Why is ice slippery? When I was a kid, the textbook answer was pressure melting. This answer is incorrect. Then it was said that the correct answer was frictional heating. Then it was a film of liquid or disordered water on ice. I remember Gabor Somorjai announcing this discovery to the applauding audience: the problem has been solved, mission accomplished. After 25 years of exploring this layer, no consensus has emerged as to its properties or even its thickness.http://www.lptms.u-psud.fr/membres/trizac/Ens/L3FIP/Ice.pdf
The latest papers tell that the liquid film is too thin to explain why ice is slippery, so it must be frictional, by default.http://prola.aps.org/abstract/PRB/v61/i11/p7760_1http://www.uoregon.edu/~rempel/12_RMP06re.pdf
Others still maintain it is viscoelasticity of the liquid filmhttp://pubs.acs.org/doi/abs/10.1021/la9001994
The answer is surely unknown. I wonder if any aspect of friction is truly understood, even the fact that the force is proportional to the load. Most theories tell it really shouldn't except for the hardest materials. http://www.tau.ac.il/~klafter1/ar3.pdfhttp://www.phys.appstate.edu/coffey/nano/nanotribologyencyclopedia.pdf
Every time I hear a talk on friction, it is painfully obvious how little is known about it. One would think it should be much harder to explain the formation of a star or prove the Poincare conjecture than explain slippery ice. But it is just the opposite. The problems that look hard are first to go. The problems that look easy are the ones that stay without any hope of resolution. A mathematician friend of mine told me the same: if it is a complex conjecture with a page of qualifiers alone and looking positively impossible to prove, chances are it will be solved quickly. The hardest problems come in the simplest form; that's where you waste your life with almost no progress. I vouch the same for every field I've worked in. It is not complex that is hard, it is simple that kills you. We often do these complex things out of sheer frustration at our inability to tackle the simplest thing. I know I do, and I've observed it in the others. I am also convinced that the real goodies are there, in these simple problems.
Why is it the simplest question that is invariably the hardest and most intractable one? Is it the law of Nature, a fundamental flaw of mind, or what? I have thought about this question long and hard, and I do not know the answer. No one seems to. It must be one of those intractable simple questions. But if this one is answered, maybe we'll be able to answer all of them?