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Frictionless
Motion Observed in Water
03/30/06
Discovery has fundamental
implications for chemistry, according to researchers at USC and
Brown University.
By Carl Marziali
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 Artist’s
conception shows a molecule in a liquid suddenly kicked into
rapid rotation pushing away molecules that surround it,
destroying its own friction.
[Image: Stephen Bradforth,
USC]
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Free rotation
can occur in gases, where molecules are far apart,
researchers said.
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Researchers at USC and
Brown University said they have achieved near-frictionless motion
in water by using lasers to spin a molecule like a propeller.
Free rotation can occur in gases, where molecules are far
apart. This is the first known demonstration of friction turning
off in a room temperature liquid, the authors report in the March
31 issue of Science.
Graduate student Amy Moskun and her
adviser Stephen Bradforth, associate professor of chemistry in
the USC College of Letters, Arts and Sciences, used ultra-short,
high-energy laser pulses to spin a CN “diatomic” –
a simple molecular stick with a carbon at one end and a nitrogen
at the other.
Within the first quarter-turn, the
molecular stick creates a shock wave, throwing back the water
molecules that had been crowding it.
“If you give
it enough spin, it pushes all the guys around it away,”
said Bradforth, “and it builds itself a little bubble. It’s
destroyed the friction in the liquid around it by completely
reshaping its environment.”
Bradforth likened the
phenomenon to a passenger swinging a suitcase around in a crowded
airport terminal, minus the real-life bruises and hurt feelings.
As with the airport analogy, after some time –
about 10 complete rotations of the CN molecule – the shock
dissipates and the water molecules rush back in.
Even so,
Bradforth said, the length of friction-free spinning was far
greater than expected.
“Everyone’s prediction
was that it wouldn’t even complete a few degrees of free
rotation,” he said.
The discovery has no immediate
practical use, but since 90 percent of reactions take place in
liquid solutions, Bradforth said his group’s study “impacts
how we think about the vast majority of chemical reactions.”
In chemistry, friction is a useful phenomenon that
transfers energy between molecules and allows reactions to
proceed. But what if a reaction needs to be stopped or delayed?
Chemists have long sought to manipulate reactions, which usually
yield useless byproducts along with the desired compound.
The
Science paper provides a potential new tool, since one way to
influence the progress of a reaction is to isolate a molecule
from its surroundings.
“Most people thought this
was hopeless in a liquid,” Bradforth said.
It took
the researchers more than a year to grasp the significance of
their work. After Moskun and Bradforth figured out how to spin up
CN molecules and to observe the rotation with a strobe-like
apparatus, they sent their data to Richard Stratt, professor of
chemistry at Brown University.
Stratt’s group of
theoretical chemists showed that the data represents a violation
of linear response theory, a key model for liquid behavior that
states the effects of friction and molecular energy are scalable.
Under the theory, a mere increase in the rotational speed of a
molecule should not cause a fundamental change in its
environment.
“People have seen linear response
theory fail in simulations before,” Stratt wrote in a
summary for the researchers’ funding agency, the National
Science Foundation. “I believe these studies represent the
first time anyone has ever seen that a particular, well-defined,
molecular event was responsible for suddenly turning off a
liquid’s linear response behavior.”
The USC
group then verified their collaborators’ interpretation
with a new round of experiments.
“The theory fails
in this case," Bradforth said, "and we can see why it
fails.”
Bradforth and Stratt were co-authors on the
Science paper, along with graduate students Moskun and Askat
Jailaubekov from USC, and Guohua Tao from Brown.
Additional
funding for Bradforth’s group came from the David and
Lucile Packard Foundation.
Source
/ credit: Brown University / University of Southern California
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