Nanotubes and nanowires are not as amenable to manipulation as macroscopic commodities, however, their promise as building blocks for future electronics, sensors, and electromechanical devices, means that researchers are keen to find ways to handle these tiny entities easily.

Now, an international team has measured the different frictional forces experienced by carbon nanotubes as they slide across a surface both in the direction of their long axis or perpendicular to it. [Lucas et al. Nature Mater. (2009) DOI: 10.1038/NMAT2529]. The study not only explains the so-called soft lateral distortions that nanotubes can undergo but could offer a practical solution to controlling and assembling nanotubes into devices.

At the fundamental level, studying these forces also reveals information about the handedness, or chirality, of the nanotubes, which cannot be obtained easily using other techniques.

Marcel Lucas of the Georgia Institute of Technology and colleagues there and in Italy and Germany used an atomic force microscope (AFM) tip to scan transversely across a multi-walled carbon nanotube deposited on a flat silicon substrate as well as molecular dynamics calculations to simulate these scans. The nanotubes are held stationary on the surface by van der Waal’s forces. The team then compared the forces measured with a transverse scan with the results of a longitudinal AFM scan.

They found that, surprisingly, the transverse friction is twice the magnitude of the friction seen with a longitudinal scan. This, they explain, is due to “hindered rolling” as the nanotube has a tendency to roll as the AFM tip strokes across it rather than along its length and this distorts its cross section.

This study provides the first detailed information about the frictional forces at work when an AFM tip interacts with a nanotube. The significant difference in energy needed to move a nanotube with an AFM tip, suggests a possible way to control the assembly of carbon nanotubes for nanoelectronics, sensors and other applications.

The computer models also suggested that it might be possible to discern chiral as opposed to non-chiral nanotubes, whether the nanotube has a clockwise or anticlockwise thread depending on the forces experienced by the AFM tip as it scans in different directions. This could allow researchers to develop a way to sort chiral and non-chiral nanotubes as well as controlling the large-scale self-assembly of these entities into sophisticated composite materials and architectures.