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  #1  
Old 02-10-2010, 07:14 PM
Claude Remains Claude Remains is offline
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Bonding of perfectly flat surfaces

Many years ago I worked in an optical fabrication shop. My purpose was to slice man made boules of various crystals. I had full access to the departments down the line from me from milling and grinding all the way to the polish and coating areas. I was shown that two perfectly flat lenses would stick together upon contact and could not be separated without damaging both optics.

Questions are - what is this called? and can someone point me to some cites that will help me explain this to a friend who is calling B.S. on me?
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  #2  
Old 02-10-2010, 08:04 PM
Telperion Telperion is offline
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I've never heard of cold-welding glass, but many metals can be joined together solely through pressure. Normally this is done in a vacuum to minimize oxidation, however.
http://www.welding-advisers.com/Cold-welding.html
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  #3  
Old 02-10-2010, 08:17 PM
Bill Door Bill Door is online now
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I've put together gauge blocks like that in a machine shop. We called it wringing them together. It was due to the fact that the blocks were so flat that you could slide them together with no air between them. They were held together by ambient air pressure.
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Old 02-10-2010, 08:27 PM
JWT Kottekoe JWT Kottekoe is offline
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It's called optical contacting or optical contact bonding and, no, they are not held together by ambient air pressure, they are held together by intermolecular forces, commonly going under the name of Van der Waals forces.
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Old 02-10-2010, 08:36 PM
Crafter_Man Crafter_Man is offline
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Originally Posted by Bill Door View Post
I've put together gauge blocks like that in a machine shop. We called it wringing them together. It was due to the fact that the blocks were so flat that you could slide them together with no air between them. They were held together by ambient air pressure.
Here is a really cool photo (from Wiki) of 36 gauge blocks wrung together. The description says the photo was taken 103 years ago. I am amazed they had the precision machinery to manufacture these back then.

Last edited by Crafter_Man; 02-10-2010 at 08:41 PM..
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Old 02-10-2010, 08:50 PM
Claude Remains Claude Remains is offline
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Originally Posted by JWT Kottekoe View Post
It's called optical contacting or optical contact bonding and, no, they are not held together by ambient air pressure, they are held together by intermolecular forces, commonly going under the name of Van der Waals forces.
Thank you. So the optics DO become one? My friend is laughing at me and tying to disprove me by using pieces of broken glass here around the garage as proof that I am full of it. I keep telling him that it is on the molecular level and that common glass will not do the trick.
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  #7  
Old 02-10-2010, 09:02 PM
CalMeacham CalMeacham is offline
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I've heard of this, too. The problem with using ordinary glass is that it's not flat enugh -- nowhere near. You need optically flat glass which is perfect enough that you can see interference fringes. And both surface have to be scrupulously clean, as well.


I've never seen it myself, though. Most of the time, you can put an optical flat directly on a test piece without any fear of their bonding -- the surfaces probably aren't clean enough.
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  #8  
Old 02-10-2010, 09:10 PM
Claude Remains Claude Remains is offline
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I once tried to join the crystal fab shop. I spent day after day cutting plates on the lathe and trying to use diamond paste to make such sufaces. The microscope and the team leader both had nothing good to say about my skills.
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  #9  
Old 02-10-2010, 09:40 PM
Claude Remains Claude Remains is offline
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Originally Posted by CalMeacham View Post
the surfaces probably aren't clean enough.
I was up to my elbows in Methanol and acitone.
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  #10  
Old 02-10-2010, 09:54 PM
Claude Remains Claude Remains is offline
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Originally Posted by CalMeacham View Post
I've heard of this, too. The problem with using ordinary glass is that it's not flat enugh -- nowhere near. You need optically flat glass which is perfect enough that you can see interference fringes. And both surface have to be scrupulously clean, as well.


I've never seen it myself, though. Most of the time, you can put an optical flat directly on a test piece without any fear of their bonding -- the surfaces probably aren't clean enough.
Yes, optical flats fall short of perfection in that they are lying around for use as a measurement tool.
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  #11  
Old 02-10-2010, 11:09 PM
billfish678 billfish678 is offline
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I've personally had optical surfaces lock together. And depending on the circumstances it could be air pressure, some sort of large scale mechanical lock, or the Van Der Walls forces if the surfaces are really clean and precise.

Now, they probably arent "welded enough" together that its like they are just like one piece of glass. But it is bad enough you have a high risk of breaking stuff trying to get them apart and sometimes WILL break em doing so.

So, it really depends on how "stuck" is "stuck" to decide whether the OP is right or his friend is. I am leaning towards the OP being right, because I know you can get things stuck petty well.

And, in the amatuer telescope making/optics community the problem of things getting stuck is well known enough that people worry about.

