Researchers may have unlocked the salty secret as to why some icicles develop "Michelin Man-like" ripples.
"Nobody has systematically investigated what causes the ripples so we began growing them in the lab," PhD candidate Antony Szu-Han Chen, lead author of a paper, said, a University of Toronto news release reported.
Past theories have suggested that the ripples are caused by surface tension when the thin film that drips down the icicle, forming its structure. The news study concluded the answer was entirely different.
The research team took "ambient temperature, flow rate of water and the motion of the air surrounding [the icicle]," the news release reported. The scientists also looked at the source of the icicle-forming 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, who participated in the study, said. "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."
The team used samples of distilled water (which has traces of sodium chloride) and Canadian tap water (which contains sodium chloride along with other impurities) for their study. They then grew 67 samples under a variety of conditions. A time-lapse video was created of the icicles development.
The researchers found the distilled samples did not form ripples under any circumstances, but water that contained salt would develop the strange ridges.
"The ripples were seen to move slowly upward during the icicle growth, though the researchers note that both the speed and direction of the ripple motion could vary depending on the concentration of dissolved salt," the press release reported.
The ripples only formed when the water had about 20 mg of salt per liter, which is lower than most tap water.
"We even added a non-ionic ingredient to the distilled water to reduce the surface tension of the thin film of water flowing over the icicle, and it didn't produce ripples," Chen, said. "Instead, ripples emerge only on icicles grown from water with dissolved ionic impurities."
"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."
Morris said the actual cause of the ripples was still a mystery, but they "now know that a little salt is required in the recipe."
