Magazine article Science News

To Branch or Not to Branch

Magazine article Science News

To Branch or Not to Branch

Article excerpt

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. …

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