New Scientist - Last Word
contributions by Dr Hugh Hunt

17 July 2008

Q: "Ice Maths", I took this photo of an impressive icicle that was hanging from a roof. It had formed over a couple of days in sub-zero but sunny conditions as melting snow running from the roof froze as it trickled down the icicle. What caused the horizontal ripples, and what properties of the water and air determined the precise wavelength of the ripples?

Blog Commentary found at NS Blog):

I suspect that the effect is produced by freezing water recording the rising heated air waves. A bit like sand on a drum, or a laser on a RW-CD. Water is an excellent energy storage substance. I think that what we see in your icicles is the frequency of rising heat in that locale. Just like heat is rendered visible in air above a burning candle, or on a hot road, freezing water is making the wave structures of rising heat visible. The relative stillness of the air helps this process. By Anonymous Paul Fanali on July 17, 2008 2:33 PM

I think the ripples on the ice are a caused by two factors. One is the melted water supply which comes from water melting on the roof(by sun or internal heat leakage). The other is the air temperature in the shaded area beneath the eaves and water accretes there. If the icicle was in the sunlight in warmer air, it would lose mass by melting. Changes in sunlight, diurnal air temperature variations and internal home heat leaking. By Blogger aurizon on July 17, 2008 6:03 PM

There is an interesting instability that gives rise to ripples on the surface of icicles. It is believed to be related to a similar process that causes ripples on stalactites in caves, and other ripply patterns on limestone deposits near hotsprings. In order for ice to form, the latent heat of freezing must be removed. In the case of stalactites, CO2 must come out of solution and be carried away for CaCO3 to form. In each case, a thin water film flows over a growing surface. The deposition onto the surface changes its shape, and the shape feeds back onto how the water flows. The rate of deposition is controlled by how the heat or CO2 is transported through the flowing water and into the surrounding air. In the end, an icicle or stalactite is unstable to the growth of a "Michelin Man"-like system of ripples. On icicles, the length scale of the ripples is very nearly 1 cm, and is apparently not strongly dependent on parameters like flow rate or undercooling. By Blogger Francesco on July 18, 2008 4:45 AM

Francesco's comment would appear to be a lift from Stephen Morris's web site (see Morris) The page has more information on the topic plus links to other sites and scientific papers studying the phenomena.

(these are found at NS Blog)

Back to the TV, Radio and Popular Press page: TV and Radio
For fun stuff on spinning things go to my Dynamics movies page