Piddly Puddle Peril: Little water pools foil road friction

The friction between a car tire and the underlying asphalt drops off dangerously in rainy weather. Now, physicists in Germany and Italy have proposed an explanation for how even slight wetness can cut road-to-rubber friction.

Most, if not all, of the friction between a tire and a typical road has nothing to do with attractive intermolecular forces between the tire rubber and the road material, says theoretical physicist Bo N.J. Persson of the Research Center Jülich in Germany. Instead, by a mechanism dubbed bulk friction, the rubber of a moving tire briefly presses down into small pits in the road surface.

As those ups and downs of road topography jostle the tire, they cause vibrations. Those vibrations heat the rubber, dissipating some of the tire’s kinetic energy and causing a drag on the car’s motion, scientists have theorized.

Unlike most other forms of friction, which take place at the interface between two materials, “rubber friction is very special, at least on rough materials. The energy is dissipated a little [way] inside the rubber,” Persson says.

Small amounts of water reduce this friction. Persson and his colleagues have now simulated this effect by using an optical scan of a sample—1.5 centimeter by 1.5 cm—of actual road. The researchers calculated the consequence of filling that surface with just enough fluid to top off all the small divots without causing them to overflow.

Persson notes that the actual contact region between a tire and the road is a narrow strip that runs the width of the tire. As the strip pushes down on the water-filled roadway, the tire rubber seals the edges of each of the pools, preventing the water from leaking out. That sealing converts each pool of trapped water into an incompressible barrier beneath the tire rubber.

“Then, the rubber becomes unable to reach out and touch the bottom of the puddle. If it doesn’t touch the bottom, that contribution to the vibration is canceled,” says Erio Tosatti of the International School for Advanced Studies in Trieste, Italy, a coauthor of the new road-friction report. Multiply that loss by many puddles, and both road friction and driver control take a dive.

The team proposes that friction-thwarting mechanism in the December Nature Materials.

Gert Heinrich of the Leibniz Institute of Polymer Research in Dresden, Germany, a theoretical physicist who co-developed a different version of the bulk-friction theory, calls the new report “an important contribution” to understanding why tires are more apt to slip when roads are wet or even just damp.

Calculations based on the new theory agree with the 20 to 30 percent friction losses actually measured for tires braking without skidding on wet roads, Persson and his colleagues claim.

However, the new findings apply only to vehicles moving slower than 60 kilometers per hour on slightly wet surfaces. At greater speeds and on inundated roads, a well-understood type of more-drastic friction loss known as hydroplaning can occur.

In that case, a thin film of water coats the road surface, Tosatti notes. The car’s tires “fly” along on that film, he adds, with no road contact and total loss of driver control.