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Home > Press > Researchers Use Smallest Pipette to Reveal Freezing "Dance" of Nanoscale Drops

Abstract:
Using what is thought to be the world's smallest
pipette, two researchers at the U.S. Department of Energy's
Brookhaven National Laboratory have shown that tiny droplets of
liquid metal freeze much differently than their larger counterparts.
This study, focused on droplets just a billionth of a trillionth of a
liter in size, is published in the April 15, 2007, online edition of
Nature Materials.

Researchers Use Smallest Pipette to Reveal Freezing "Dance" of Nanoscale Drops

UPTON, NY | Posted on April 16th, 2007

"Our findings could advance the understanding of the freezing
process, or 'crystallization,' in many areas of nature and
technology," said Eli Sutter, a scientist at Brookhaven's Center for
Functional Nanomaterials (CFN) and the lead author of the study.

Melting and crystallization are so-called phase transformations --
fundamental processes by which most substances change between a
disordered liquid state (such as liquid water) and an ordered solid
state (e.g., ice). When a liquid droplet is cooled, the motion of its
atoms gradually slows until they come to a stop, resulting in a
solid. For large droplets, this crystallization usually starts at a
small impurity (e.g., a speck of dirt), from which it rapidly spreads
over the entire droplet. However, very pure substances lack such
crystallization centers and have difficulty starting the phase
transformation.

"The accepted theory of crystallization, developed in the first half
of the previous century, predicts that without impurities, a small
solid core generated at random in the interior of the droplet
initiates the phase transformation," Sutter said. "Our experiments on
very small droplets challenge this theory."

To study the freezing process at the ultra-small scale, Eli Sutter
and fellow researcher Peter Sutter use what is thought to be the
world's smallest pipette, a device capable of producing liquid
droplets of a gold and germanium alloy with a volume of only a few
zeptoliters (a billionth of a trillionth of a liter). Operated inside
an electron microscope, which creates an image of the sample by
bombarding it with a beam of electrons, this zeptoliter pipette
suspends the tiny droplets so their phase transformations can be
studied with high magnification.

To achieve a liquid state, the metallic alloy must be kept at a
temperature above 350 degrees Celsius. When the temperature is
lowered to about 305 degrees Celsius, the researchers observe a
striking phenomenon: The liquid droplets develop surface "facets,"
which are straight, planar sections on the otherwise spherical-shaped
structures. The facets continually form and decay in an "ethereal
dance" of the droplet shape. This "dance" can last for hours, but
quickly stops if the temperature is lowered any further. At this
point, the droplet solidifies into a structure that is determined by
the ending positions of the dancing surface facets.

"In our experiments, solid-like properties first develop in a thin
skin at the surface, while the interior remains liquid," Eli Sutter
said.

This finding counters the traditional idea that all crystallization
originates in the interior of a liquid droplet, instead showing that
the process may differ based on the size of the sample. This work
lays the foundation for a better understanding of freezing processes
in the environment as well as in nanotechnology. For example, the
balance of solid and liquid water in upper-atmosphere clouds -- an
important factor in climate models -- greatly depends on the exact
way water droplets freeze. Such parameters might be predicted more
accurately with a better picture of the freezing mechanism.

This research was funded by the Office of Basic Energy Sciences
within the U.S. Department of Energy's Office of Science. The CFN at
Brookhaven Lab is one of five Nanoscale Science Research Centers
being constructed at national laboratories by the DOE's Office of
Science to provide the nation with resources unmatched anywhere else
in the world for synthesis, processing, fabrication, and analysis at
the nanoscale.

Note to local editors: Eli Sutter and Peter Sutter live in
Westhampton Beach, NY.

####

About Brookhaven National Laboratory
One of ten national laboratories overseen and primarily funded by the
Office of Science of the U.S. Department of Energy (DOE), Brookhaven
National Laboratory conducts research in the physical, biomedical,
and environmental sciences, as well as in energy technologies and
national security. Brookhaven Lab also builds and operates major
scientific facilities available to university, industry and
government researchers. Brookhaven is operated and managed for DOE's
Office of Science by Brookhaven Science Associates, a
limited-liability company founded by the Research Foundation of State
University of New York on behalf of Stony Brook University, the
largest academic user of Laboratory facilities, and Battelle, a
nonprofit, applied science and technology organization. Visit
Brookhaven Lab's electronic newsroom for links, news archives,
graphics, and more: http://www.bnl.gov/newsroom .

For more information, please click here

Contacts:
Media & Communications Office (631) 344-2345 phone
Community, Education, Government (631) 344-8350 phone
& Public Affairs Directorate (631) 344-3368 fax
Brookhaven National Laboratory
Upton NY 11973

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