Technical Thinking By Mark Jones
Slippery and misunderstood The temperature outside is in the low single digits. I’m freezing some water bottles for use in a cold therapy device by placing them outside. Three are frozen solid. One remains liquid until I pick it up. As the thin plastic crinkles under my touch, crystallization begins. The clear water goes cloudy and begins to stiffen. After mere seconds, the bottle is a solid mass of ice. The explanation is easy. Supercooled water remains liquid until perturbed by shaking or touch, triggering a nucleation event. Rapid ice formation follows. About one quarter of the time, I observe supercooling. All four bottles freeze solid, but most of the time, one of the four remains liquid, and it isn’t always the same bottle. The triggered freezing is still super cool — pun intended. It brings Cat’s Cradle to mind. While supercooling is well understood, ice still holds some secrets. Winter in Michigan comes with the telltale sound of antilock brakes. Traction is at a premium. Ice is slippery — an undisputable fact. There is, however, no consensus on why. The explanation I first heard is that pressure causes melting, and the liquid water formed acts as a lubricant. It makes sense. Ice expands when it freezes, so compressing it should lower the freezing point. Lord Kelvin demonstrated freezing point
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depression way back in the mid1800s. It isn’t enough to explain slipperiness. Tons of force are required. A water layer produced by friction was proposed in the 1930s. Experiments showed materials that conduct heat well create more friction on ice than insulators, consistent with heat flow being a driver — heat from friction. More experiments followed, this time looking at friction when surfaces are spun against ice. Rotating a piece of metal against ice at different speeds while measuring the forces showed that friction is not what creates the water layer. Frictional heating increases with speed, but the experiment showed no dependence on the speed of rotation. Friction-induced melting isn’t the cause. More recent explanations both invoke changes in ice structure at the air interface. Both use computer models as part of their explanation. Note I didn’t use “prove.” I’m not sure the models are testable. Throughout my career, I’ve learned all models are wrong, but some are useful. I purchase shoes and tires for winter with ice traction in mind. Thousands of patents devoted to rubber compounds and tread design show this remains an active area of research. The research and patents clearly show that what is touching the ice matters. My own experience
shows that two identical-looking shoe soles can behave very differently on ice. I am a believer in snow tires — the traction difference can be remarkable. The empirical evidence is undeniable. Understanding seems to be lagging. There are computer models of ice traction that focus on friction forming a water layer. This seems at odds with more recent models and experiments illustrating why ice is slippery. There is a widely quoted statistic that one million falls and 17,000 deaths are due to ice and snow. Dig as I might, I can’t find a source. It appears to be a zombie fact, one likely inflated. Extrapolating U.S. Bureau of Labor Statistics data gives an estimated 140,000 falls and 150 deaths due to falls on ice and snow. A total of 536,000 crashes, 117,000 injuries, and 1,300 deaths are attributed to snowy and icy pavement, according to Federal Highway Administration data. Not a million injuries — but stopping even one death would be a positive. Leveraging better understanding into better technology for keeping us upright and in our lanes would be a great step forward. It would be a great outcome for what today is curiosity-driven research. Models of why ice is slippery might strike some as frivolous. The models are surely wrong but hopefully will prove useful. DW
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3/31/26 2:36 PM