For over a century, scientists have been trying to understand one of winter’s greatest mysteries: why is ice so slippery? The old textbook explanation pointed to a thin film of water turning every sidewalk into a potential skating rink. However, research suggests the answer is more complex.
Challenging the Thin Water Film Theory
For years, many physics and chemistry books explained ice’s slickness by the presence of a thin layer of liquid water. This film, supposedly formed by pressure, heat, or friction, was thought to help skates and shoes glide across ice. However, this explanation doesn’t fully account for the reality of ice’s slipperiness at very cold temperatures.
- People can ski or skate at temperatures far below freezing—around -4 °F—without any measurable temperature increase on the surface, according to observations.
- The liquid water explanation struggles to explain why ice remains slippery even in these conditions.
A Molecular Perspective: New Clues on the Ice Surface
To address this, a team led by Martin Müser at Saarland University conducted large-scale numerical simulations of ice at the molecular level. Using the TIP4P/Ice model, they replicated the properties of ice and liquid water, offering insights into how ice behaves at extremely cold temperatures. The researchers simulated two perfectly flat ice crystals pressed together at temperatures as low as 10 kelvins above absolute zero.
Before any sliding occurred, the researchers observed areas on the ice surface where the molecular structure was less stable than the surrounding crystal. These regions, corresponding to alignments of the water molecules’ electric dipoles, became local breakpoints when sliding began. As these zones deformed, the crystal structure gradually became disorganized, without traditional melting or significant heating.
Instead, a dense, disordered amorphous layer formed, resembling supercooled liquid water at the molecular level. This transformation was accompanied by a slight local decrease in volume, reflecting the higher density of this intermediate state. The surface becomes slippery and disorganized without turning into ordinary liquid water.
The Growth of Disorder
The scientists discovered that the thickness of the amorphous layer increased with sliding distance, following a square-root relationship. This suggests a mechanism driven by mechanical deformation rather than temperature. Each lateral movement allows water molecules to move out of their crystalline structure. This process involves ice becoming mechanically disorganized.
- The team also investigated the superlubricity hypothesis, questioning whether two perfectly flat but misaligned ice crystals could glide without friction. The results indicated that shear stress remained high until the amorphous layer formed.
Surprisingly, the disordering process occurs faster at extremely low temperatures than at -10 °C (14 °F). At 10 kelvins, the process is approximately six times faster. Cold ice is not harder to glide on simply because it doesn’t melt, but because the amorphous layer that forms is more viscous and offers more resistance to flow, even as it forms quickly.
Real-World Friction: Hydrophilic vs. Hydrophobic Surfaces
To understand the implications for slipping and sliding, Müser’s team simulated a rigid surface moving over ice. Surfaces that attract water (hydrophilic) generated high friction, aligning with real-world measurements. Conversely, surfaces that repel water (hydrophobic) produced less resistance to sliding. This difference depends on how water interacts with the moving surface, influencing energy dissipation without radically changing the microstructure. The relationship with water may be a factor when selecting winter boots.
The slipperiness of ice is related to mechanical changes at the molecular level, a process of disorder that facilitates sliding even at low temperatures. The next time you’re gliding (or falling) on an icy surface, remember that it involves molecules making your day interesting.