I am pretty sure I have had pieces of regular plate glass stick together a fair bit as well. Not stuck good, but stuck enough it took some work to get em apart. Clean em and prep em right and bet you could get a good stick with plate glass as well.
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  #12  
Old 02-11-2010, 11:36 AM
Interconnected Series of Tubes Interconnected Series of Tubes is offline
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Originally Posted by Claude Remains View Post
Thank you. So the optics DO become one? My friend is laughing at me and tying to disprove me by using pieces of broken glass here around the garage as proof that I am full of it. I keep telling him that it is on the molecular level and that common glass will not do the trick.
To expand slightly on Van der Waals:

As you probably know, electrons don't really "orbit" around a nucleus. One can roughly approximate their behavior to instantaneously blipping in and out of existence in various locations around the positively charged center. We generally think of this as a probability distribution or electron cloud.

Now, at any one moment, all the electrons around a given nucleus are happy to stay within their defined orbital which we usually define as a shell within which we have a 90% chance of finding the given electron. However, those electrons might, due to sheer chance, momentarily 'exist' in an unequal distribution around the nucleus. Very briefly, electrons might pile up to one side of the nucleus. Being negatively charged, this inequality will - just for a moment - induce a dipole in the atom/molecule in question, giving it a positive side and a negative side.

This momentary dipole can induce other dipoles in the materials surrounding it, polarizing nearby molecules in the opposite direction. Opposites attract, and so simply due to the random motion of electrons, every molecule experiences an attractive force to other nearby molecules. This 'dispersion' force is usually swamped by other, more powerful effects like permanent dipoles or other electrostatic/polar interactions, but in some materials and at very close distances, the effects can be profound.

Additionally, by placing two very smooth plates together one can exclude air from the interface between them, resulting in what is essentially suction between the pieces.

To really see these interactions on a macroscopic scale, you need (as has been mentioned) two very smooth, very clean, very close surfaces. You can see a similar sort of effect with new microscope slides, although air pressure is playing the larger role here too.

I don't know which forces are dominating in your case or what kind of glass you're using, but your friend is, bluntly, wrong. Try looking at a piece of common glass under a microscope, if you have one available.
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  #13  
Old 02-11-2010, 01:53 PM
Washoe Washoe is offline
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Originally Posted by Interconnected Series of Tubes View Post
This momentary dipole can induce other dipoles in the materials surrounding it, polarizing nearby molecules in the opposite direction. Opposites attract, and so simply due to the random motion of electrons, every molecule experiences an attractive force to other nearby molecules.
Stupid question time—is this how Saran Wrap works?
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  #14  
Old 02-11-2010, 01:58 PM
Contrapuntal Contrapuntal is offline
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Originally Posted by Claude Remains View Post
I was up to my elbows in Methanol and acitone.
Sounds like something Hunter Thompson would say.
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  #15  
Old 02-11-2010, 02:03 PM
md2000 md2000 is offline
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Originally Posted by billfish678 View Post
And, in the amatuer telescope making/optics community the problem of things getting stuck is well known enough that people worry about.

I am pretty sure I have had pieces of regular plate glass stick together a fair bit as well. Not stuck good, but stuck enough it took some work to get em apart. Clean em and prep em right and bet you could get a good stick with plate glass as well.
Yeah, I almost did that to my 6-inch telescope blank when I was making a telescope mirror in college. After I rinsed off the grinding grit, I put it against the grinding blank to see how close they were; it took a while to get them apart - after which I remembered the warning NOT TO DO IT.

Of course, the problem is worse if they are wet; so I suspect water and surface tension had something to do with it too. I had been polishing down to extremely fine grit - so the two pieces fit very well together.

Imagine trying to tear apart two boards coated with velcro. Either (a) they bend slightly, so you only tear apart a bit at a time, or (b) They don't bend at all, so you need enough force to tear apart the whole lot at once (which is more likely to break something).

My guess had to do with precise fit and the inability of air to get between them. I could see the van der Waals maybe being a factor, but I have trouble imagining that being the entire explanation; it just does not seem to be a striong enough connection on its own. Try the trick with two unpolished big sheets of glass or arborite, or even plywood, and you will sometimes hear a "whoosh" sound as you lift the top one and the air rushes in.
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Old 02-11-2010, 02:23 PM
billfish678 billfish678 is offline
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Originally Posted by md2000 View Post
Yeah, I almost did that to my 6-inch telescope blank when I was making a telescope mirror in college. After I rinsed off the grinding grit, I put it against the grinding blank to see how close they were; it took a while to get them apart - after which I remembered the warning NOT TO DO IT.

Of course, the problem is worse if they are wet; so I suspect water and surface tension had something to do with it too. I had been polishing down to extremely fine grit - so the two pieces fit very well together.
Yep. Been there done that.

If you read the Amatuer Telescope Makers, Volumes I,II,III (a large collection of DIY articles on telescope making/design), there is more than one scary story of optics being worked being stuck together. That series is the BIBLE of handmade optics. If you had only one reference source for making optics to choose, that would probably be it.

