On the atomic scale, friction is a curious beast and explaining exactly how it arises (and why in certain circumstances it appears to be absent) has stumped tribologists.
For the growing number of engineers designing and building nanomachines, one important question is how friction scales with the contact area between nanoscale components.
In the macroscopic world, this is easy to answer: dry friction is independent of contact area, according to the second law of friction developed by the 17th century French scientist Guillame Amonton.
Not so on the nanoscale, say Dirk Dietzel at the University of Münster in Germany and friends who have spent many happy hours measuring the force needed to push nanoparticles around using an atomic force microscope.
And their results are at first glance quite counterintuitive. They say that in some circumstances the frictional force increases linearly with surface area. And, get this, in other circumstances friction is absent entirely.
Friction free sliding is actually predicted between surfaces that are perfectly smooth, atomically flat and inert. That turns out to be feasible only for very small surface areas. The evidence for this effect has been patchy so far so Dietzel’;s team can pat themselves on the back.
More interesting perhaps is their assertion that the frictional forces they have measured are the result of contamination between nanosurfaces.
What they’re implying is that the problems that many engineers have with friction on that scale could be solved by reducing contamination. That’s an interesting take. The only trouble is that cleanliness on the atomic scale is not a simple thing to achieve.
Ref: arxiv.org/abs/0805.2448: Frictional Duality Observed during Nanoparticle Sliding