Why do super smooth surfaces have large friction?

Question

If we can't touch anything really because of repulsion forces of electron clouds then how can super smooth surfaces have Large Friction?- as objects will hover on each other.

Answer 1

If we can't touch anything

Ok, so what do you really mean by "touch"? Because...

because of repulsion forces of electron clouds

I'd say that this is pretty much the definition of "touch". When the particles on you start having interactions with the particles in that other thing... presto, you're touching it.

But what you're really asking is...

repulsion forces of electron

This is fundamentally wrong.

Yes, any single electron will repel another single electron. But atoms don't have single electrons, they have lots. And molecules often share electrons among their atoms. So it's much more complex. And on top of that, think about a hydrogen atom holding its electron... what happens when you bring another close to it? Well, the attractive force extends past the original electron and starts pulling on the other guy. So even with just two electrons on two atoms you can have attractive forces... ever notice H comes in H2?

What actually happens when you have lots of electrons bound to something is that the repel among each other. This causes them to have certain preferred locations. When they're in these locations they are not evenly distributed. When you put atoms together into molecules the patterns get even more complex.

So even though an atom or molecule is overall electrically neutral, the distribution of that charge definitely isn't. And when you have charges that are not evenly distributed in space, you get all sorts of secondary forces. Those can be repulsive or attractive, depending on the arrangement.

And that's why flat surfaces can have friction even though they're covered in electrons. Imagine a substance where you have layers where one layer has electrons on the left and the other on the right, so in a chunk of it you have stripes.

If you brought two of those together, they might repel if the "right" bits are aligned with each other, or they might bind if the "right" ones are aligned with the "left" bits of the other. In that case they will have a whole lot of friction. Such a thing exists, it's called salt. If you were to bring two perfectly flat pieces of salt together in a vacuum, they'll have essentially infinite friction.

A friend of mine did his grade 13 science fair project on this. He took two pieces of steel and milled them so flat they would stick to each other. Why? Because think of an electron around a iron atom... in this case the electrons at the outside are so far from the atom that when you place another one beside it, it's almost equally attracted to the atom beside it. So simply bringing metal atoms together will generally make them stick. That's (basically) why metals are so strong, and also why they make good conductors, because you only need a little push to move an electron from one atom to another.

Now practically, none of this has anything to do with real world friction. Here are the macroscopic level, what you call flat looks like the himalayas. You should google up electron microscope images of knife edges! And those bumps and ridges fill up with all sorts of crap, not the least of which is air. So when you bring them together, the actual reaction can be very difficult to predict.

So in that same science fair experiment, what he would do is get rid of all that stuff. How? By rubbing margarine on the metal. That pushed out all the air and water and dirt, leaving a really flat surface that stuck to both pieces of metal really strongly. That's basically all glue does too, it gets stuck inside those imperfections on both sides and then solidifies.

Answer 2

"If We Can't Touch anything really because of repulsion forces of electron clouds then, How can super smooth surfaces have Large Friction?"

In brief: you're forgetting the nuclei!

The surface layers of objects close together don't always repel. For example the electron clouds can become modified in shape and density to increase the probability of finding electrons in the region between the nuclei of atoms in the two surface layers, attracting both sets of nuclei towards this 'very negative' region, so we actually have an attractive force between the surfaces. This is really chemical bonding. The formation of such bonds in places where the surfaces touch is a reason for increased friction in some cases.

Note that the surfaces have to be very clean. Even so we rely on contact being at only a few points, so that local pressure is high and surface films are punctured.