Winter sports like ice skating or luge, depend on ice to be the perfect amount of slipperiness. Buy why is ice slippery? What is the perfect temperature of ice for winter sports? Researchers from the University of Amsterdam and MPI-P have a theory.

In the experiments, a steel ball slides over the ice surface which consists of rapidly tumbling mobile water molecules that are only loosely bounded to the underlying ice. (Source: Nagata/MPI-P)

The first scientific explanation came from Irish physicist John Joly who theorized in 1886 that the slipperiness is caused by a layer of water forming when an object touches the surface of the ice. He believed the resulting local contact pressure was so high that it melted the ice, creating a liquid layer that made the ice slippery. The most recent theory is that the liquid water is melted off the ice by friction rather than by pressure.

The team's macroscopic friction experiments at temperatures ranging from 0 to -100˚C found that slipperiness is temperature-dependent.

By using spectroscopic measurements, the researchers discovered two types of water molecules at the surface. The first are molecules that are stuck to the underlying ice and are bound by three hydrogen bonds. The second are mobile water molecules that are only bound by two hydrogen bonds and roll over the ice when experiencing thermal vibrations. When the temperature increases, these two types of water are interconverted. This means that the fixed water molecules are turned into mobile water molecules, thereby creating more mobile water molecules and making the surface of the ice more slippery. The greater the mobility at the surface of the ice, the slippier the ice gets, while lower water mobility means more friction. All of this research lead the team to decide that there isn’t a thin liquid layer on the surface of the ice, but rather water molecules with high mobility that are responsible for the perfect conditions for winter sports.

While this is a huge discovery, the researchers also answered another question: what temperature creates the perfect conditions for winter sports like ice skating? The answer is -7˚C, the exact temperature setting used at speed skating rinks. From -6˚C to 0˚C the ice starts to soften and skate blades dig deeper into the ice.

The study on this research was published in the Journal of Physical Chemistry Letters.