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Water purity key to wavy icicle riddle

Bumpy conundrum Scientists may be closer to unlocking the mystery of why icicles have ripples of exactly the same wavelength no matter how big they get.

"As far as we know it's the same everywhere in the world," says study senior author Professor Stephen Morris of the University of Toronto.

"The fascinating part of it, is that they're always found to be the same wavelength. It's very hard to explain why there's ripples at all, and it's even harder to explain why they don't change at all."

The study reported in the New Journal of Physics, has at least opened the door to solving the ripple riddle, by finding they're caused by small impurities in the water.

Morris and colleagues found pure distilled water formed icicles without any ripples.

"Only icicles made from slightly salty or dirty water formed ripples, so it turns out to be an impurity affect, which was completely unexpected," says Morris.

It was previously thought ripples formed on small defects in the ice, which allowed heat to be removed more quickly than in surrounding areas, and therefore cause ice to build up more quickly, and form a ripple.

An alternate idea, suggests they're caused by surface tension within the freezing water, flowing in a thin layer over the icicle. Higher surface tension cause ripples to form more slowly, causing lower amplitude waves.

"But neither of these theories involved any salt, and we found you have to have salt present in order to make ripples in the first place," says Morris.

Frosty finding

In their experiments, Morris and colleagues carefully controlled the conditions in which icicles were formed.

"We controlled all the temperatures, and all the flow rates, and we can stir the air or make it windy... the only thing that makes a real difference for the ripples growing, was the saltiness," says Morris.

An analysis of 67 icicles found the ripples grew at a much faster pace, and reached higher amplitudes, when there was a higher concentration of salt in the water. The overall shape of the icicle also became more distorted.

"We know as you add more salt, they grow faster and get larger, so we know the salt is necessary," says Morris.

"But we don't really know why, and we don't have a very good explanation for the wavelength of the ripples."

Researchers also found that the presence of a surfactant, which lowers water surface tension, didn't increase the growth rate of ripples, contradicting one of the earlier theories.

"We changed the surface tension by adding soap, but that wasn't enough to produce ripples," says Morris.

"The next thing is to try different kinds of salt, and different kinds of soap, combinations of salt and soap. We're also working on the theory but that's not complete yet."

Serious side

According to Morris, while the research was motivated purely by the curiosity about patterns in nature, a deeper understanding of the formation of icicles could have applications, specifically in engineering.

Ice build-up on structures during freezing rain events is a serious hazard. Power lines, ships, bridges and aircraft all need to be protected from, and designed to withstand, ice accumulation.

"This is what we call blue sky research, we're curious about anything we see in nature that occurs regularly and needs an explanation," says Morris.

"Science is really about curiosity and just being curious is quite motivating and interesting."