As the icicle turns

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A team of Canadian iciclologists has put to rest the notion that one frozen cone of drips is exactly the same as the next. By growing lots of icicles in controlled laboratory conditions, the scientists have uncovered evidence that runs counter to an earlier theory saying that all icicles should, by and large, assume the same uniform, platonic icicle shape. They posted their observations online August 11 at arXiv.org, with a supplementary series of videos on YouTube.

Physicists Stephen Morris and Antony Szu-Han Chen of the University of Toronto set out to test the existing theory’s prediction — that most icicles should assume a conical shape. Break off one of these perfect icicles anywhere along its length, and the fragment will be the exact same shape as the whole thing.

“As far as we know, no one has really systematically studied the shape of icicles and how they grow,” Morris says. “Nobody has really tried to fill in the physics of how the shape emerges.”

To grow icicles, the researchers built a frosty contraption made of a refrigerated box with a water drip at the top. The growing icicles rotated once every four minutes, like a rotisserie chicken, so that on average, the whole surface got the same treatment.

The lab-grown icicles grew to about half a meter long, and while some assumed the  iconic icicle shape, others came out looking much less than perfect. What’s more, the earlier theory posited that growing a perfectly shaped icicle required still air. But Morris and Chen found the opposite in their experiments: Most of the icicles grown in still air sprouted odd little legs at their tips, while those grown in the presence of moving air tended to be more ideally shaped.

Water quality also matters, Morris and Chen found. Icicles grown from distilled water were more likely to be perfectly conical than those grown from tap water, the team found. Tiny impurities in the tap water might be responsible for the lumpy and rippled shapes. “This is Toronto tap water, which is not especially wonderful,” Morris says.

Icicles are interesting in their own right (perhaps even more so to Canadians), but studying how they form is important for engineering problems such as ice on airplane wings or power lines. “If you can’t understand an icicle, you’re not going to understand how ice develops on a fast-blowing airplane wing,” Morris says.

A nearly perfect icicle, created with distilled water in the presence of gently stirred air, forms over the course of about 10 hours. The icicle rotates about once every four minutes.
Credit: A. Szu-Han Chen and S. W. Morris, Experimental Nonlinear Physics, University of Toronto

An icicle grown with tap water turns bulbous and crooked as it grows in the presence of gently stirred air.
Credit: A. Szu-Han Chen and S. W. Morris, Experimental Nonlinear Physics, University of Toronto