Young man slides and falls on frozen lake

Ice is slippery because molecules in the surface layer are only loosely bound to each other. Credit: Getty

Physical chemistry

Why your feet slip and slide on ice

Experiments and simulations disprove an old theory.

Skaters slide across ice because they’re riding atop a layer of rolling molecules — not because the skates melt the ice as they go, as was previously thought.

Daniel Bonn at the University of Amsterdam and his collaborators measured the friction of a metal ball sliding on ice at temperatures from −100 °C to 0 °C. At the colder end of that range, friction was high. But at around −70 °C, friction began to decrease, reaching a minimum at −7 °C — the ice temperature at typical speed-skating rinks.

Molecular dynamic simulations suggested that each molecule in a microscopic layer on the ice surface forms a bond with two to three others, compared with four bonds in the bulk of the ice. As temperature warms above −70 °C, the proportion of molecules with just two bonds begins to increase. These act like a layer of rolling logs, allowing a skater — or an unlucky pedestrian — to glide along their surface.