"Here's the mystery," ruminates associate professor of
chemistry Dr. James D. Martin. What's the structure of something
that's not supposed to have structure?"
Liquids and glass have long been understood by scientists to be amorphous,
meaning without structure. Basic chemistry textbooks frequently present
cartoon representations showing liquids to be much like gasesa
collection of randomly moving atoms or molecules. But Martin has discovered
a few things about the nature of liquids and glasses at the atomic
and molecular levels that suggest the need to revise many of those
books. Martin's breakthrough, featured in a September 2002 issue of
the journal Nature, could lead to the development of totally new materials
with valuable optical and electronic properties for computing and
communications technologies, where the ability to direct movement
of light and/or current through matter is critical.
Like many discoveries, Martin's was an unforeseen result of other
basic research. Several years ago, he noticed that as he designed
and synthesized crystals, he also produced a lot of liquid and glassy
blobs. He originally dismissed the blobs as trash, but became curious
about them because they appeared so frequently. His curiosity led
him into the study of the molecular structure of liquids and glasses,
an area not well understood by science.
Did the blobs have a common structure? An analogy occurred to him
one day as he watched his children "swim" through big playpens
filled with plastic balls. "No matter how kids moved around in
the playpen, the balls always touched each other in about the same
way, Martin says. And the arrangement of the balls looked
very much like my tennis ball models of the molecules in crystals."
|Martin deduced that if similar bonding interactions hold molecules
in liquids, glasses and crystals, then it should be possible to engineer
the structure in liquids and glasses just as it's possible to engineer
the structure of crystals. And he was right. "If you understand
the network's structure and the chemical bonds within the structure,
you can manipulate the structure," he explains. "And if
you change the structure, you change the properties."
In the lab, Martin and graduate student Steve Goettler have proven
this by introducing molecules of a different substance into glasses
and liquids, thereby changing the original properties. The foreign
molecules were engineered at the atomic level to fit within the liquid's
structure and interact with the liquid's own molecules. The presence
of these foreign molecules changes the liquid's characteristics, such
as the melting point, viscosity, and manner in which light travels
through the material. Control of these properties is important in
mechanical applications such as lubrication and liquid crystal displays.
"Just as a symphony is much more than a collection of random
notes," says Martin, "the atoms and molecules in a liquid
are quite organizedmore like those in a crystal than a gas."
With this new understanding of the structural organization in amorphous
materials comes the ability to engineer specific atomic and molecular
arrangements. In essence, Martin and his colleagues have discovered
chemical principles that allow them to "write new symphonic masterpieces"
opening a new area of scientific researchamorphous materials
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