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Model growth: simulations expose branching nature of polymer crystals.

The intricate shapes of snowflakes snowflakes

small patches of gray or white hair acquired after birth. Skin color is unchanged. See also achromotrichia, vitiligo.
 have long fascinated scientists, but water isn't the only fluid that freezes into elaborate crystal structures. Polymers and metal alloys do the same. Now, scientists in the United States United States, officially United States of America, republic (2005 est. pop. 295,734,000), 3,539,227 sq mi (9,166,598 sq km), North America. The United States is the world's third largest country in population and the fourth largest country in area.  and Hungary have uncovered previously unknown facets of the physics underlying such crystal growth.

Using computer models, the researchers simulated a polymer liquid as it solidifies into a crystal. In one scenario, the researchers added increasing amounts of dirt particles, representing impurities, to the liquid and confirmed previous observations that the more dirt present, the more complex the resulting polymer crystal.

Normally, a polymer in solution begins solidifying by forming tiny needle-shaped crystals. These grow along one direction, eventually packing into a polycrystalline Adj. 1. polycrystalline - composed of aggregates of crystals; "polycrystalline metals"
crystalline - consisting of or containing or of the nature of crystals; "granite is crystalline"
 structure. However, when one of these needle crystals encounters an impurity im·pu·ri·ty  
n. pl. im·pu·ri·ties
1. The quality or condition of being impure, especially:
a. Contamination or pollution.

b. Lack of consistency or homogeneity; adulteration.

, "it has to figure out how to incorporate the particle into its structure," says James Warren James Warren may refer to any of the following people:
  • James Warren (1726-1808) - President of the Provincial Congress of Massachusetts and a general during the American Revolution.
  • James Warren - British engineer who patented the Warren-type truss bridge in 1848.
 of the National Institute for Standards and Technology in Gaithersburg, Md.

He and his colleagues at the Research Institute for Solid State Physics and Optics in Budapest found that in their model, the position and orientation of a particle can make the polycrystal growth veer in a new direction, creating a branch point. Therefore, the more dirt particles it encounters, the greater the randomness of the branching structure. The researchers dub polycrystals that follow these branching growth patterns "dizzy dendrites."

Warrens group also reports a surprising discovery: Dirt isn't required for branching structures. When the team modeled a dirt-free solution and simulated it being chilled below its normal freezing point freezing point

Temperature at which a liquid becomes a solid. When the pressure surrounding the liquid is increased, the freezing point is raised. The addition of some solids can lower the freezing point of a liquid, a principle used when salt is applied to melt ice on
, the polycrystals that formed were as highly branched as were the dizzy dendrites produced from solutions with impurities.

Warren notes that many small crystals form independently in a polymer solution. Generally, a small crystal rotates to line up with a growing polycrystal before joining it. However, in a supercooled solution, the small crystals rotate so slowly that they become incorporated in whatever orientation they happen to be in when they encounter the polycrystal. The researchers describe their simulations in the September Nature Materials Nature Materials is a monthly multi-disciplinary journal aimed at bringing together cutting-edge research across the entire spectrum of materials science. The journal’s Impact Factor of 19. .

"The simulations employed in this paper are truly state of the art," says Mark Asta, a computational materials scientist at Northwestern University Northwestern University, mainly at Evanston, Ill.; coeducational; chartered 1851, opened 1855 by Methodists. In 1873 it absorbed Evanston College for Ladies.  in Evanston, Ill. Because a material's microstructures determine its mechanical properties, the findings could help scientists design improved materials--for use in products ranging from plastic grocery bags to airplane wings--by controlling the way a material crystallizes, researchers contend.
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Title Annotation:This Week
Author:Goho, Alexandra
Publication:Science News
Geographic Code:1USA
Date:Sep 11, 2004
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