The usual way to overcome friction is with a liberal coating of lubricant, whether it be some kind of oily liquid or a solid such as graphite.
But that’s a distinctly macroscopic solution. In recent years, however, physicists have become increasingly concerned with friction on the nanoscale. The problems arise in devices such as computer disc drives which can suffer stick-slip friction that is hard to overcome.
Today, Andrea Vanossi, at the University of Modena and Reggio Emilia, and a couple of pals investigate an interesting way of reducing friction on the nanoscale.
The idea, which has been around for a few years now, is to shake the surfaces involved. That makes sense on an intuitive level but exactly how this might reduce friction has never been fully investigated.
Vanossi and co examine the behaviour of a single tip, such as an atomic force microscope tip, in contact with a one-dimensional surface. In ordinary circumstances, the tip and the atoms on the surface arrange themselves in a way that minimises their energy. It is this energy barrier that causes stick-slip friction.
Overcoming friction is really a question of overcoming this barrier.
That’s where the oscillations turn out to be important. Shake the surface (or the tip) and this immediately raises the tip out of this minimum, allowing it to explore the energy landscape. This is equivalent to smooth sliding, or at least smoother sliding. Vanossi and co study the relationship between the friction and vibrations of various different frequencies and amplitudes.
So the vibration dramatically reduces friction. In fact, it essentially allows friction to be switched on and off.
But Vanossi and co have another interesting result. They say that once the oscillations have overcome stick-slip friction, they can help to maintain motion. In effect, the tip can ride the oscillations, like a surfer rides ocean waves.
That raises the prospect that the same mechanism that reduces friction could also help to move particles around surfaces, with appropriately designed waves.
That could turn out to be especially important for microelectromechanical devices. MEMS were once heralded as machines that would change the world but we are still waiting for this revolution largely because of the problem of ‘stiction’. Very often, these machines stick and remain stuck.
It’s just possible that a little careful shaking could help.
Ref: arxiv.org/abs/1106.2430: Controlling Microscopic Friction through Mechanical Oscillations
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