I seem to recall one story where this guy got a mirror and tool stuck together quite well. He tried all kinds of shit to get em apart without breaking them. Can't remember if he finally succeeded or not. The editor had a good final solution. Throw the both of them against a sturdy fire hydrant, then walk away and start over

IIRC WHAT exactly what causes a mirror and tool to stick together has not been resolved to anyones satisfaction. Plenty of ideas, but hard to determine what the dominate factor really is.
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Old 02-11-2010, 05:40 PM
Spatial Rift 47 Spatial Rift 47 is offline
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Originally Posted by Washoe View Post
Stupid question time—is this how Saran Wrap works?
Yes and no. It's certainly present, but the other thing about saran wrap is it is extraordinarily light. When you pull it off the roll and rip it from the box, that removes some of the plastic's electrons just through ordinary friction. Since the wrap is so very light, that tiny electrical imbalance (note this is a permanent imbalance as opposed to Van der Waals' temporary dislocation) is enough to stick it to things. And, irritatingly, to itself.
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Old 02-11-2010, 06:13 PM
BrandonR BrandonR is offline
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Adhesion/Cohesion? Adhesion is part of the collective force that causes what we generally think of as friction, since it causes attractive forces between unlike molecules (cohesion is between similar molecules). If you have two flat surfaces where the atoms line perfectly with one another, it can cause uber-high values of friction between the surfaces (1010 higher than unaligned systems) in what becomes known as a commensurate system.
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Old 02-11-2010, 06:31 PM
PatriotGrrrl PatriotGrrrl is offline
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Originally Posted by CalMeacham View Post
I've heard of this, too. The problem with using ordinary glass is that it's not flat enugh -- nowhere near. You need optically flat glass which is perfect enough that you can see interference fringes. And both surface have to be scrupulously clean, as well.

I've never seen it myself, though. Most of the time, you can put an optical flat directly on a test piece without any fear of their bonding -- the surfaces probably aren't clean enough.
I've done it with some optical flats our college Physics Club had. They were about 3 or 4 inches in diameter and it was sometimes quite difficult to get them apart. I don't remember doing anything in particular to clean them. The interference fringes were cool.
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Old 02-11-2010, 06:53 PM
Interconnected Series of Tubes Interconnected Series of Tubes is offline
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Originally Posted by Spatial Rift 47 View Post
Yes and no. It's certainly present, but the other thing about saran wrap is it is extraordinarily light. When you pull it off the roll and rip it from the box, that removes some of the plastic's electrons just through ordinary friction. Since the wrap is so very light, that tiny electrical imbalance (note this is a permanent imbalance as opposed to Van der Waals' temporary dislocation) is enough to stick it to things. And, irritatingly, to itself.
It's worth (barely) mentioning that electrostatic attraction between charged molecules is actually a Van der Waals force. What I was describing was, in particular, "London" dispersion which is another Van der Waals attraction.

But yes, plastic wrap's stickiness is primarily due to the "static" cling with which we're all familiar.
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  #21  
Old 02-11-2010, 07:26 PM
Chronos Chronos is offline
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Nor do van der Waals forces actually require a perfectly smooth surface: A rough surface will work, too, provided that you can match the roughness of the surface precisely enough. That's how geckos can walk on walls: Their feet have oodles of tiny hair-like structures that match the shape of the wall precisely, and stick to it via van der Waals forces.
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  #22  
Old 01-10-2014, 07:33 AM
Mangetout Mangetout is online now
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Originally Posted by Claude Remains View Post
Thank you. So the optics DO become one? My friend is laughing at me and tying to disprove me by using pieces of broken glass here around the garage as proof that I am full of it. I keep telling him that it is on the molecular level and that common glass will not do the trick.
Quote:
Originally Posted by CalMeacham View Post
I've heard of this, too. The problem with using ordinary glass is that it's not flat enugh -- nowhere near. You need optically flat glass which is perfect enough that you can see interference fringes. And both surface have to be scrupulously clean, as well.
OK, but what if you try to reunite a piece of glass broken in two? Should not those two surfaces be a perfect fit for one another? (I imagine the answer is no, due perhaps to difficulty of alignment, or minute loss of material during the break)

Last edited by Mangetout; 01-10-2014 at 07:35 AM..
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  #23  
Old 01-10-2014, 08:01 AM
randompattern randompattern is offline
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They're not permanently bonded but, can't be lifted straight away from each other without twisting or sliding them apart. They would separate by doing this but would be damaged (scratched) in the process, which is bad for optics.
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  #24  
Old 01-10-2014, 08:14 AM
CalMeacham CalMeacham is offline
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Originally Posted by Mangetout View Post
OK, but what if you try to reunite a piece of glass broken in two? Should not those two surfaces be a perfect fit for one another? (I imagine the answer is no, due perhaps to difficulty of alignment, or minute loss of material during the break)
You'd think so, but I've never heard of this happening. I suspect bits of material falling off the broken sides and getting between the two halves would be enough to spoil it, along with random dirt that easily gets in there.

Incidentally, I've polished a LOT of flat surfaces to 0.3 micron grit, and never seen this effect of van der Waals bonding. I know I've heard of it, but never seen it myself.


I HAVE had a problem getting microscope slides apart. When you try to separate the ones boxed together, they frequently stick, and you can see interference fringes between adjacent ones. But they aren't van der waals sticking -- they can be separated with enough effort. And I'm pretty certain microscope slides aren't ground and polished to perfect flatness. I believe they used to be float glass, but I don't know if it's still done that way.
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