No two snowflakes are alike, according to common wisdom. Icicles, on
the other hand, are all alike--that is, the ripples that embellish the
surfaces of most icicles are similar regardless of variations in air
temperature, humidity, icicle thickness, or growth rate.
An icicle grows when thin sheets of water flow down the icicle shaft.
A portion of the flowing water freezes and the rest drips from the icicle
tip. But the ice that's left behind doesn't build up uniformly; instead,
it is selectively deposited at certain locations.
As a result, icicles are covered in ring-like ripples extending along
their lengths, which always measure about 1 cm from peak to peak.
Researchers at Hokkaido University's Institute of Low Temperature Sciences
in Japan (Naohisa Ogawa and Yoshinori Furukawa: firstname.lastname@example.org,
email@example.com) have developed a theoretical model that explains
the surprisingly universal structure of icicles.
According to the new model, two effects are important as an icicle
grows. The first effect is the Laplace instability, which is related
to the latent heat released from an icicle's surface and dispersed into
the air through the thin water layer. The instability arises because
heat is more rapidly lost from the convex surfaces than that from the
concave surfaces, which makes ice build up faster on an icicle's convex
protrusions than on the concave indentations, thus amplifying ripples.
The second factor is the fluid effect. Flow in the thin water layer
decreases the temperature distribution along the layer, making it uniform
and thus inhibiting the Laplace instability.
As it happens, these two competing effects ensure that all icicle ripples
have the same wavelength, although the ripple height can vary from one
icicle to another. The theory also predicts that the ripples should
migrate down an icicle at about half the speed that the icicle grows--a
prediction the researchers hope will soon be verified experimentally.
In addition the researchers expect that their model should be helpful
in explaining the structures of mineral stalagmites commonly found in
limestone caves. (N.
Ogawa and Y. Furukawa, Physical Review E, October 2002)