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Mystery Solved

Ripple effect

SALTY SHAPES  Frozen alone, distilled water produced an unrippled, carrot-shaped icicle (left). When scientists added a pinch of sodium chloride, or table salt, to the freezing water, regularly spaced ripples formed (center). Adding more salt made the ripples become wildly irregular (right).   

If you want ripples in your icicles, just add salt. This recipe comes from physicists reporting in the October New Journal of Physics. Antony Chen and Stephen Morris of the University of Toronto built a tabletop machine that allowed nearly ice-cold water to drip through a nozzle onto a slowly rotating support, where the water froze.

Distilled water produced an unrippled, carrot-shaped icicle. When the scientists added a pinch of sodium chloride, or table salt, regularly spaced ripples formed. When they added more, the ripples became wildly irregular.

The researchers have not been able to find a theory to explain why salt is crucial to ripple formation. Fortunately, nature doesn’t need a theory; the team found that water running off Toronto roofs had enough dissolved ions to make ripples on its own.

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