Researchers from Saarland University have overturned a nearly two-century-old physics theory about why ice is slippery, according to a groundbreaking study published on August 8 in the journal Physical Review Letters. The work, led by Professor Martin Müser and colleagues Ahraf Atila and Sergey Sukhomlinov, demonstrates that the slippery liquid layer causing people to fall on ice results from molecular dipole interactions, not pressure or friction.

This discovery challenges the established theory proposed in 1849 by James Thomson, brother of Lord Kelvin, who suggested that pressure and friction melt the ice surface. This explanation has been taught to students worldwide for over 150 years, positing that the sliding layer on ice forms due to the body’s weight pressing through the soles of shoes.

Revolutionary Understanding of Ice Structure

The German research team used computer simulations to study ice at the molecular level and found that the slippery layer forms due to “cold, shear-induced amorphization” rather than thermal melting. When ice is structured below zero degrees Celsius, water molecules align in a highly ordered crystalline lattice, with molecules neatly aligned with each other.

According to Professor Müser from Saarland University, “It turns out that neither pressure nor friction plays a particularly significant role in forming the thin liquid layer on ice.” Instead, when someone steps on ice, dipoles in the shoe sole interact with dipoles on the ice surface, disrupting the ordered crystalline structure.

“In three dimensions, these dipole-dipole interactions become ‘frustrated,’” Müser explained, referring to a physical concept where competing forces prevent the system from reaching a stable configuration. The interaction between dipoles in the ice and the contacting material causes the ice to become disordered, amorphous, and ultimately liquid at the contact boundary.

Implications for Extremely Cold Conditions

The study also debunks another long-standing assumption about winter sports. Previously, scientists believed that skiing at temperatures below minus 40 degrees Celsius was impossible because a lubricating liquid film could not form at such low temperatures.

“Dipole interactions persist even at extremely low temperatures. Remarkably, a liquid film still forms at the interface between ice and a ski—even near absolute zero,” Müser noted. However, at such extreme temperatures, this film becomes more viscous than honey, making skiing practically impossible despite its presence.

The findings have direct practical applications beyond academic understanding. According to the researchers, the discovery could influence the design of winter sports equipment, footwear, and protocols for assessing risks on slippery surfaces.