FOR decades physicists have been stumped by an everyday problem: why do even slow-moving cars skid so easily on wet roads? Now a team in Italy and Germany has come up with an explanation that could make wet-weather driving safer.
A vehicle on wet a road typically requires double or triple its normal stopping distance, and skids more easily. If the road is flooded or the vehicle is travelling faster than 60 kilometres per hour, the cause is usually aquaplaning, which occurs when the tyre is moving too fast to flush water from the road beneath it.
But aquaplaning does not explain the loss of grip at lower speeds, which measurements have shown can fall by as much as 30 per cent compared with the grip in the dry. One suggested explanation is that water forms a partial barrier between the tyres and the road; another that the water weakens the bonds that form briefly between the molecules in the tyre and road.
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But neither of these factors can account entirely for the drop in friction, according to Erio Tosatti of the International School for Advanced Studies in Trieste, Italy, and Bo Persson from the Institute of Solid State Research, in J眉lich, Germany. The roughness of a road surface means only about 1 per cent of a tyre is in contact with it, and water does not reduce this area significantly at low speeds.
Tosatti鈥檚 theory focuses on small irregularities in the road surface, which set up vibrations in the tyre that are known to make a significant contribution to friction between the tyre and the road. According to Tosatti and Persson, tyres trap small pools of water in the road鈥檚 surface, which effectively smooth the surface and reduce the vibrations. Persson and Tosatti calculate that this effect should cut friction by 30 per cent, close to the reduction measured on wet roads (Nature Materials, DOI: 10.1038/nmat1255). 鈥淚t鈥檚 an extremely simple idea,鈥� he says. 鈥淪mall puddles and large puddles smooth out the road, and that removes oscillations at some short and some long wavelengths.鈥�
Tosatti believes that having a realistic model of dry and wet friction should help in the design of safer tyres and better roads. For instance, it might be possible to create road surfaces that prevent puddles smoothing off the road鈥檚 roughness, or tyres that do not seal water into tiny pools. 鈥淜nowing where to aim is always a big step forward in trying to find a cure for the problem,鈥� he says.
