Tiny icicles grow in electric fields.

Ice crystals can assume many complex shapes, as the delicate, intricate patterns of snowflakes clearly show. Scientists are attempting to understand better the process by which ice crystals extend their tendrils outward to grow like branches on a tree.

Now, Kenneth G. Libbrecht and Victoria M. Tanusheva of the California Institute of Technology in Pasadena have found that in a strong electric field, ice crystals abandon their conventional branching patterns and grow into long, sharp needles. Moreover, the electric field stimulates the crystals to grow more than ten times faster than normal.

This technique may help in producing uniform ice crystals that can be studied systematically. "It could be a very useful tool--the problem is control, to get crystals to behave in the way that you want." says Peter G. Kusalik of Dalhousie University in Halifax. Nova Scotia.

The Caltech team grew ice crystals on the tip of a tungsten wire inside a cold chamber filled with water vapor, By attaching the wire to a power source. the researchers could observe the influence of an electric field on the growing crystals. In the July 6 Physical Review Letters, Libbrecht and Tanusheva present a theory to predict such effects and describe their findings.

With no applied field the crystals grow at their normal speed, about 3 micrometers ([micro]m) per second, and form treelike branches known as dendrites (SN: 7/21/90, p. 47). A low electric field accelerates the growth somewhat, says Libbrecht, but the branched shape of the crystals remains essentially the same.

Above a certain voltage, however, the growing crystal shoots forward, lengthening at 20 to 70 [micro]m per second and sharpening into a thin, smooth needle.

The observed behavior fits well with what their new theory predicts, says Libbrecht. An electric field emanating from the sharp tip of the crystal draws water molecules toward it, speeding up the growth (SN: 5/9/92, p. 311). The pointed tip, however, also has high surface tension which tends to inhibit attachment of fresh molecules and slow down the growth.

At low voltages, the crystal "sharpens up until the two effects balance" Libbrecht says. At high voltages, the attraction of water molecules to the tip overwhelms surface tension launching the crystal into a "runaway growth regime."

Another force must stabilize the growth under these conditions. Libbrecht says, since the ice tip doesn't grow infinitely sharp as the theory would indicate. He suspects that the rapidly lengthening crystal heats up at its point, keeping the growth in check.

The researchers grew the needles at -15 [degrees] C. Other temperatures produce different shapes, such as flat plates, and this variation is not well understood.

To further explore this crystallization process, the researchers might study a different material. "Water is not the best system," says Libbrecht. "It's too crazy." For now though, he adds. "we're happy with ice. It's fascinating stuff."