Though it's barely the beginning of autumn, scientists at the University of Toronto are one step closer to explaining why winter's icicles form with Michelin Man-like ripples on their elongated shapes.
Experimental physicist Stephen Morris and PhD candidate Antony Szu-Han Chen were spurred to investigate by the ripples that appear around the circumference of icicles that occur naturally. It has been theorized that the ripples are the result of surface tension effects in the thin water film that flows over the ice as it forms. Their investigation revealed that the actual culprit is salt.
"Nobody has systematically investigated what causes the ripples so we began growing them in the lab," said Chen, lead author of a paper published online this week in New Journal of Physics. Accounting for key factors that influence the shape of an icicle as it forms in nature -- ambient temperature, flow rate of water and the motion of the air surrounding it -- the researchers experimented with the composition of the source water.
"We had already tried Toronto tap water and found that it formed ripply laboratory icicles, when distilled water didn't," said Morris. "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."
Using pure distilled water, distilled water with small quantities of sodium chloride added, and Toronto tap water -- which contains sodium chloride as well as many other impurities -- they produced 67 samples grown under a broad range of conditions. The evolution of the icicle shapes over time was acquired from digital images using detection of their edges, which were then analyzed with computer image processing.
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 millimetres. 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.
Morris and Chen found that ripples only became apparent at the remarkably low salinity of the water with 20 mg of salt per litre. This level, in fact, is a considerably lower level of impurity than found in common 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," said Chen. "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," said Morris. "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."
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