"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."
Another week, and news of another carbon nanotube advance in our rapidly unfolding new carbon age. While this advance is seemingly trivial, in fact it reconciles two different laws in carbon nanotechnology that produced very different results. Which law was correct? It turns out that both of them are, and that opens up a whole new line of scientific enquiry and with it great possibilities. In effect, it doubles the possibilities of new discoveries in cutting edge, no pun intended, carbon nanotube research.
While this materials handling advance is more next decade than this, in terms of bringing new products and benefits to the consuming public, by doubling the possible area of carbon research, my guess is that this small step will not turn out to be small at all.
Tiny Bubbles Snap Carbon Nanotubes Like Twigs
ScienceDaily (July 9, 2012) — 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.
—- "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."
Competing mechanisms and scaling laws for carbon nanotube scission by ultrasonication
Dispersion of carbon nanotubes (CNTs) into liquids typically requires ultrasonication to exfoliate individuals CNTs from bundles. Experiments show that CNT length drops with sonication time (or energy) as a power law t-m. Yet the breakage mechanism is not well understood, and the experimentally reported power law exponent m ranges from approximately 0.2 to 0.5. Here we simulate the motion of CNTs around cavitating bubbles by coupling Brownian dynamics with the Rayleigh–Plesset equation. We observe that, during bubble growth, CNTs align tangentially to the bubble surface. Surprisingly, we find two dynamical regimes during the collapse: shorter CNTs align radially, longer ones buckle.