Ripples on Icicles Created with Salt: Uncovering the Physics of Water

Why do some icicles have ripples while others remain smooth? Good question--and one that scientists decided to investigate in order to learn more about the physics of water. Now, they've uncovered the answer to the question, revealing a little bit more about the dynamics of water.

In the past, researchers believed that the ripples on icicles might be the result of surface tension effects in the thin water film that flows over ice as it forms. Yet researchers hadn't confirmed that this was indeed the case.

"Nobody has systematically investigated what causes the ripples, so we began growing them in the lab," said Antony Szu-Han Chen, one of the researchers, in a news release.

Using pure distilled water, distilled water with small quantities of sodium chloride and Toronto tap water (which contains many impurities), the scientists grew 67 samples under a broad range of conditions. They then examined the evolution of the icicle shapes over time by taking digital images and analyzing them with computer image processing.

So what did they find? It turned out that ripple growth was not observed on distilled water icicles, whereas saltier icicles showed clear ripples that appear in a patchy way and sometimes grew as large as a few millimeters. The ripples also moved slowly upward during icicle growth. That said, the scientists noted that both the speed and direction of the ripple motion could vary depending on the concentration of the dissolved salt.

"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," said Stephen Morris, one of the researchers, in a news release. "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. No theory accounts for the effect of salt, so the shape of icicles and the reason for their ripples are still mysteries. Except we now know that a little salt is required in the recipe."

The findings could eventually help researchers develop ways in order to prevent the accumulation of ice on vehicles. In addition, it could allow them to account for different dynamics while an aircraft is in flight.

The findings are published in the New Journal of Physics.