The experiment showed that greater forces were required to move the tip in the transverse direction. Using molecular dynamics simulations, collaborators Erio Tosatti and Xiaohua Zhang at the International Centre for Theoretical Physics, International School for Advanced Studies and CNR Democritos Laboratory analyzed the phenomenon to understand what was happening.
"In principle, there seems to be no reason why the frictional forces required to move the AFM tip would be different in one direction," Riedo noted. "But the theory confirmed that this 'hindered rolling' and soft mode movement of the nanotubes are the sources of the higher friction when the tip moves transversely."
Because the nanotube-tip system is so simple, it offers an ideal platform for studying basic friction principles, which are important to all moving systems.
"This kind of system gives you the opportunity to explore friction using an ideal experiment so you can really probe the energy dissipation mechanism," Riedo explained. "The system is so simple that you can distinguish between the dissipation mechanisms, which you can't usually do well in macro-scale systems."
Based on the molecular dynamics simulations, Riedo and Tosatti believe that the friction anisotropy will be very different in chiral nanotubes versus non-chiral left-to-right symmetric nanotubes.
"Because of the chirality, the tip moves in a screw-like fashion, creating hindered rolling even for longitudinal sliding," Tosatti said. "Thus, the new measuring technique may suggest a simple way to sort the nanotubes; among the next steps in the research will be to show experimentally that this can be done."
In addition to the researchers already menti
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