Ripples in Icicles Caused by Salt

Ripples in icicles are caused by salt, researchers from the University of Toronto recently discovered.

In the past, theories pointed to surface tension in the thin film of water that flows over ice as it forms as the likely culprit. However, as PhD candidate Antony Szu-Han Chen explained, "Nobody has systematically investigated what causes the ripples so we began growing them in the lab."

The researchers accounted for key factors influencing icicle shape, including ambient temperature, the rate at which the water flows and the motion of the air surrounding it, and then experimented with the composition of the water.

"We had already tried Toronto tap water and found that it formed ripply laboratory icicles, when distilled water didn't," experimental physicist Stephen Morris said in a statement. "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."

In all, the scientists produced 67 icicle samples grown under a variety of conditions and representing three compositions: pure distilled water, distilled water with small quantities of sodium chloride added and Toronto tap water, which contains sodium chloride and other impurities.

They were able to monitor the evolution of the icicle using digital images that detected the edges of the ice formations. These were then analyzed with computer image processing.

Ripples did not form in the case of distilled water icicles, the scientists found, but in the case of the saltier icicles, clear ripples as large as a few millimeters were observed.

"Our motivation is pure curiosity about natural patterns, but the study of ice growth has serious applications, including ice accumulation on airplanes, ships and power lines," Morris said. "This result is totally unexpected, not just by us before we did this, but by theorists and experimentalists in our field who study ice dynamics and pattern formation."