Ice provides a slippery surface unlike most other materials. Whether you are ice skating, sledding, skiing, or simply walking on an icy sidewalk, it doesn’t take much for your feet or vehicle to slide out of control. But why is ice so slippery in the first place?
Pressure and Friction Melt Ice
For over a century, scientists believed ice was slippery because pressure from skates, sleds, or feet caused the ice to melt. This melting created a thin layer of water that allowed for smooth gliding.
In the 1850s, scientist James Thomson discovered that under high pressure, ice melts and turns back into water. This is because solid ice is less dense than liquid water. When you squeeze ice, it becomes unstable and melts.
So it was believed that the pressure from a skate blade on ice caused melting that reduced friction for gliding. However, studies have shown that typical pressures from skating or walking do not cause nearly enough melting to explain the slipperiness.
Frictional Heating Melts a Surface Layer
In the 1930s, scientists proposed that simply rubbing two surfaces together generates friction that melts the ice surface, similar to warming your hands by rubbing them together. This frictional heating concept better explained the liquid layer allowing ice to be slippery.
But friction still did not account for slipperiness when standing still on ice. So other theories emerged.
A Quasi-Liquid Layer Forms on the Surface
More recently, the dominant explanation involves a “quasi-liquid” layer that exists on the ice surface even below freezing temperature. This means the surface layer exhibits properties between solid ice and liquid water, while not being fully either.
Water Molecules Are Less Ordered at the Surface
At the molecular level, water molecules at the ice surface experience fewer bonds with other water molecules compared to those within the ice. This makes the surface layer more disordered and mobile like a liquid.
So even well below freezing, the top layer of molecules provides a slick surface on which objects can slide with little resistance or friction. This liquid-like layer has been verified down to just 1-94 nanometers thick through advanced imaging techniques.
Combination of Factors
The slipperiness of ice likely involves a combination of pressure melting, frictional heating, and the quasi-liquid surface layer. Depending on factors like temperature and speed, these mechanisms work together to create a uniquely smooth and slippery surface.
So next time you glide effortlessly across the ice, you’ll know it’s thanks to the complex molecular behavior of frozen water. The science behind skating, sledding, and slipping on ice involves decades of research on this common but deceptively complex material.