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Impurities in water are behind the ripples seen around an icicle's circumference.
Ripples or ribs form naturally in icicles, an effect that previous theories attributed to surface tension in the thin film of water that flows over the ice. Antony Szu-Han Chen and Stephen Morris at the University of Toronto, Canada, analysed 67 icicles grown under a broad range of conditions in the laboratory. They found that whereas icicles made from pure water were ripple-free, even small amounts of salt dissolved in the water — less than is found in most tap water — caused ripples to emerge. The ribs also grew faster in saltier water.
Existing theories do not account for the effects of impurities in ripple formation, leaving salt's role in the process a mystery.
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Pinch of salt makes for bumpy icicles. Nature 503, 441 (2013). https://doi.org/10.1038/503441f
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DOI: https://doi.org/10.1038/503441f
