# Two sheets of glass stick together. Why?

• I
• RubinLicht
In summary: Apart from the summary, in summary, the conversation discusses the force required to pull apart two sheets of glass that are pressed together strongly. The force is determined by the atmospheric pressure and the area of the sheets, but adding water between the sheets can make it significantly harder to pull them apart due to the elimination of air gaps and adhesion forces between the molecules of the glass. There is also a debate about the role of gravity in this scenario, with some arguing that it is a significant force while others believe it is negligible.
RubinLicht
When the sheets are very smooth and are pressed together strongly, I guess they could be viewed somewhat as a unit, so the force to pull them apart would be approximately equal to or less than the atmospheric pressure times the area of the sheets, but definitely not more than, is this correct?

Now if you add a few drops of water between the sheets of glass, it becomes significantly harder to pull apart. I can see two reason for this, please tell me if I'm missing anything or if something I say is wrong.
1. Eliminates any air gaps, and so decreases the pressure between the plates of glass
2. Adhesion forces, for smaller distances, the glass might actually pull together because of the adhesion forces.

RubinLicht said:
When the sheets are very smooth and are pressed together strongly, I guess they could be viewed somewhat as a unit, so the force to pull them apart would be approximately equal to or less than the atmospheric pressure times the area of the sheets, but definitely not more than, is this correct?
I'm not sure the upper limit is that high with just dry glass. For very precisely ground Jo Blocks on the other hand it might be that high.

Now if you add a few drops of water between the sheets of glass, it becomes significantly harder to pull apart. I can see two reason for this, please tell me if I'm missing anything or if something I say is wrong.
1. Eliminates any air gaps, and so decreases the pressure between the plates of glass
2. Adhesion forces, for smaller distances, the glass might actually pull together because of the adhesion forces.
1 is definitely true and I THINK 2 is also true but don't know that for a fact.

Just a thought for what it's worth. I expect it would be gravity... That is the gravity produced by every atom in each of the two thin sheets. With each sheet being pressed to the other they would tend to draw themselves even closer... Obviously not the same as if they were all in one sheet but they might not know to what extent they are not in one sheet.. :) ?

photo_guy said:
Just a thought for what it's worth. I expect it would be gravity... That is the gravity produced by every atom in each of the two thin sheets. With each sheet being pressed to the other they would tend to draw themselves even closer... Obviously not the same as if they were all in one sheet but they might not know to what extent they are not in one sheet.. :) ?

Er... try estimating the gravitational force between the two plates. Do you really think it can be as strong as what has been described? And do you think this gravitational force that you are subscribing to changes significantly when you moisten the surface that it causes the force to separate it to change by THAT much?

Zz.

PeterDonis
photo_guy said:
Just a thought for what it's worth. I expect it would be gravity... That is the gravity produced by every atom in each of the two thin sheets. With each sheet being pressed to the other they would tend to draw themselves even closer... Obviously not the same as if they were all in one sheet but they might not know to what extent they are not in one sheet.. :) ?
The force of gravity between two sheets of glass is approximately zero so no, I don't think so.

EDIT: I see ZZ beat me to it.

Ok... I understand water eliminating air gaps... But what exactly are adhesion forces..? Glass is not my idea of an adhesive...

photo_guy said:
Ok... I understand water eliminating air gaps... But what exactly are adhesion forces..? Glass is not my idea of an adhesive...

photo_guy said:
Ok... I understand water eliminating air gaps... But what exactly are adhesion forces..? Glass is not my idea of an adhesive...

Air pressure!

Zz.

ZapperZ said:
Air pressure!
I disagree. Air pressure is specifically a DIFFERENT force than adhesion in this discussion.

Is that not gravity..? What other force is there..? Atoms produce gravity. Molecules are made of atoms...
I just think we underestimate gravity... It's pretty strong, balancing and holding the cosmos together as it does...
Not to disagree if there is another force of course...

