[wireties]

I think encoders in the drive train are good enough to see slippage in the direction you are trying to go (just look for unnatural acceleration of a single wheel). Detecting slippage normal to the direction of travel is much tougher. I'd try a gyro before an optical mouse kind of thing. This is a tough surface to get an optical mouse to work well.

A couple of posts talk about the increase in traction if the floor is not smooth. This might be true but it is not because there is more surface area or a change in the coefficient. The reason is because there is now something to push against (bump/s in the material). There is an additional force term in the direction of travel.