The earliest ice skates were made by Finns about 4,000 years ago by attaching sharpened bones to the soles of shoes. Around the 14th century, the Dutch invented steel ice skates, and ice skating gradually became a fashionable sport in Europe, with speed skating and figure skating attracting widespread attention. So, why do ice skates help people skate better? When we walk, we rely on the friction between shoes and the ground. Without friction, we can't move at all. However, for ice skating, the less friction there is, the smoother the skating will be. Ice skates can not only quickly gain forward momentum during the start and acceleration phases, but also reduce friction during skating and maintain stability. But why is there so little friction between ice and ice skates, which are both solid? If a layer of fluid is placed between two solids, friction will be greatly reduced, which is the principle of lubricant operation. Scientists studied the contact surface between ice skates and ice and found that the reason for the low friction is that there is a layer of water film between the ice skates and the ice surface, which acts as a lubricant.
Initially, it was believed that the water between the ice skates and the ice surface originated from the phase change of solid-liquid: ice turned into water under high pressure. But later, it was known that when the temperature is below -22℃, no matter how high the pressure is, ice will not melt, but will only change from one phase of ice to another phase of ice, not into water. So why can people still skate freely at low temperatures of -30℃? Later, it was realized that even below the melting point, there is a layer of liquid film on the surface of natural ice, which is called surface melting. The closer to the melting point, the thicker the liquid film. Between -35 and 0℃, the thickness of the liquid film is about 10 nanometers. Of course, the presence of impurities will also affect the thickness of the liquid film. For example, the presence of salt will make the liquid film thicker. So why does this layer of liquid film exist? In 2004, two Japanese scientists used molecular dynamics simulations to explain this. The water molecules inside the ice are subject to the forces of neighboring molecules in all directions, so a stable lattice is formed. However, the water molecules on the surface are exposed to one side and are unstable, so it's not easy to form a regular lattice, but tend to an amorphous form, that is, a liquid film. From the perspective of molecular thermal motion, water molecules vibrate near their equilibrium position. Due to the instability of the surface molecules, their vibration amplitude is larger than that of the inner molecules. This difference in vibration amplitude becomes larger and larger as the temperature approaches the melting point, so the thicker the liquid film is near the melting point, the closer it is to pure liquid water. According to current research results, there are three reasons for the formation of the water film: high-pressure melting, friction heating, and the presence of a liquid film on the ice surface. At different temperatures, the contribution of these three mechanisms to the formation of the film will vary. When the temperature is very close to the melting point of ice (such as greater than -3.5℃), the contribution of high-pressure melting is the largest; when the temperature is far from the melting point (such as -30℃), the contribution of friction heating is the largest; when the temperature is far from the melting point and at rest, the contribution of the liquid film is the largest. It is precisely because of this layer of water film between the ice skates and the ice surface that people can skate smoothly at extremely low temperatures.