Overview of What’s Happening On The Slippery Surface Of Ice?
This Science Friday episode (host Ira Flatow) explores why ice is slippery, featuring Dr. Robert Karp (tribology expert, Univ. of Pennsylvania). The discussion reviews historical ideas (thin liquid layer, pressure- or friction-induced melting), highlights new computational work suggesting a surface-driven molecular disordering mechanism, and describes recent applied research on detecting and preventing ice and on the physics of curling stones.
Key points and main takeaways
- Ice is a mineral (when naturally occurring) — a solid crystalline “rock.”
- Traditional explanations for slipperiness: a thin liquid layer on ice produced by:
- Pressure-induced melting,
- Frictional heating (melting during sliding),
- Or an always-present quasi-liquid surface layer.
- New computational results (Martin Müser and colleagues, Saarland University) suggest a different mechanism:
- Water molecules at the ice surface sense a nearby surface via electrostatic dipole interactions.
- That interaction disorderly rearranges the surface molecules into an amorphous, soft layer that is intrinsically slippery.
- This disordering can occur even at very low temperatures (though slipperiness increases closer to the melting point).
- These findings are simulation-based and need experimental validation.
- Practical effects:
- Ski wax reduces friction because it repels water and weakens attraction between the ski base and the surface water/disordered layer.
- Triboelectric nanogenerators (TENGs) can detect ice formation (via charge produced by the freezing contact line) and potentially de-ice surfaces—useful for aircraft and drones.
- Curling physics: high-precision sensor studies show friction between granite stones and ice falls sharply with increasing speed, rises at the lowest speeds, and the stone’s rotation center shifts during motion—insights that can inform play strategy.
Topics discussed
- Definition and origin of “tribology” (from Greek tribos, “rubbing/sliding”; term promoted by Peter Jost in the 1960s).
- Historical debate about why ice is slippery (Faraday and others considered the problem).
- Details of molecular/dipole-based surface-disordering hypothesis from large-scale computer simulations.
- Role of ski wax and surface chemistry in reducing friction.
- Ice-detection and de-icing technology using triboelectric nanogenerators (Kevin Golovan’s group, Univ. of Toronto).
- Physics of curling stones — speed-dependent friction behavior and implications for athletes.
Notable insights and quotes
- “Tribology” — the field studying friction, wear and lubrication — comes from Greek tribos (rubbing/sliding).
- New simulation idea: surface water molecules “sense” nearby surfaces via their electric dipoles and become disordered, creating a soft, slippery layer.
- Practical combo: detection and mitigation via the same TENG technology (detect freezing and use generated charge to help melt ice).
Practical recommendations / implications
- For athletes (skiers, curlers): base-surface chemistry and sliding speed matter—waxing and appropriate speed choices can significantly change frictional behavior.
- For engineers/aviation: consider TENG-based sensors for real-time ice detection and integrated de-icing on critical surfaces (aircraft, UAVs, wind turbines).
- For researchers: experimental validation of the dipole-induced surface-disordering hypothesis is a high priority; further study on temperature dependence and surface chemistry effects is needed.
Open questions / future research
- Can experimental work confirm the simulated dipole-driven surface disordering and quantify its contribution versus pressure- or friction-induced melting?
- How does surface chemistry (different materials contacting ice) quantitatively alter the disordered layer and friction?
- What are the best designs to deploy TENGs widely for robust ice detection and active de-icing in different environments?
- How can curling and other ice-sport findings be translated into actionable coaching/strategy tools?
Who to follow / credits
- Guest: Dr. Robert Karp, John Henry Towne Professor, Dept. of Mechanical Engineering and Applied Mechanics, University of Pennsylvania.
- Researchers mentioned: Martin Müser (Saarland University) — simulations; Kevin Golovan (University of Toronto) — TENG ice detection.
- Episode produced by Charles Berquist; host Ira Flatow.
If you need a one-paragraph version or a bulleted “what to remember” cheat sheet for quick reference, I can provide that next.