Friction is determined by history

Friction has memory, a team at Harvard University in the US has discovered.

Researchers found that the force which keeps your coffee cup from sliding about on the table is a changeable phenomenon. Mostly, friction increases the longer your cup sits still, but sometimes it can decrease – for example when its load lightens rapidly as the coffee is consumed.

Most surprisingly, the team found that, given the right circumstances, friction can even decrease for a while, then change direction and start to increase again – and that it all depends on what’s happened before.

This memory effect reveals friction to be a much more complex interaction than previously realised. Understanding it could change the way we think about a huge range of material behaviour – from earthquakes caused by sliding tectonic plates down to the precision operation of industrial machinery.

Sam Dillavou and colleagues stacked up two polymer blocks and set up a spring to push down on top of them. They then partially released the spring tension and measured the friction.

“Real surfaces are rough at small scales – think zooming in until our flat surface looks like a mountain range,” says Dillavou.

“When you press two surfaces together, only a small portion of the apparent area is in true contact, typically less than 1%. 

“This real area of contact is what dictates the resistance to sliding of the interface … two surfaces in static contact are not truly static; these small areas of real contact are under enormous pressures and deform and grow over time.”

To understand the way surfaces mould to one another, the scientists conceptualised the two blocks covered in tiny springs that stuck up at different heights. As they forced the blocks together, the tallest springs compressed, allowing some of the shorter ones to come into contact.

When the force was lessened, the springs slowly decompressed and pushed the blocks apart slightly, which was why the friction decreased – sometimes for hours – before starting to increase again.

It’s a model that’s successfully been used to understand disordered materials, such as glass, which do not have a regular crystal structure, Dillavou adds.

“We managed to quantify these behaviours using a theory that describes numerous glassy disordered systems such as crumpled paper and elastic foams,” he explains.

To quantify the actual contact area between the polymer blocks, the team shone a light into the lower one at a low angle. Where the surfaces touched, light was able to pass into the upper block. The brightness of light seen in the upper block gave a good indication of the contact area.

While the friction force was mostly correlated with the surface area, the experiment held another surprise, says Dillavou.

“We showed that under the right conditions, it is possible to have the real area of contact shrink while the frictional strength is growing,” he said.

“We believe we reconciled this apparent paradox by showing that frictional interfaces evolve non-homogeneously, and that certain regions become more important than others, exerting an oversized influence on the frictional strength.”

The research is published in the journal Physical Review Letters.