Why won't two objects stick when put tightly together?

[lirkepirk]

Hey, I was just thinking a bit about vacuum and air pressure and suddenly came over something really simple I cannot explain.

I know that it is the external air pressure that makes two halves of a hollow sphere containing vacuum stick together like in this picture: http://www.radicalart.info/physics/vacuum/guericke/GuerickeHorses-S.jpg

But, what if the two halves were made of solid metal? Like, if I put a book on my desk, so that there is no air between them, why won't the pressure from the atmosphere make it stick? Why is it necessary with a vacuum?
As far as I know, there should not be any additional pressure other than the normal force due to gravity , coming from the table, pushing against the atmospheric pressure, so this is bothering me a bit :/

Thanks!

 

[rumborak]

Funny, just today I watched a Youtube video about "wringing" of gauge blocks. The metal blocks are so perfectly flat that indeed, when you put them together the air can not get between them and they stick together.

Which I think answers your question. It's all about the impurities. Those impurities create enough "channels" between the two objects in order to let the air in.

 

[lirkepirk]

That is interesting! I was trying some gauge block a while ago, but they did not have to be perfectly clean in order to work.
Wikipedia also says this, implying that air pressure is (probably) not the biggest factor:

• Air pressure applies pressure between the blocks because the air is squeezed out of the joint.
• Surface tension from oil and water vapor that is present between the blocks.
• Molecular attraction occurs when two very flat surfaces are brought into contact. This force causes gauge blocks to adhere even without surface lubricants, and in a vacuum.

It is believed that the last two sources are the most significant.But why would a plunger stick when you create a vacuum inside it by pressing it against something, but not something else (say a plastic bag) when doing the same?

 

[rumborak]

You can do it with a plastic bag! Just not as easily, the seal will be broken much more easily because the bag deforms much more.

 

[A.T.]

But, what if the two halves were made of solid metal?

http://en.wikipedia.org/wiki/Cold_welding

 

[DaveC426913]

But why would a plunger stick when you create a vacuum inside it by pressing it against something, but not something else (say a plastic bag) when doing the same?

A plastic bag is not rigid. Thus, when you pull one part of it out (which increases the internal volume and lowers the pressure), another part will simply collapse, keeping the internal volume (and pressure) constant.

 

[russ_watters]

Which I think answers your question. It's all about the impurities. Those impurities create enough "channels" between the two objects in order to let the air in.

Right: a book is neither flat enough, nor well enough sealed, nor is there a way to remove all the air from under it just from setting it on a table.

 

[lirkepirk]

Thanks for the answers! The examples with the book and the plastic bag were just to create pictures of what I was talking about - the thing is, more generally, that I cannot think of any two objects that will stick together that way. Not even two smooth surfaces made of roughly the same material as a plunger (tight and fairly rigid). Also, I can for example suck the air out of a bottle and make it stick to my skin, but I cannot push a piece of plastic or glass to my skin and make it stick the same way? Shouldnt the seal be the same? Or is it because that with a bottle, the surface (bottle opening, O-shape) pushed against the skin is smaller, and given the same force, creates a tighter seal?

But regarding the book: I guess there should be some contact points between a book and the table where there is no air present. Given the net-surface where air pressure can push is greater above the book than between the table top and the book, shouldn't objects stick, if ever so sligthly, when put together?

And again, thanks for answering. I can appreciate why I don't see this effect so often, I am just trying to build up some understanding of it :)

 

[bhobba]

that I cannot think of any two objects that will stick together that way.

You must remember solidity actually has its basis in QM. When objects 'touch' really what's going on is Van Der Waals forces initially attract them, but because of the Pauli Exclusion principle it can't penetrate the electron cloud so it's pushed back (technically its called electron degeneracy pressure). When those two forces balance objects 'touch':


If they were to stick together somehow that Van Der Waals force must be so large its hard to pull apart - some kind of chemical bond maybe?

Thanks
Bill

 

[Andy Resnick]

Thanks for the answers! The examples with the book and the plastic bag were just to create pictures of what I was talking about - the thing is, more generally, that I cannot think of any two objects that will stick together that way.

Microscope coverslips (thin glass) are notorious for sticking together in just this way.

 

[greypilgrim]

I once disassembled an old hard drive. If I put two of the disks together, they can very easily be moved against each other parallelly to the surface, but stick to some extent if I try to take them apart vertically.

However I'm not sure if this is due to the air pressure or molecular forces.

 

[A.T.]

 

[DaveC426913]

However I'm not sure if this is due to the air pressure or molecular forces.

Air pressure.

 

[CWatters]

Try spreading a large plastic bag on a smooth flat surface and then picking it up quickly from the middle.

 

[Svein]

About 40 years ago I automated a combination of metalworking machines. The system was supposed to pick up a large steel plate, put it on a roller table, push it against a stop, punch some holes in it and then feed it into a roller press in order to create a cylinder. What stumped me for a while was the fact that my system seemed unable to pick up just one plate at a time - they stuck together as if glued. The reason was (of course) that these heavy plates were very flat and they had been packed tightly into crates and then transported quite a long way. Thus, every scrap of air had been squeezed out - and since the plates were very smooth, there was almost no opening for air to enter the the stack.

The solution was according to physics - I had a large electromagnet made and put it outside the far end of the plate stack. When it was time to pick up a plate, I energized the electromagnet which in its turn magnetized the steel plates - all of them in the same direction - which caused the upper plate to jump a couple of millimeters into the air. Problem solved.