Ice crystal growth: an electric finding.

Sometimes even the simplest materials can baffle scientists. Take ice, for example. Researchers still know very little about how this seemingly mundane substance forms.

Studies conducted over the past four decades have shown that ice crystals sometimes grow around molecular "seeds," substances that give ice a geometric template to mimic and build upon. Scientists first suggested this idea in 1947 after observing that smoke containing silver iodide, a chemical with an ice-like crystal structure, caused ice formation in clouds. Since then, researchers have shown that other substances also seem to act as templates.

Now, Leslie Leiserowitz and his co-workers at the Weizmann Institute of Science in Rehovot, Israel, describe another growth mechanism. They report in the May 8 SCIENCE that an electric field appears to cause ice crystals to form.

In their experiment, the group paired various combinations of amino acid crystals. Some combinations resulted in polar, or electrically charged, crystals; others remained electrically neutral. The researchers placed water vapor in the microscopic crack of both kinds of crystals and cooled them. They found that tiny ice crystals started to form at temperatures 4[degrees] to 5[degrees]C higher in the polar crystals than in the nonpolar crystals. Since amino acid crystals bear no structural resemblance to ice, the team attributes the ice growth to the electric field.

Scientists have suspected since 1879 that an electric field might trigger ice formation, but until now the theory had never been well tested, notes Yale University chemist J. Michael McBride. "This is the kind of experiment you like to see - where you have as many things controlled as possible," he says.

"It is refreshing that careful observation of simple materials can still yield original insight," McBride writes in a review article accompanying the research report. Nonetheless, more experiments are needed to confirm the electric-field theory, he says. "The evidence they've provided is very suggestive, but I don't think they've nailed it down yet."

Exactly how electricity might promote ice growth remains a matter of speculation. One explanation, says McBride, may hinge on ice's structural flexibility, which allows its crystals to form in many molecular arrangements.

Learning more about how ice forms may help investigators discover new ways of stunting its growth. This would have many applications, says Leiserowitz, from preventing icing of airplane wings to keeping cells of frozen donor organs from bursting.