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To branch or not to branch.

To branch or not to branch

The feathery forms glistening on a frosted windowpane and the branched structures sometimes evident on the polished surface of a chunk of metal are both examples of the kind of intricate patterns that can form when a pure substance or an alloy freezes. Guided by a number of novel theoretical models and the results of several ingenious experiments, researchers have made considerable progress in the last few years toward understanding the factors that determine the type of branched, or dendritic, patterns into which a material can crystallize. Two sets of experiments now shed light on the origin of sidebranches -- offshoots from the main branches that form and grow as a substance freezes into a tree-like shape.

X.W. (Chester) Qian and Herman Z. Cummins of City College of the City University of New York studied the effect of applying a brief, tiny heat pulse near the tip of a growing dendrite. They discovered that although the initial, heat-induced deformation at the solid-liquid boundary was too small to observe directly, it initiated the formation of a new branch growing out from the dendrite's side. This observation, reported in the June 18 PHYSICAL REVIEW LETTERS, supports the notion that an amplification of minute, random temperature fluctuations leads to sidebranching during dendritic crystal growth.

"Until our work, nobody had actually seen the evolution of an individual sidebranch," Cummins says. "There's a very small amount of spontaneous temperature fluctuation that's there all the time and gets amplified to make the ordinary sidebranches. But if we put on a slightly bigger [heat] pulse right at the tip, then that will get amplified faster. So you should see a sidebranch grow out quite close to the tip where the ordinary noise is still too small to see."

In another recent set of experiments, he says, researchers in Paris, France, have shown that by periodically changing the overall temperature pattern surrounding a dendrite, they can produce sequences of sidebranches in a way that matches the pattern of temperature perturbations. "That's also an important piece of the story," Cummins says.

Cummins and his colleagues are now studying the effcts of applying periodic heat pulses to a dendrite tip in order to determine how the pulse rate affects the formation of sidebranches. Theory predicts that the efficiency of producing a train of sidebranches synchronized with the pulse rate should be highest at some frequency characteristic of the system.

These experiments and related theoretical work should provide insights into the way metals and other substances solidify. "It's not inherently different whether you do the growth in a carefully thermostatted microscope or whether it happens in the atmosphere or in the foundry," Cummins says. "The physical processes are still the same."
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Title Annotation:patterns in crystallization
Publication:Science News
Date:Jul 21, 1990
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