Understanding why ice is slippery has evaded researchers for more than 150 years. Slipperiness is usually attributed to the formation of a thin layer of liquid water generated by friction, enabling the slider to glide over the liquid layer. However, this remains a hypothesis and raises many questions: What is the thickness of this layer, and what are its properties? In contrast to oil, water is a bad lubricant, so why would it reduce friction in this case? We develop a new force instrument that opens a new window on the origin of ice friction and allows us to probe this interfacial film at the nanometer scale for the first time.

The force apparatus is based on a tuning fork to which we glue a millimeter-sized bead of glass. By exciting the tuning fork and measuring the horizontal and vertical displacement of the bead, we are able to investigate the liquid layer that is produced as the bead slides over the ice. We find that the liquid film is far smaller than expected, just hundreds of nanometers thick. More unexpectedly, the interfacial meltwater is far from any “simple water”—it is as viscous as oil with complex viscoelastic properties. This “slimy” water film explains why ice is so slippery.

Such behavior requires a fundamental overhaul of existing frameworks describing ice friction, and our experimental results will allow researchers to build such a theory. We hope that any new theory may provide clues on how to increase slipperiness for winter sports, and it may lead to useful solutions for increasing friction, such as in automobile applications.