A team of scientists at the University of Toronto has discovered that the salt in water is responsible for the distinctive ripples seen in the ice stalactites that grow from eaves and on bridges during the winter.
Other contaminants as well probably contribute to the formation of the characteristic bumps, says senior author Stephen Morris, an experimental physicist at the University of Toronto.
"We didn't expect this, but it turns out that very slightly dirty water — like Toronto tap water — produces nice ripply icicles," says Morris of the research, which is published this week in New Journal of Physics.
"And pure water, or even just distilled water which is pretty pure but not super pure, produces smooth icicles with no ripples on them."
His team was trying to figure out why icicles form with ripples.
It may be blue sky research, Morris acknowledges, though it's completely serious. Figuring out how ice forms and why it takes the shapes it does is important for dealing with ice buildup on planes, ships and bridges, among other things.
"There's a huge engineering field concerning ice buildup and this is directly connected to ice buildup," Morris says.
"In fact, the icicle is just about the simplest kind of ice buildup you could ask for. And we don't understand it. It's surprising."
The thinking has been that the ripples are the result of surface water tension effects on the thin water film that flows over the ice as it forms. Surface tension is what allows small insects to dance on the water of a lake, for instance.
It's known that adding soap to water reduces the surface tension, so Morris's group added soap to water to see if it affected the shape of icicles. But icicles made from soapy water didn't form ripples.
As the work progressed, however, the team realized there was a difference between icicles made from distilled water and from regular water from the tap.
"Toronto tap water is very close to pure water and we didn't believe initially that it would make any difference using tap water or really pure water. But it does," Morris says.
The effect is noticeable in a picture of three icicles formed in the experiment — a smooth icicle made of distilled water, a moderately ridged one made with distilled water plus a little salt and a carbuncled icicle made from distilled water plus a lot of salt. The image can be seen on a Flickr page Morris set up at http://www.flickr.com/photos/nonlin/sets/72157619114347064/.
He says it's not clear why salt has this effect, but it's worth studying. Morris does other research into why substances that are smooth develop bumps — roads, for instance — and he says the work is all linked.
"Everything is connected in physics. Even the most trivial phenomenon can turn out to be important," he says.
"Crystal growth — and an icicle is a crystal — is a huge field in engineering."