Besides its potential for fundamental research and
commercial applications, Giapis says that the result
is an opportunity to set the record straight. "We
have found that when measuring the properties of
carbon nanotubes in contact with liquid metals, researchers
need to take into account that the application of
a voltage can result in electrically activated wetting
of the nanotube.
"Ever since carbon nanotubes were discovered in
1991, people have envisioned using them as molds
to make nanowires or as nanochannels for flowing
liquids. The hope was to have the nanotubes act like
molecular straws," says Giapis.
However, researchers never got liquid metal to flow
into the straws, and eventually dismissed the possibility
that metal could even do so because of surface tension.
Mercury was considered totally unpromising because,
as anyone knows who has played with liquid mercury
in chemistry class, a glob will roll around a desktop
without wetting anything it touches.
"The consensus was that the surface tension of metals
was just too high to wet the walls of the nanotubes," adds
Collier, the co-lead author of the paper. This is
not to say that researchers have never been able
to force anything into a nanotube: in fact, they
have, albeit by using more complex and less controllable
ways that have always led to the formation of discontinuous
wires.
Collier and Giapis enter the picture because they
had been experimenting with coating nanotubes with
an insulator in order to create tiny probes for future
medical and industrial applications. In attaching
nanotubes to gold-coated atomic force microscope
tips to form nanoprobes, they discovered that the
setup provided a novel way of making liquid mercury
rise in the tubes by capillary action.
Casting far beyond the nanotube research papers
of the last decade, the researchers found an 1875
study by Nobel Prize-winning physicist Gabriel Lippmann
that described in detail how the surface tension
of mercury is altered by the application of an electrical
potential. Lippmann's 1875 paper provided the starting
point for Collier and Giapis to begin their electrowetting
experiments.
After mercury entered the nanotubes with the application
of a voltage, the researchers further discovered
that the mercury rapidly escaped from the nanotubes
immediately after the voltage was turned off. "This
effect made it very difficult to provide hard proof
that electrowetting occurred," Collier said. In the
end, persistence and hard work paid off as the results
in the Science paper demonstrate.
Giapis and Collier think that they will be able
to drive various other metals into the nanotubes
by employing the process at higher temperature. They
hope to be able to freeze the metal nanowires in
the nanotubes so that they remain intact when the
voltage is turned off.
"We can pump mercury at this point, but it's possible
that you could also pump nonmetallic liquids," Giapis
says. "So we now have a way of pumping fluids controllably
that could lead to nanofluidic devices. We envision
making nano-inkjet printers that will use metal ink
to print text and circuitry with nanometer precision.
These devices could be scaled up to operate in a
massively parallel manner. "
The paper is titled "Electrowetting in Carbon Nanotubes." In
addition to Collier and Giapis, the other authors
are Jinyu Chen, a postdoctoral scholar in chemistry,
and Aleksandr Kutana, a postdoctoral scholar in chemical
engineering.
Contact: Robert Tindol (626) 395-3631 tindol@caltech.edu
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