It’s a bumpy road ahead

This has nothing to do with the parlous state of the British economy. The subject is in fact friction, which no doubt had you enthralled as you timed wooden blocks sliding down inclined planes in your high school physics laboratory. Exciting stuff.

The interface between seemingly smooth surfaces is in fact formed by a multitude of isolated points of contact (source: Jay Fineberg/Hebrew University of Jerusalem).

Think of friction and you might well imagine two smooth surfaces rubbing against each other, with a resulting drag force due to the material nature of the surfaces in question. Focus on these surfaces at the microscopic scale and you notice that they are not in fact smooth, but lumpy. And that is way before you get down to the molecular level.

Frictional drag is due to the interaction between relatively rough surfaces, with the contact taking place in only a tiny area. That much has long been understood, but what isn’t known is exactly how the so-called slip-stick phenomenon works in detail.

“Although friction plays such an important role in so many aspects of our lives, it is surprising that many key processes embodied within frictional motion have been far from understood.”

The quote is from Jay Fineberg, Max Born Professor of Natural Philosophy at the Racah Institute of Physics at the Hebrew University of Jerusalem, who with graduate students Oded Ben-David and Shmuel Rubinstein has just published in the journal Nature a paper in which they discuss the evolution of frictional strength from short to long timescales.

Given the roughness of real-world materials, sliding contact between them takes place in only a tiny area, and it is the behaviour of the surface bumps which controls friction. Fineberg and his colleagues show that millionths of a second before surfaces begin to slide, a miniature ‘earthquake’ tears through the interface and ruptures the contact points. From the moment of that contact rupture, four distinct and interrelated phases of frictional evolution can be identified.

First, within a few microseconds, the local contact area reduces with the passage of a crack-like front across the interface. This is followed by a rapid slipping, which then reduces sharply in speed. The sequence ends with a re-strengthening of the material over a timescale of up to 100 seconds as the contact area once again increases.

Understanding the fundamental nature of friction should have major practical benefits, and the researchers identify examples such as the read/write cycle of computer hard drives, mechanical energy dissipation within internal combustion engines, and the dynamics of earthquakes.

Further reading: Ben-David et al., “Slip-stick and the evolution of frictional strength”, Nature 463, 76 (2010)