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  • Regularly once a month or so I've had an email saying "Have you come across gauge blocks?

  • Do you know what gauge blocks are? You know they stick together? How do they stick together?

  • Do they do..what's going on there? Is it due to a vacuum? Is it due to grease? Is it due to something else?"

  • (Brady) So, so Phil what are gauge blocks? I don't even know you talking about.

  • Great, so I had very little knowledge of what a gauge block was until actually this morning. When I did, I decided

  • I'd finally take a look, and go down to the workshop and talk to our wonderfully talented technicians.

  • And they have got a number of boxes of these gauge blocks.

  • So these are like measurement standards. Very precisely machined to be a certain number of millimeters.

  • So the surfaces are incredibly smooth, incredibly flat. Machined to be really, really flat and

  • what you can do is you can take two gauge blocks and just by pushing them together get them stick. No glue, no

  • jiggery-pokery, no magnetism. They'll just stick. (Brady) And what are they made of?

  • Just pure steel, which seems quite magical and so I want to try and attempt to do this.

  • It's a bit of a skill sometimes it seems to work sometimes it seems to not work.

  • I am sure the Brady has ever will leave in all the attempts that don't work.

  • But let's let's see. What I'm going to do first of all is just just put a little bit of acetone on

  • And clean off any crud that's on

  • On the smooth face. That's relatively clean now, and let's clean this one off. (Brady) You're drying it so there's no acetone left?

  • No, there's no acetone left now. I'm using the acetone to just take off any crud that's on there.

  • No, I don't think this is gonna work that time. No that time didn't.

  • No! So the way you're meant to do it is actually twist around to 90 degrees, but I've never

  • nearly!

  • I think I can get that to work this right before. There we . . .[expletive]

  • We'll just clean it again.

  • (Brady) Technicians know of the trick?

  • Yeah, no technicians use it all the time

  • It's not just a trick, it's

  • a way of stacking them up. And the reason why

  • it was introduced like this is because they didn't want to have a whole set of different calibration gauges

  • and this makes it much more straightforward to just add them up--if you can get it a bloody work!

  • Almost . . .

  • there we go! (Brady) Keep still! Keep still for me.

  • (Brady) So they're like stuck there, Phil? (Phil) They're stuck together, yep, and you know, force of gravity doesn't care. So that's you know,

  • it's a fairly hefty block and it's being held on there against the force of gravity. [Laughter]

  • What makes it happen? So that's been a question now people have--I'm getting quite okay with it now--

  • That's a question that people have asked a lot, an awful

  • lot over the last century, a 120 years or something like that, as to what the heck is going on.

  • So it's an effect that you see

  • when you have very smooth surfaces. Now, it just doesn't have to be steel.

  • Over here

  • I've got a couple of silicon wafers. These are very smooth as well, and what I'm going to do is

  • I'm going to put one on top of the other and apply a bit of pressure,

  • hopefully without cracking that.

  • Here we go.

  • Okay same idea.

  • (Brady) What's going on there? Two very smooth surfaces?

  • (Phil) Two very smooth surfaces. So there are number of different things. First of all, everything around us to a greater or lesser extent is covered

  • with a thin film of water.

  • Depends on how

  • hydrophobic--how much it doesn't like water--or hydrophilic a surface is. To a certain extent all

  • surfaces were covered with a molecularly thin film of water.

  • So that's one aspect of this on a molecular level that acts as a sort of glue, but even beyond that what's remarkable,

  • great

  • aspect of, how will we put it--the statistics of quantum physics in action.

  • But in this case the silicon wafers for example have got a thin film of oxide that's relatively hydrophilic. It likes water.

  • Similarly with the metal here,

  • relatively hydrophilic, quite likes water. However in the case of the silicon, what you can do is you can take a silicon wafer,

  • dip it into hydroflouric acid,

  • strip off all the oxide, and end up with a very hydrophobic surface, so as soon as you put water on it,

  • it beads up.

  • Those will still bond. And the reason they still bond, and these will still bond in vacuum even when you remove the water, the

  • reason they still bond is over here you've got fluctuating electrons, over here

  • you've got fluctuating electrons, and it's something called the van der Waals effect. In any given instance of time, there's a little dipole.

  • There's a little imbalance of charge so that you've got a region

  • which is slightly more positive compared to another region.

  • And those dipoles interact and snap things together so a whole area in the semiconductor industry called wafer bonding based on this.

  • (Brady) It's not like one's positive and one's negative? (Phil) No, no, absolutely not. It's exactly the same effect happens in atoms. So xenon,

  • for example,

  • you can condense that to form a xenon solid, and that's not because electrons are being shared,

  • or electrons are being transferred as in a covalent or an ionic bond. What's happening is it's this van der Waals effect.

  • It's, it's because there are fluctuations of the electrons, which

  • means that it's not like you've added a positive charge. It means that if the

  • fluctuations of electrons then this bit slightly got more electrons over here than it is over here,

  • so there's a tiny, tiny dipole. And it's those dipoles that give rise to this, this force.

  • Even though there's no transfer of charge, even though there's no sharing of electrons,

  • it isn't an ionic bond. It isn't a covalent bond. It's called the van der Waals force.

  • It's exceptionally important.

  • And that's what's certainly a contributor alongside any liquid film that might be there to what's holding these things together.

  • So you might be wondering why doesn't it sort of work in the opposite direction as well?

  • Why doesn't it correlate so that they just fly apart in terms of the way the charges line up?

  • So a key thing that can happen is that the dipole that forms here, on that instant moment of time, it can induce, it can

  • influence the electrons over here and includes them in such a way so that the net attraction that that happens there.

  • (Brady) Why did the surface have to be so smooth? Why don't you and I stick to each other?

  • (Phil) So that's a really good question. So you need those smooth because you want those--if you have asperities there, if you have sharp bits,

  • then what happens is your forces don't add up in the right way.

  • You don't get the interactions. Moreover, if the liquid film is important, the fact that you've got a

  • very rough surface means that that water film that liquid film can't act as a glue in the same way.

  • In some state called an entangled state

  • That's all we need to go into you

  • Send one particle to one end of the universe if the universes ends the other particle to the other end of the universe you make

  • A measurement on this one and here's the weird bit the bit, but course causes so many physicists so many sleepless night

Regularly once a month or so I've had an email saying "Have you come across gauge blocks?

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Gauge Blocks (Van der Waals forces) - Sixty Symbols

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    林宜悉 posted on 2020/03/30
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