  Loss of braking power and rubber skidding on a wet road is still an open
physics problem, since neither the hydrodynamical effects nor the loss of
surface adhesion that are sometimes blamed really manage to explain the 20-30%
observed loss of low speed tire-road friction. Here we advance a novel
mechanism based on sealing of water-filled substrate pools by the rubber. The
sealed-in water effectively smoothens the substrate, thus reducing the
viscoelastic dissipation in bulk rubber induced by surface asperities, well
established as a major friction contribution. Starting with the measured
spectrum of asperities one can calculate the water-smoothened spectrum and from
that the predicted friction reduction, which is of the right magnitude. The
theory is directly supported by fresh tire-asphalt friction data.
