Rubbing up the right way!

Microscopic string like structures cause friction among surfaces

Published: Wednesday 15 May 2002

-- when you move a piece of chalk on a blackboard, a nerve tingling screeching sound is produced. This is an example of the 'stick-clip' motion that occurs when two surfaces that are not lubricated move on each other. According to R Budakian and S Putterman of the University of California, usa, one of the main reasons behind this kind of motion or friction is the formation of microscopic string like structures between the two surfaces when they are brought into contact with each other. The researchers say that these structure need to be broken before the surfaces can slip on each other.

At the most obvious level, roughness of the surface is responsible for most of the 'stick-clips'. But for scientists, smooth looking surfaces have ridges and irregularities that interlock with each other and provide the friction and hence the stick-clip motion. But with the new research results such theories would be revised.

To prove their theory, the two researchers experimented on two gold balls. One, about 0.2 millimetres in diameter, was attached to an optical fibre, while the bigger one (about two millimetres in diameter) was fixed to a mechanism whose motion could be precisely studied and controlled. The scientists could precisely measure the smaller ball's displacement mainly because of the optical fibre's presence. Changes in the forces between them, and also in their area of contact helped the researchers assess the displacement. They first studied the pulling apart motion of the two balls.

What they found was that when the balls were held together, narrow bridges of gold formed between themand these broke when they were pulled apart. Surprisingly, when the balls were made to slide on each other, a similar phenomenon was again noticed. Strings were formed when the balls came in contact with each other. The strings got stretched and finally snapped when the balls were made to slide on each other.

The finding of Budakian and Putterman would help a lot in building miniature mechanical devices. For these devices, conventional methods of lubrication have failed till now. A better understanding of what makes surfaces stick together as shown by the researchers' experiment would pave way to make considerable progress in manufacturing these devices.

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