5 Minutes
For nearly two centuries, textbooks have explained ice’s slipperiness as a result of melting caused by pressure or friction. A new study from Saarland University, led by Professor Martin Müser with colleagues Achraf Atila and Sergey Sukhomlinov, challenges that long-standing picture. Their simulations indicate the thin, lubricating layer that forms at the ice interface arises primarily from interactions between molecular dipoles at the contact surface—for example, the molecules in a shoe sole or ski base—rather than heat generated by pressure or friction.
The team’s work revises an idea proposed almost 200 years ago by James Thomson (brother of Lord Kelvin), who argued that pressure, friction and temperature were the key drivers of surface melting. Instead, the researchers find that electrostatic dipole-dipole forces disrupt the ordered crystal lattice of surface ice, producing disorder and a liquid-like film even under conditions where traditional melting should not occur.
The physics of dipoles and the ice surface
What is a dipole?
A molecular dipole occurs when a molecule has regions of partial positive and negative charge, creating a directional polarity. Water molecules (H2O) are polar: below 0 °C they lock into a highly ordered crystalline lattice that defines solid ice. When another material contacts that lattice, the orientation of its surface dipoles interacts with the ice’s dipoles.
The illustration shows what happens on the surface of ice when another object, such as skis, ice skates or shoe soles, comes into contact with it: the previously orderly crystal structure of the water molecules is suddenly disrupted. Credit: AG Mueser
According to the Saarland simulations, these dipole-dipole interactions can become "frustrated" in three dimensions—a physics term for when competing orientational forces prevent a system from reaching a single, low-energy ordered state. At the ice–contact interface this frustration destabilizes the crystalline arrangement, creating an amorphous, disordered layer that behaves like a viscous liquid. Importantly, the simulations show this process does not require significant heating by friction or pressure-induced melting: the electrostatic interactions alone are sufficient to generate the lubricating film.
Key findings, low-temperature behavior, and implications
One striking implication overturns another common assumption: that a lubricating film cannot form at extremely low temperatures (for example, well below −40 °C). Müser and colleagues report that dipole-driven disorder persists even at very low temperatures. While the interfacial film becomes increasingly viscous—approaching the consistency of honey near extreme cold—the layer still exists. Practically, this means some degree of interfacial mobility is present across a wider temperature range than previously thought, although the mechanical consequences (e.g., how easily someone slips or how skis glide) depend on the film’s viscosity.
The discovery has broad relevance for surface science, tribology (the study of friction and lubrication), winter safety, and materials engineering. If dipole orientation and surface chemistry control interfacial melting, then designing shoe soles, skate bases, or ski materials with specific dipole characteristics could modify slip risk or performance. Similarly, this mechanism could inform anti-icing coatings and cryogenic surface treatments where controlling interfacial disorder is critical.
Expert Insight
Dr. Elena Park, materials scientist and science communicator, University of Cambridge: "This study reframes a familiar phenomenon by moving the causal explanation down to molecular electrostatics. It opens a practical route: if dipole alignment at the interface governs liquidity, engineers can target surface polarity to either suppress or promote thin-film formation. That could influence everything from safer footwear to optimized winter sports equipment."
The Saarland research relied on advanced atomistic computer simulations to capture dipole-dipole interactions at nanometer scales—regimes difficult to probe experimentally. The team’s computational approach provides testable predictions: for example, varying the polarity or dipole orientation of an contacting material should predictably alter the thickness and viscosity of the interfacial film.
Conclusion
The long-held belief that pressure and friction alone cause ice to become slippery is incomplete. Saarland University’s simulations demonstrate that dipole interactions at the molecular scale can destabilize surface ice, producing a lubricating film even at very low temperatures. This shift in perspective has both scientific and practical consequences: it reframes textbooks, suggests new experimental tests in cryophysics and tribology, and points toward engineered surfaces that control slip by tuning dipole behavior rather than relying only on thermal or mechanical factors. As experimentalists and materials scientists follow up, the discovery promises to refine our understanding of cold-surface physics and inform new approaches to safety and performance on ice.
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