If I recall correctly, when I studied physics I was taught that the reason we can ice skate and curl “the rock” on ice was due to friction and pressure. In short either pressure, friction or both, create a thin layer of water to glide on. While listening to a recent edition of Science Friday, I was intrigued. This is not actually the case. While the podcast is always very informative, I seek a more detailed explanation.
In the past, scientific observations gave good reason to believe that pressure and friction explained why we can skate and curl. Let’s start there. Take a few minutes to watch this National Geographic video from which one could easily conclude that it is due to pressure. If you took the time to watch, it is pretty clear that the cheese wire cut right through the block of ice. It also seems logical that the weights used in the video were far lighter than that of the average ice skater. So this explanation seems to make sense.
According to the podcast, the pressure and/or friction generated by an ice skater is not an adequate explanation. If the temperature were more than a few degrees below freezing, the math of the physics does not add up. It also fails to explain how we can end up on our butts while wearing flat bottomed boots and standing still.
Dr. Robert M. Rosenberg, emeritus professor of chemistry at Lawrence University, states the same. The weight of a 150 pound skater standing on 32° F ice would only melt the ice by 3 hundredths of a degree! Figure skating and hockey are on ice that is between 22 and 16 degrees Fahrenheit. The pressure theory does not hold enough weight (pun alert!).
Ice is a mysterious substance. (The New York Times article I quote below even describes different types of ice.) For starters, it is less dense when solid than when liquid while most solids are denser when compared to their liquid state. And what other solid substance would you even consider gliding across with thin metal blades? Should the question be “why is ice slippery?”
A theory, proposed by Michael Faraday in 1850 is that ice always has a thin layer of water on its surface. His simple experiment involved pressing two ice cubes together. They fused form one solid piece of ice. Indicating that the thin layer turned to ice. He concluded that the layer was just so thin that it could not be easily detected.
More recent research seems to support that this water layer is very real even at extremely cold temperatures:
“In 1996, Gabor A. Somorjai, a scientist at Lawrence Berkeley Laboratory, bombarded the surface of ice with electrons and watched how they bounced off, producing a pattern that looked at least partially liquid at temperatures down to minus 235 degrees. A couple of years later, a team of German scientists bounced helium atoms off ice and found results that corroborated the Lawrence Berkeley findings.”
Again, we have a theory that seems to make sense, liquid is always on the surface of the ice. The problem is that, even with pressure and friction, this thin layer of liquid is not thick enough to account for skating or curling. In fact, more recent research indicates that ice is not that slippery.
In 2002, Dr. Miquel Salmeron and his team, also from Lawrence Berkeley Laboratory, dragged an atomic force microscope across ice. This ultra-tiny-phonograph-like needle indicated that the surface of the ice has a pretty high frictional co-efficient.
The research seems to yield contradictory findings. (Add to it the fact that it is at the same facility, and I can’t get this image out of my head of coffee breaks erupting into conflicts often associated with hockey games.) How can this be?
So, is ice slippery? In layman’s terms, the best answer seems to be both yes and no.
Dr. Somorjai is quite the expert on surface chemistry. So much so that he was actually a consultant for the 2002 Olympics to make better ice. When he first examined the surface of ice at the molecular level, his observations indicated that every other molecule of water was missing! Knowing that this was not possible, he looked into the matter further.
The hypothesis is fascinating. It seems that these molecules are vibrating so fast that they are not easily detected. While ice is solid, the vibrating molecules have the characteristics of a liquid. The molecules are vibrating fast enough where they should turn to a liquid. However, they don’t.
The surface of ice is believed to be in a “quasi-liquid” phase. These molecules are only vibrating up and down. If the molecules vibrated in all directions, they would turn liquid, but, by not vibrating to and fro or side to side, the thin layer atop ice is both solid and liquid. While chemists and physicist are still trying to figure out all the details, it seems that ice skaters and curling rocks are sliding along on these vibrating molecules.
If you are interested in reading more on this topic, here are additional articles I read on it but did not cite above: