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?

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.

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.

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.

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.

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.

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 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.

I was up to my elbows in Methanol and acitone.

Yes, optical flats fall short of perfection in that they are lying around for use as a measurement tool.

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.

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.

Stupid question time—is this how Saran Wrap works?

Sounds like something Hunter Thompson would say.

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.

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 :slight_smile:

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.

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.

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 (10[sup]10[/sup] higher than unaligned systems) in what becomes known as a commensurate system.

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.

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.