natural Sciences and Engineering Research Council of Canada.[3]

References:

  1. Antony Szu-Han Chen and Stephen W Morris, "On the origin and evolution of icicle ripples," New Journal of Physics, vol. 15, no. 10 (October, 2013), article 103012.
  2. Want ripples on your icicles? University of Toronto scientists suggest adding salt, University of Toronto Press Release, October 10, 2013.
  3. Water impurities key to an icicle's ripples, Institute of Physics Press Release, October 10, 2013.

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