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Physics tip sheet #29 October 23, 2002

1) How icicles get their ridges
N. Ogawa, Y. Furukawa
Physical Review E (Print issue: October 2002)

Icicles often have circular ridges around them but nobody has been able to explain the details until now. A new theory described how the ridges appear because of the interplay of different effects. If there is some sort of bump on the icicle, as can randomly happen, it will tend to enlarge because the bump increases the area through which the icicle can dump the heat in water flowing over it into the colder air. So more water freezes on the bumps. This effect is more pronounced for thinner ridges. On the other hand, the water flowing down the icicle tends to spread out the ridges, making them wider. Curiously, these two effects almost always balance out to cause ridges every 8 mm or so. The theory also predicts that the ridges should travel down the icicle at half the rate of the icicle growth, something that still has to be checked experimentally.

Journal article: http://link.aps.org/abstract/PRE/v66/e041202

2) Speed of virus infections correctly predicted by physical models
J. Fort, V. Mndez
To appear in Physical Review Letters

Spreading of viruses happens slower than predicted by past physical models of the process. Many believed this is because biological factors needed to be taken into account. However, a new model shows that physical principles are enough to accurately predict the speed of virus spreading. The model is based on a reaction-diffusion equation, in which the virus "reacts" by reproduction and then "diffuses" by infection. Details of both processes, including the time it takes for a virus to reproduce, are needed to give an accurate model. Similar models have only previously been applied to historical social settings such as the Neolithic transition in Europe and the spread of the Black Plague. This model is the first to give predic
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Contact: David Harris
harris@aps.org
301-209-3238
American Physical Society
23-Oct-2002


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