Researchers use smallest pipette to reveal freezing 'dance'
of nanoscale drops
UPTON, NY - 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.
“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.
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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.
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