Friction, at a microscopic level, is due to electromagnetic forces and the exchange interaction between atoms. Gears, bearings and lubricants can reduce friction in the macroscopic world, but nanoscale mechanical devices require other solutions.
And solutions are certainly required, as the enormous surface-to-volume ratio in such devices leads to severe friction and component wear, reducing working lifetime and increasing costs. Liquid lubricants are of no use as they become highly viscous when confined to molecular-scale layers.
Exploiting an effect known as superlubricity, a group of researchers led by Anisoara Socoliuc and Enrico Gnecco from Basel University have found a way of reducing friction between sliding nano-contacts to negligible values. Socoliuc now works with Zürich-based scanning probe microscopy specialists Nanonis.
Superlubricity, which can be found in many natural phenomena from biological systems to tectonic plates, occurs when two crystalline surfaces slide over each other in dry contact. The atoms in the surfaces are arranged in a hill and valley landscape, somewhat like an egg carton, and, depending on the orientation of the atoms, the frictional force is either high or low.
The researchers found that by applying an AC voltage to the tip of an atomic force microscope moving over a surface, and tuning the current to a mechanical resonance frequency of the system, the tip and surface slide past each other without jumping from atom to atom in ‘stick-slip’ fashion and dissipating energy. And unlike with purely mechanical superlubricity, the crystal surfaces need not be well-defined.
There are some technical challenges to be overcome before the technique can be exploited in real-world situations. For one thing, nanoscale mechanical devices must be designed to accept AC excitation. ‘Electrodes are already implemented in nanoscale electro-mechanical systems,’ says Gnecco. ‘Minor modifications are presumably required to get well defined resonance frequencies in contact, but I don’t think that’s a problem from the point of view of design and micro-fabrication.’
The research is reported in the journal Science (subscription required).
Figure: Friction in a nano-scale contact, in the form of stick-slip instabilities (left), is dramatically reduced (right) when a modulation in the normal force is applied to the interface (top-right). Taken from Science (subscription required).
Article first published in Nanomaterials News.