October 10, 2013

Why Do Icicles Ripple As They Form?

Michael Harper for redOrbit.com – Your Universe Online

A group of Canadian researchers says they’ve finally figured out why icicles grow with those characteristic ripples. The answer, they say, is the amount of impurities in the water, particularly salt. Experimental physicist Stephen Morris and PhD candidate Antony Szu-Han Chen say they were intrigued by the way ripples consistently grow on icicles, regardless of geographic location or how large the spear becomes. After growing 67 icicles in the lab, the researchers found ice grown from pure, distilled water was free of the wavelength ripples. Yet when water with salt impurities was frozen, the icicles grew as they do in the wild. What’s more, the researchers say icicles grow faster and longer as salt concentrations are higher.  How icicles grow has long perplexed scientists, but Morris and Chen feel they’ve sorted it and have written their findings in a paper published in the New Journal of Physics.

[ Watch the Video: On the Origin and Evolution of Icicle Ripples ]

“Nobody has systematically investigated what causes the ripples so we began growing them in the lab,” said Chen, the lead author of the published paper and researcher with the University of Toronto.

“We had already tried Toronto tap water and found that it formed ripply laboratory icicles, when distilled water didn’t,” said Morris.

“We also confirmed that melted rippled icicles taken from Toronto garages were very slightly salty, so that’s what led us to pursue the composition factor.”

As it stands there are two commonly understood theories about icicles and why they form the way they do. First, there are some who believe icicles form ripples due to heat displacement and gravity. Cold water dripping from a rooftop, for instance, freezes before it can hit the ground, thus laying the groundwork for an icicle. As it grows, it develops a point at which any remaining heat in the water droplet can be removed more quickly. As the droplet falls it freezes on this point, creating a ripple.

A second theory holds surface tension is responsible for growing the ripples. According to those who subscribe to this theory, higher surface tension means ripples will grow more slowly and be much more defined.

“Existing theories of how icicle ripples form have been supported by very few observations of natural or lab-grown icicles. We have performed a really controlled and complete study of icicles ripples that has never been done before,” said 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.”

Though it may sound less-than-important, Chen and Morris believe their icicle research could have real world applications. One of the many dangers of winter weather is falling shards of ice when the temperatures climb past freezing. Understanding how these icicles are formed and what shape they take could help engineers as they design roads, bridges and more.

“Ice build-up on structures during freezing rain events is a serious hazard. Power lines, ships, bridges and airplanes must all be protected from, and designed to withstand, ice accumulation. In most engineering applications, only the total amount of ice really matters, not its precise shape. We are interested in the shape, which is much less understood,” said Morris.