Why is Ice Slippery? 

If you have ever been skating you will have felt, perhaps painfully, that ice is slippery. However, there is a common misconception that ice is inherently slippery. This is not true; in fact, ice is not always slippery and nor indeed is any other material. As a result, the investigation into this property of ice is often focused on a situation where its slipperiness has been tried and tested - the physics of skates on ice.

 It has to do with the melting point of ice being reduced when subjected to high pressure (a result of water’s unique hydrogen bonding), and increased when the pressure is removed. This theory is often demonstrated using the setup shown in Figure 1. However, the pressure required to sufficiently reduce the melting point would be very extreme, effectively ruling out this mechanism as the sole reason for ice’s slipperiness.

Another theory suggested that the thermal energy created by the friction between the skates and the ice as you move along the ice causes the ice to melt and form a thin layer of water. However, this also does not provide the complete answer as it does not explain why ice is slippery even if you are just standing on it.

On May 9, 2018 Mischa and Daniel Bonn published a paper in the Journal of Chemical Physics  The paper presents their investigation of the surface of ice with a more convincing reason for its slipperiness. They found that rather than a layer of liquid water on the ice, there were loose water molecules. These loose molecules were described by Mischa Bonn as “marbles on a dance floor” where slipping across the surface of ice is analogous to rolling on these marbles. The molecular structure of ice is regular and neat with each water molecule being bound to three others. However, the surface molecules of the ice are only bound to two other molecules and this weak bonding allows them to attach and detach themselves from different sites on the ice crystal. This layer of molecules is still not the same as a layer of liquid water as these molecules, and the resultant slipperiness, can be observed far below the freezing point of ice. Daniel Bohn regards these particles as a “two-dimensional gas” rather than a three-dimensional liquid due to the way in which the molecules freely diffuse across the surface of the ice.

Nevertheless, even this property of loose surface molecules is not unique to ice. So, it is still not what makes ice uniquely slippery. The most unique property of ice is it being the only material where we have the solid, liquid and gas states within our regular climate ranges. Since we almost always encounter ice so close to its melting point, mechanisms such as thermal energy from friction, regelation and loose surface molecules provide a noticeable combined effect to make ice uniquely slippery at such temperatures, a luxury ice skating enthusiasts should be thankful for.