Friction and Relation of Coefficient of Friction to Smoothness

[eagermind]

Friction and Relation of Coefficient of Friction to "Smoothness"

In my physics class today my teacher began discussing a very peculiar characteristic concerning the static and dynamic coefficients of friction. Apparently, when one makes an object's surface "smoother" (meaning with less impurities and a more uniform surface), the coefficient decreases to a certain point. However, past that point of "smoothness," the smoother the object becomes, the greater the coefficient of friction becomes. Apparently Japanese scientists have machines for this purpose unlike anywhere else in the world that make the surface of an object look uniform, even to a scanning electron microscope, and yet have a quite high coefficient of friction.

My teacher said that there were several theories as to why this strange phenomenon occurred, such as a theory that the uniformity of the object's surface creates a uniform electron spin that interferes with the spin of another object placed upon it. However, others say that this and other theories don't hold any water.

If anyone has any examples or other theories that explain why this weird and surprising (to me) quirk appears, I would appreciate your input. This is my first post, so I hope you all can cut me some slack.

 

[cheahchungyin]

Hi! :D Welcome! I'm new to this forum too haha.

Just making a guess :P, maybe its because of the total contact surface area. When the surface is not too smooth, the contact surface is not 100% as some molecules will be higher while some will be lower which won't come in contact with the object. Friction is related to its contact surface. We know that when friction happens, the objects tend to "rub" off some tiny particles from the surface. So it would be better if the contact surface is not 100% smooth so object won't have so much contact with the suface cause if its smooth it will fit the surface so perfectly that it will have to "rub" off the whole surface haha :)

 

[Alkim]

Hi, it is not just the surface area but the number of particles per unit area that are in close proximity. When particles are close to each other, Van der Waals forces arise between them. To check this yourself try putting together two polished glass plates (e.g. microscope slides) and lifting the upper one. The lower one seems to stick to the upper one. This is also how insects, geckos etc. stick say to vertical surfaces. Adhesion and friction are related properties.

 

[0xDEADBEEF]

You can actually weld stuff together without any glue or heat if the surface is very clean http://en.wikipedia.org/wiki/Optical_contact_bonding