A carbon nanotube is 100 times stronger than steel, and has 1/6 the weight. But it can be snapped like a twig by a minuscule air bubble. A new study by scientists at Rice University shows just how the much-studied nanomaterials can snap when exposed 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 Matteo Pasquali, Rice engineering researcher and lead scientist on the study, published in the July Proceedings of the National Academy of Sciences.
Carbon nanotubes are hollow tubes of pure carbon approximately as wide as one strand of DNA. They are one of the most highly studied materials in nanotechnology.
Sonicated Nanotube Behavior
Scientists have used ultrasonic vibrations to separate and prepare nanotubes in the lab for more than a decade, and in the study, Pasquali and his colleagues demonstrate how this process works, and why it’s a disadvantage to long nanotubes. This knowledge is significant for researchers who want to make and work with long nanotubes.
“We found that long and short nanotubes behave very differently when they are sonicated,” said Pasquali. “Shorter nanotubes get stretched while longer nanotubes bend. Both mechanisms can lead to breaking.”
Carbon nanotubes, discovered over 20 years ago, are one of the original marvel materials of nanotechnology. They are close relations of the buckyball, the particle whose 1985 discovery at Rice helped jump-start the nanotechnology upheaval.
“Processing nanotubes in liquids is industrially important but it’s quite difficult because they tend to clump together. These nanotube clumps won’t dissolve in common solvents, but sonication can break these clumps apart in order to separate, i.e., disperse, the nanotubes,”
said co-author Micah Green.
Power Law Discrepancies
Nanotubes applications include paintable batteries and sensors, diagnosing and treatment of diseases, and next-generation power cables in electrical grids.
Freshly grown nanotubes can be a thousand times longer than they are wide, and even though sonication is exceptionally effective at breaking up the clumps, it also makes the nanotubes shorter. 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. As power and exposure time increase, the nanotubes become shorter.
“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. 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,”
Pasquali and study co-authors Pagani, Green and Poulin decided to investigate the discrepancy. They proceeded by aiming to model the interactions between the nanotubes and the sonication bubbles accurately.
A computer model, run 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.”
The model uncovers how both power laws can 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. But the upshot is that we have a very accurate description of what happens when nanotubes are sonicated,”
Reference: Competing mechanisms and scaling laws for carbon nanotube scission by ultrasonication. Guido Pagani, Micah J. Green, Philippe Poulin, and Matteo Pasquali. 2012, doi: 10.1073/pnas.1200013109 PNAS June 29, 2012. FERMILAB-PUB-14-032-A, MIT-CTP 4533
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