I w
phinds said:
I disagree. Air pressure is specifically a DIFFERENT force than adhesion in this discussion.
I was thinking air pressure as well, but will look at the search that you suggested. Could you compare the two contributions in magnitude? (Yes, I'm being lazy here...)

photo_guy said:
Is that not gravity..? What other force is there..? Atoms produce gravity. Molecules are made of atoms...
I just think we underestimate gravity... It's pretty strong, balancing and holding the cosmos together as it does...
Not to disagree if there is another force of course...
No, in the context of this discussion, gravity is utterly trivial and you vastly OVERestimate it. And you have not answered ZZ's request that you do the math.

photo_guy said:
Is that not gravity..? What other force is there..? Atoms produce gravity. Molecules are made of atoms...
I just think we underestimate gravity... It's pretty strong, balancing and holding the cosmos together as it does...
Not to disagree if there is another force of course...

This is where you need to adhere to the principle that physics is MORE than just saying "what goes up, must come down". It must also say "when and where it comes down", meaning that there must be a QUANTITATIVE ASPECT of what you are saying.

I asked you to make an estimate of what the gravitational force is between two glass plates. Did you attempted doing that before continuing to insist that it is gravity? This is something elementary that one can do, and it will save you from having to keep on insisting this line of thought.

And, just in case you are not aware of it, the electromagnetic forces are the most dominant, as in occurrences, force in our everyday lives. The atoms in your body are held together into a solid NOT by gravity!

Zz.

conscience and berkeman
berkeman said:
I w

I was thinking air pressure as well, but will look at the search that you suggested. Could you compare the two contributions in magnitude? (Yes, I'm being lazy here...)
As far as I'm aware air pressure is WAY more of a contributing factor but I have read (sorry, no citation) that molecular attraction is measurable for something like extremely precisely milled jo blocks.

EDIT: ZZ beat me to it again !

berkeman
Well I'm never able to do math to disprove myself... It never seems to come out right... :)
Others can do the math to disprove me and I may see where I went wrong...
I'm not insisting it's gravity... It was a thought... And I didn't argue air pressure or the addition of water.., or even molecular adhesion...
Although air pressure is only effective when there is less on the other side... Water takes away that possibility... And molecular adhesion seems strikingly similar to gravity... And I did look it up in Google... It didn't say much more than I already knew...

I offered a thought... Who's is wrong on the strength of gravity remains to be seen afai'm concerned... Cause we're in our own particular inertial frame, at rest, and basically balanced out with the cosmos... We may well be underestimating it's cosmic wide strength... Just another thought...

I see two glass surfaces with a flat plane of differentiation between sheets of atoms but basically in the same mass.. Holding themselves together pretty much as they do in each single sheet... Singly or constituting molecules... ?

P.S. I understand EMF... I wasn't referring to how atoms hold themselves together...

photo_guy said:
Well I'm never able to do math to disprove myself... It never seems to come out right... :)
Then you should ask for our help in showing you the right equations to understand the magnitude of the forces involved. We are happy to show you the relevant equations to use and the values of the constants involved. We can then check your math to be sure you got the answers correct.
photo_guy said:
Others can do the math to disprove me and I may see where I went wrong...
No, we do not have the obligation to disprove anything. A a poster, you have the obligation to prove any claims that you are making. And you must prove them using mainstream science.
photo_guy said:
Although air pressure is only effective when there is less on the other side... Water takes away that possibility.
No, this is incorrect. Without water between the sheets, it is easier for air to "leak" into the space between them. With water between the sheets, it is harder for air (and the resulting link to the surrounding air pressure) to make it between the sheets.
photo_guy said:
I offered a thought... Who's is wrong on the strength of gravity remains to be seen afai'm concerned... Cause we're in our own particular inertial frame, at rest, and basically balanced out with the cosmos... We may well be underestimating it's cosmic wide strength... Just another thought...
This is complete nonsense, and is not allowed here.

Please re-read the PF rules (under INFO at the top of the page). We do not allow nonsense and non-mainstream posts here. The PF is a very valuable place to learn about physics and the real world. And that is true precisely because we do not allow nonsense posts here.

Last edited:
Thread re-opened for mainstream science discussion and questions.

photo_guy said:
Well I'm never able to do math to disprove myself... It never seems to come out right... :)
Others can do the math to disprove me and I may see where I went wrong...
I'

I think i recall approximately that the force of an infinite charged plane on a charge L away from the plane is the same as the force of a sphere of radius L/2 on the point, if the point is right on the surface of the sphere. It should be trivial to extend this to mass, since the gravity and electrical forces are pretty much analogous. So I'll be using this to approximate the force of gravity one sheet on the other...

