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Surfaces and interfaces

Surfaces and interfaces

A sticky problem

01 Dec 2006

Physicists in the US have made a peculiar discovery about how a small object slides over a rapidly rotating lubricated disk. They have found that if the object is tilted so that it only touches the disk at its corner, then the friction is greater when the disk rotates in one direction rather than the other. However, the low-friction direction is not the one you might expect -- a counterintuitive finding that the researchers say is caused by the properties of a meniscus that forms around the object (Phys. Rev. Lett. 97 216104).

Liquid friction

In simple systems, the atomic origin of friction is fairly well established. But in more complex systems, such as the movement of a computer’s read/write head over a rapidly spinning hard disk, a generalized understanding of friction at the nanoscale has so far remained out of reach. The friction depends on numerous factors, including roughness, lubrication, contact geometry, speed and vibration.

Now a team of researchers at electronics giant Hitachi have looked at what happens when the corner of a tilted oblong slider moves over a rotating carbon disk coated with polymeric lubricant. In their experiments, the team spun the disk under the slider at speeds of up to 12 m/s and measured the amount of friction using a strain gauge mounted on the slider’s suspending arm. Curiously, the team found that the friction opposing the spinning of the disk is greater when the disk moves “away” from the tilted slider rather than “towards” it (see figure: “Liquid friction”).

Most people would naturally assume friction to be greater in the latter case, thinking that the pointed edge of the slider would “dig” into the surface. In fact, the lubricant not only prevents this from happening, but it also adds its own friction where it builds up as a meniscus in front of the slider. When the disk is spinning towards the slider this preceding meniscus so small that it has just a negligible effect, but in the opposite direction it is large enough to significantly hinder the disk’s rotation.

However, this counterintuitive friction could also manifest itself on surfaces where there is no applied lubricant. When the team reduced the amount of “mobile” molecules in the lubricant (in other words, the slipperiness of it) from 50% in the original experiment down to less than 10%, they discovered again that friction was still greater for the disk moving away from the slider. And this low mobility of lubricant, they say, would be comparable to the trace amounts that reside even on surfaces supposedly considered to be “dry”.

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