|
What’s 100 times stronger than steel, weighs one-sixth as
much, and can be snapped like a twig by a tiny air bubble? The answer is a
carbon nanotube—and a new study by Rice University scientists details exactly
how the much-studied nanomaterials snap when subjected to ultrasonic vibrations
in a liquid.
“We find that the old saying ‘I will break but not bend’
does not hold at the micro- and nanoscale,” said Rice engineering researcher
Matteo Pasquali, the lead scientist on the study, which appears in the Proceedings of the National Academy of
Sciences.
Carbon nanotubes are one of the most-studied materials in
nanotechnology. For well over a decade, scientists have used ultrasonic vibrations
to separate and prepare nanotubes in the laboratory. In the new study, Pasquali
and colleagues show how this process works—and why it’s a detriment to long
nanotubes. That’s important for researchers who want to make and study long
nanotubes.
“We found that long and short nanotubes behave very
differently when they are sonicated,” said Pasquali, professor of chemical and
biomolecular engineering and of chemistry at Rice. “Shorter nanotubes get
stretched while longer nanotubes bend. Both mechanisms can lead to breaking.”
Discovered more than 20 years ago, carbon nanotubes are one
of the original wonder materials of nanotechnology. They are close cousins of
the buckyball, the particle whose 1985 discovery at Rice helped kick off the
nanotechnology revolution.
Nanotubes can be used in paintable batteries and sensors, to
diagnose and treat disease, and for next-generation power cables in electrical
grids. Many of the optical and material properties of nanotubes were discovered
at Rice’s Smalley Institute for Nanoscale Science and Technology, and the first
large-scale production method for making single-wall nanotubes was discovered
at Rice by the institute’s namesake, the late Richard Smalley.
“Processing nanotubes in liquids is industrially important
but it’s quite difficult because they tend to clump together,” co-author Micah
Green said. “These nanotube clumps won’t dissolve in common solvents, but
sonication can break these clumps apart in order to separate, for example,
disperse, the nanotubes.”
Newly grown nanotubes can be a thousand times longer than
they are wide, and although sonication is very effective at breaking up the
clumps, it also makes the nanotubes shorter. In fact, researchers have
developed an equation called a “power law” that describes how dramatic this
shortening will be. Scientists input the sonication power and the amount of
time the sample will be sonicated, and the power law tells them the average
length of the nanotubes that will be produced. The nanotubes get shorter as
power and exposure time increase.
“The problem is that there are two different power laws that
match with separate experimental findings, and one of them produces a length
that’s a good deal shorter than the other,” Pasquali said. “It’s not that one
is correct and the other is wrong. Each has been verified experimentally, so
it’s a matter of understanding why. Philippe Poulin first exposed this
discrepancy in the literature and brought the problem to my attention when I
was visiting his lab three years ago.”
To investigate this discrepancy, Pasquali and study
co-authors Guido Pagani, Micah Green, and Poulin set out to accurately model
the interactions between the nanotubes and the sonication bubbles. Their
computer model, which ran on Rice’s Cray XD1 supercomputer, used a combination
of fluid dynamics techniques to accurately simulate the interaction. When the
team ran the simulations, they found that longer tubes behaved very differently
from their shorter counterparts.
“If the nanotube is short, one end will get drawn down by
the collapsing bubble so that the nanotube is aligned toward the center of the bubble,”
Pasquali said. “In this case, the tube doesn’t bend, but rather stretches. This
behavior had been previously predicted, but we also found that long nanotubes
did something unexpected. The model showed how the collapsing bubble drew
longer nanotubes inward from the middle, bending them and snapping them like
twigs.”
Pasquali said the model shows how both power laws can each
be correct: One is describing a process that affects longer nanotubes and
another describes a process that affects shorter ones.
“It took some flexibility to understand what was happening,”
Pasquali said. “But the upshot is that we have a very accurate description of
what happens when nanotubes are sonicated.”
Source: Rice University
