[Mike Betts]

Tristan,

A "sticky" subject indeed. All references are correct. For most all applications, mu is indeed less than one, however, it is not a physical requirement.

As an example, I ask that you consider one of nature's most amazing substances... Water.

When a skater is gliding across the ice, the weight of the blade of her skate causes the ice to melt and this lubricates the ice/blade boundary to a very small value of mu.

Now for an old fashioned dinner table experiment. Take a standard teaspoon, clean both the spoon and the end of your nose with a clean napkin (this gets rid of grease). Next, breathe heavily on the spoon to create a light layer of condensation. Now you can "hang" the spoon off the tip of your nose demonstrating a mu of > 1.0 for the very same substance (water).

In our first example, water is a "lubricant" and in the second it is a "glue". Both actions must be examined on an extremely small scale to be understood.

In general, Ken is correct. For mu > 1 there is usually a hook or glue mechanism at work. Just remember, the action might be taking place at the molecular level...