Snowflake growth puts on electrifying show.
The symmetrical, lacy shapes of snowflakes dramatically illustrate how simple building blocks such as water molecules can settle into intricate geometric patterns. But the details of how this process occurs remain elusive. Now two researchers have discovered that minute traces of naturally occurring ionic salts, such as sodium chloride, play a crucial and hitherto largely unsuspected role in determining the symmetry, structure and chemistry of snowflakes and ice crystals.
William G. Finnegan and Richard L. Pitter of the Desert Research Institute in Reno, Nev., suggest that growing ice crystals can incorporate certain ions, leaving their oppositely charged partners behind in a thin liquid layer surrounding the ice core. In effect, a growing ice crystal acts like a battery, separating positive from negative ions to generate a potential difference across an ice-water interface amounting to 30 volts or more.
This charge-separation mechanism, which operates only while the ice crystal is growing, may not only determine a crystal's shape but also initiate important electrochemical reactions within ice crystals, leading to the production of chlorine and the reduction of carbon dioxide to formate ions. Such snowflake chemical reactions potentially furnish a natural source of chlorine and an unrecognized sink for carbon dioxide in the atmosphere.
"This is all new," Pitter says. "Most of what we have found is contrary to what the textbooks say." Finnegan and Pitter described their findings at a meeting this week in Washington, D.C., of the American Association of the Advancement of Science.
The initial impetus for this research came from studies of how and why ice crystals clump together. Researchers had observed that pairs of cylindrically shaped ice crystals often join point to center to form T-shaped aggregates. Moreover, in cloud-seeding experiments, an ionic salt such as silver iodide generally produces such aggregates much more readily than dry ice.
Using a cloud chamber, Finnegan and Pitter studied the formation of ice crystals from tiny, supercooled water droplets laced with trace amounts of different salts. By examining samples of the resulting crystals collected on microscope slides, they showed that a significant portion had joined together to produce T-shaped aggregates.
To explain their results, the researchers drew on research done in 1950, which demonstrated that charge separation by selective incorporation of ions into ice can occur during the freezing of bulk water when low concentrations of salts are present in the liquid. Finnegan and Pitter applied this mechanism to ice crystals of various shapes and reasoned that because a cylindrical ice crystal's tips and center would have opposite charges, such crystals would naturally join tip to center.
The same charge-separation mechanism may account for how a snowflake maintains its hexagonal symmetry while it grows, Finnegan and Pitter say. The electric charge left in the thin liquid film that typically surrounds a growing snowflake would continually redistribute itself so that growth proceeds more readily in certain preferred directions.
The researchers also discovered that the incorporation of particular ions influences crystal shapes in a consistent, reproducible fashion. For example, ice crystals grown in supercooled clouds of pure water at -16 [degrees] C have branches with straight, parallel edges. Under the same conditions, clouds containing low concentrations of sodium chloride produce snowflakes with a distinctively different shape, in which each branch ends in a pattern resembling a duck's foot (see photo).
Recent experiments demonstrate that electrochemical reactions can occur within ice crystals as a result of charge separation, followed by the transfer of protons or electrons to neutralize the charge. Such reactions could play important roles in atmospheric chemistry, Finnegan says. "If we are correct, then it's very important to study the mechanisms of how these processes occur."
"Why do certain ions separate into ice? How does it happen?" Finnegan asks. "We don't have all the answers yet."
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|Date:||Feb 23, 1991|
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