Antony Szu-Han Chen, Stephen W. Morris
Natural icicles often exhibit ripples about their circumference which are due to a morphological instability. We present an experimental study that exposes the mechanism of the instability, using laboratory-grown icicles. We find that, contrary to theoretical expectations, icicles grown from pure water do not exhibit ripples. The addition of non-ionic surfactants that reduce the surface tension does not produce ripples. Instead, ripples emerge on icicles grown from water with dissolved ionic impurities. We find that even minute levels of impurity are sufficient to trigger ripples and that the growth rate of the ripples increases very weakly with ionic concentration. We outline a proposed theory of the instability based on constitutional undercooling.
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http://arxiv.org/abs/1301.4734
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