Da Vinci’s sketches also feature experiments in friction. | Image: The British Library

Friction is one of the physical phenomena that continues to hide the truth. Given the context, it might make you think of the Dan Brown bestseller The Da Vinci Code. It was the very same Leonardo da Vinci who, almost six centuries ago, set out the laws of friction. In the meantime, many are the scientists who have been stumped by the mechanisms at play when two bodies rub against each other.

Despite progress, today nobody has fully cracked the code and correctly predicted the friction caused by the imperfections of two rough surfaces in contact. That hasn’t put off Tom de Geus. He is a mechanical engineer at EPFL who has conducted SNSF-supported research known as ‘The Five Ds in Friction’. At the heart of this work is a phenomenon that de Geus calls – cheekily – ‘black magic’. He is referring to the enigmatic force needed for an object to start moving suddenly.

As surrounded as it is by so many grey areas, we might imagine friction becoming forgotten in an academic drawer, accessible only to the eyes of seasoned physicists. Yet it is one of the earliest subjects of school physics lessons. Pupils experiment to find the lowest angle where a stationary object starts to slide or roll down an incline, and the threshold of force that must be surpassed for there to be sudden movement in a block drawn by way of a spring.

“Friction is a fundamental concept, because it covers forces in counterbalance, for instance, pushing and pulling forces”, says Veronique Trappe, a researcher in the physics department of the University of Fribourg. “Without friction, we’d slide freely and, unless we used a propulsion mechanism, we’d be unable to initiate any movement”. Friction is at the heart of daily gestures and activities, says de Geus. “If there was no friction, we’d be unable to walk, move around or handle any number of our interactions with or within our environment”.

Lucky strike

Despite its centrality to the normal functioning of the planet and its inhabitants, friction harbours some major challenges. “Due to friction, machinery loses a non-negligeable amount of energy”, says de Geus. This is where lubricants, which reduce friction, come into play. One even more evocative example is earthquakes. “Most earthquakes are linked to the friction of tectonic plates”. Providing tools to calculate the probabilities of earthquakes and therefore contribute to their prevention is one of the main motivations underlying the work of de Geus.

“I think I’ve resolved the problem of predicting the ‘black magic’”, says de Geus. He hastens to clarify that he was lucky to have existing research upon which to base his own, as well as complex digital simulations. His solution takes into account not only the collective collapse of surface irregularities—much like how the state of water changes during evaporation—but also their mechanical instability—much like the sudden propagation of a crack in ice. By merging them, the frictional force can be predicted.

This discovery, which he calls a ‘lucky strike’, is linked to his findings alongside a statistical physicist, which were what set him on this path. “It took unlikely bedfellows, a mechanical engineer and a statistical physicist, to gather what was necessary to develop a simple and novel approach”. The future will reveal whether this tool means researchers can finally crack the Da Vinci code.