Newswise — Researchers
have caught the first glimpse of nanometer-scale
structures at the boundary between droplets of liquid
aluminum and the solid face of sapphire. The detailed
view provides direct evidence that the sapphire's
crystal structure induces the liquid aluminum atoms
to line up in an orderly fashion, which is not normally
characteristic of liquids. These findings were published
online today by the journal Science at
the Science Express Web site.
“Basically, this means we need to think about liquid-solid
interfaces in a totally different way,” says Professor
Wayne D. Kaplan of the Technion-Israel Institute
of Technology, who co-authored the study with Technion
Ph.D. student Yaron Kauffmann and colleagues from
the Max Planck Institute in Germany.
“The findings have fundamental implications for
a variety of processes, including lubrication, the
growth of thin crystal layers and ‘wetting,' or how
well liquids spread over a solid surface. So many
current technological processes depend on our understanding
of such phenomena,” Kaplan adds. “For instance, the
processes play a key role in building semiconductor
chips and other microelectronic devices, soldering
materials, and maneuvering liquids through small
spaces.” Other applications could include "labs-on-chips," where
chemicals or biological fluids are moved through
microchannels on a small glass plate, and printing
(on fabrics and paper), which also involves the wetting
process.
The researchers were able to glimpse the dynamics
of the liquid-solid interface using a special high-resolution
transmission electron microscope. Transmission electron
microscopes work by passing a high voltage beam of
electrons through a thin slice of material. The electron
beam scatters due to interactions with atoms in the
sample, and researchers can construct atomic-scale
images of the sample based on measurements of this
scattering effect.
Kaplan and colleagues used an in-situ heating stage
on the microscope to heat a thin slice of pure, single
crystal sapphire (also known as aluminum oxide) above
the melting point of aluminum. The combination of
heat and electron irradiation knocked oxygen atoms
out of the sapphire crystal and allowed aluminum
atoms to drift to the crystal's surface to form liquid
droplets.
The researchers used images and real-time movies
to capture a dynamically evolving interface between
the sapphire and aluminum drops. Due to the interface
with the crystal, atoms in the liquid formed an ordered
structure. As a result, the atoms in the liquid adjacent
to the interface have properties differing from those
in the liquid and from those in the solid. In addition,
due to the presence of a small amount of oxygen in
the microscope, over a few hundredths of a second,
oxygen combined with the ordered aluminum atoms and
the crystal grew layer by layer into the liquid.
This allowed the researchers to determine the mechanism
of crystal growth for sapphire, an important engineering
material. The images show structures at the extremely
small scale of less than one nanometer. (A single
human hair, by comparison, is 10,000 nanometers wide.)
In some ways, producing a liquid-solid interface
was one of the easier tasks for the research team,
according to Kaplan.
“Once one side of the interface is liquid, there
are a slew of experimental challenges that follow,” Kaplan
says, including preventing the liquid's evaporation,
steadying the sample for measurement and most importantly,
analyzing the images to pinpoint the actual atoms
amongst the visual “artifacts” created by the microscopy
process.
“The approach we developed for analysis of the data
has not been applied to data from solid-liquid interfaces
in the past, and without these efforts the data would
only be pretty pictures and videos, and would not
be convincing to the scientific community,” he adds.
Kaplan
says that “one of the nice things about the
present study” is that he and his students had predicted
order among liquid atoms induced by contact with
a crystal from computer simulations and indirect
experiments conducted previously.
Kaplan and colleagues will continue their work on
the liquid-solid interface with the help of a new
transmission electron microscope with a unique, image-correcting
lens and extremely high-resolution capabilities.
The new microscope will be installed at the Technion
in January 2006.
The study was supported in part by the Russell Berrie
Nanotechnology Institute at the Technion, the German-Israel
Fund and the German Science Foundation.
The Technion-Israel Institute of Technology is Israel's
leading science and technology university. Home to
the country's only winners of the Nobel Prize in
science, it commands a worldwide reputation for its
pioneering work in nanotechnology, computer science,
biotechnology, water-resource management, materials
engineering, aerospace and medicine. The majority
of the founders and managers of Israel's high-tech
companies are alumni. Based in New York City, the
American Technion Society is the leading American
organization supporting higher education in Israel,
with 17 offices around the country.
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