Scientists say the tiny bubbles that make icicles look foggy are filled with water, not air

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Like tree rings, layers of water pockets also preserve records of icicle growth

Tiny droplets of dirty water, often mistaken for air bubbles, tell the story of an icicle.

Icicles made of pure water are smooth. But salt or other impurities make icicles form ripples , when they hang from branches, bridges and power lines. Impurities also cause the hazy appearance of icicles, which is usually attributed to air bubbles. These bubbles are actually there small portions of contaminated water researchers report in the November issue Physical Review E.

Examining cross-sections of icicles with a thickness of 3 millimeters, grown in the laboratory , University of Toronto physicists Stephen Morris and John Ladan discovered pockets of impure liquid water surrounded by relatively pure ice. “It turned out that there were very few air bubbles in the icicle,” says Morris. He calls the water pockets “inclusions” to distinguish them from air bubbles.

Ring-shaped patterns of concentrated dye can be seen on the section of the icicle, which record the growth history of the icicle.J. LADAN AND S. MORRIS/UNIVERSITY OF TORONTO

Moreover, “the inclusions record the growth history of the ripples. It’s like rings on a tree,” says Morris. Inclusions form in layers near the surface of the icicle, with older layers covered by younger ones as the icicle grows. “You can infer growth history by looking at the pattern of inclusions.”

To track the formation of icicles, the researchers mixed a fluorescent dye with water instead of other types of impurities and used the water to grow the icicles in the lab. Ultimately, the dye was concentrated to higher levels in the liquid inclusions in the icicles, like any other contaminant. It also glowed brightly under ultraviolet light, which made it easier to see the layers of inclusions.

By changing the concentration of the dye in the water, the researchers showed how the impurities affect the patterns of the icicles. According to Morris, all it took was pollution comparable to tap water “before they changed their shape from smooth to wavy.”

The main reasons why pollutants lead to ripples are not yet clear. As an experimenter, Morris says, he will leave that conundrum to theorists.

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