Images compare an AFM tip sliding longitudinally along a carbon nanotube (left) versus sliding in the transverse direction. (Credit: Christian Klinke, University of Hamburg)
Publishing in Nature Materials this week, researchers report measuring different friction forces when a carbon nanotube slides along its axis compared to when it slides perpendicular to its axis.
The findings provide a better understanding of fundamental friction issues. Besides clarifying the forces acting on nanotubes, these investigations may offer a new tool for assembling nanotubes into devices. For example, asymmetries in the friction could potentially also be used in sorting nanotubes according to their chirality, a property that is now difficult to measure with other means.
“Because the energy required to move in one direction is twice as much as required to move in the other direction, this could be an easy way to control the assembly of carbon nanotubes for nanoelectronics, sensors and other applications,” said Elisa Riedo, co-author of the study and an associate professor in the School of Physics at the Georgia Institute of Technology. “To assemble nanotubes on a surface, you need to know how they interact and what force is needed to move them.”
The study was funded by the DOE. Other institutions contributing to the project include the International Center for Theoretical Physics, the International School for Advanced Studies and the CNR Democritos Laboratory – all in Trieste, Italy – and the University of Hamburg in Germany.
Though friction has been studied before in nanotubes, this research is the first to provide detailed information about the frictional forces at work in both the longitudinal and transverse directions when the tubes interact with an AFM tip.
“As systems become smaller and smaller, it becomes more important to understand how to address friction,” said Riedo. “Surface forces can prevent micro and nano systems from operating at all.”
Experimentally, the researchers scanned an atomic force microscope tip longitudinally along a series of multiwalled carbon nanotubes held stationary on a substrate. They also conducted a series of similar scans in the transverse direction. The researchers applied a consistent force on the AFM tip in both scanning directions and relied on powerful van der Waals forces to hold the tubes in place on the substrate.
The experiment showed that greater forces were required to move the tip in the transverse direction.
“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.”