I read that:
If you take a rough surface and make it smooth, the coefficient of friction decreases. But if you make it super smooth, then the coefficient of friction increases. How come?
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$\begingroup$ What do you mean by smooth and soft? Soft is not smooth to me, while smooth is not soft. A plastic surface could be really smooth and have little friction. Some kind of fabric could be soft, and have more friction. $\endgroup$– RuudCommented Oct 18, 2011 at 14:03
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$\begingroup$ @RuudvA: Sorry, I didn't mean to differentiate soft & smooth. I've changed soft to smooth. $\endgroup$– clawsCommented Oct 18, 2011 at 14:13
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3 Answers
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What do you mean by super smooth?
I remember a note (again) by Feynmann that said if you made your test surface so clean that there is absolutely no dirt or impurities on it, then the super clean surface would actually attach to anything sliding on it, making the apparent friction coefficient higher.
Is this what you mean by super smooth?
In that case, take a copper plate. It is ideally made of copper atoms. Its surface will be dirty, filled with other molecules.
If you now imagine that you have the tool to clean it so well that just the copper atoms are on the surface and nothing else, you will actually get a very reactive - in the chemical sense - surface. "Naked" atoms will bind to anything that passes by, and if you try to make something slide over it, they will make bounds and stick very well.
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$\begingroup$ yes! yes! Thats exactly what I mean. Could you explain why? $\endgroup$– clawsCommented Oct 18, 2011 at 14:58
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4$\begingroup$ Take a copper plate. It is ideally made of copper atoms. Its surface will be dirty, filled with other molecules. If you now imagine that you have the tool to clean it so well that just the copper atoms are on the surface and nothing else, you will actually get a very reactive - in the chemical sense - surface. "Naked" atoms will bind to anything that passes by, and if you try to make something slide over it, they will make bounds and stick very well. $\endgroup$ Commented Oct 18, 2011 at 15:26
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1$\begingroup$ @Jolow: you should promote that second comment into an edit of your answer. $\endgroup$ Commented Oct 18, 2011 at 23:52
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2$\begingroup$ they say it as "cold welding".... $\endgroup$ Commented Oct 19, 2011 at 4:44
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1$\begingroup$ - cold welding - if you somehow mill two pieces of metal perfectly along some known crystal plane, then put them together, how would you later tell where they were joined? - You can't - which means you have 'cold welded' the metals. In the real world you don't get that to happen full on, but in vacuum it happens enough to prove a bother when building satellites, etc. $\endgroup$ Commented Oct 19, 2011 at 13:37
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If I think about friction geometrically, i.e. ignore for now chemical interactions, (although they are very important) you get an argument about the surface area in contact. If you take two flat smooth surfaces, they will contact along a plane. If you roughen up, one or more of the surfaces, contact will only take place at high spots, i.e. there will be a lot of gaps where there is no contact, so the area in contact is much lower. Conversely the forces/stresses on the much smaller area that is in contact will be higher.
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There is chemical reason behind it. Interlocking is the reason for the friction between rough surfaces. In case of super smooth surfaces, interlocking can't take place.
Chemically, when two super smooth surfaces comes in contact with each other, their chemical structures disturbs.
Then their outer surface shells get merged into one another so they oppose relative motion, hence friction increases.