Note that the gravitational attraction by a sphere is equivalent to that of an equivalent mass concentrated at the center of the sphere.

very very rough diagram
glass layer 1 --------------------------------------------------------------
center of equivalent sphere -----------x
glass layer 2 ----------------------------(dm)----------------------------

m = mass of a glass layer
d= distance between layer one and two
where I'll find the force on a mass element dm and then just sum over entire sheet of glass.

the force on the mass dm is just
dF= (G*dm*m)/(d/2)^2 - from the equivalent sphere
which if you integrated over the area of the glass slab, you would get approximately
F= (G*m*m)/(d/2)^2,
"approximately", because at the edge, a mass element is really only attracted to half of the apparently "infinite plane", so this is an overestimate. Honestly, I'm fairly sure that if you multiplied the answer I'm about to get by several orders of magnitude, the force would still be really small, but anyways I'll plug in some numbers.

d=1mm = 0.001 m (thickness of a thin thin sheet of glass = 1 mm)
m = (density of glass) (volume) = 2500 (N/m^3) * (0.1m)^2 (0.001m) = 0.025 kg (for 10cm x 10cm glass)
G = 10^(-11) (roughly)

F= 10^(-11) * (0.025)^2 / (0.0005)^2 = a whopping 2.5*10^(-9) Newtons of force to pull apart two 10cm x 10cm sheets of glass.

Even if I used some dubious estimations here, mind that all of them were purposefully made to be favorable to your theory, just to prove my point further. It's still off by at least 9 orders of magnitude. So no, not gravitational forces for sure.

I apologize for any sloppy math and estimations I did, this is the best I can do with my current meager high school education :p

but going back to the original question, the two most relevant forces are then just adhesion (much smaller), and air pressure, right?

RubinLicht said:
but going back to the original question, the two most relevant forces are then just adhesion (much smaller), and air pressure, right?
As far as I'm aware, that's correct. Adhesion is highly variable, depending as it does on (1) the smoothness of the surfaces and (2) the material they are mad of, and of course air pressure can be seriously affected by adding a liquid in the gap (as well as the smoothness of the materials).

RubinLicht said:
but going back to the original question, the two most relevant forces are then just adhesion (much smaller), and air pressure, right?
The average pressure at sea level is about 105 Pa in other words about 105 N on each square metre. From that it's easy to calculate the air pressure force acting on the plates.

Size of the glass has an effect.
Smaller less adhesion and less air pressure.
Big sheets of glass or boards tend to stick together more.
Had a similar discussion about fridge magnets I remember the extra sticking effect was strongly refuted as the size was said to be too small to have an effect on even the flexible type.
Still not sure if it's true or not.

## 1. Why do two sheets of glass sometimes stick together?

Two sheets of glass can stick together due to the phenomenon of adhesion. This occurs when two surfaces with similar chemical properties come into contact and form a bond. In the case of glass, the smooth and flat surfaces can easily adhere to one another, creating a strong bond.

## 2. Does the type of glass affect how easily they stick together?

Yes, the type of glass can affect how easily they stick together. For example, tempered glass, which is strengthened through a heating and cooling process, is less likely to stick together compared to regular glass. This is because tempered glass has a more uneven surface, making it less prone to adhesion.

## 3. Can environmental factors play a role in two sheets of glass sticking together?

Yes, environmental factors such as humidity and temperature can affect the adhesion of two sheets of glass. High levels of humidity or extreme temperatures can cause the glass to expand or contract, making it more likely to stick together. Additionally, dust or other particles on the surface of the glass can also contribute to adhesion.

## 4. Is there a way to prevent two sheets of glass from sticking together?

Yes, there are various methods to prevent two sheets of glass from sticking together. One way is to apply a thin layer of lubricant, such as oil or wax, on the surfaces that come into contact. This creates a barrier between the two surfaces, preventing adhesion. Additionally, storing the glass in a dry and cool environment can also help prevent sticking.

## 5. Does the age of the glass affect how easily they stick together?

Yes, the age of the glass can affect how easily they stick together. Over time, glass can develop microscopic cracks or imperfections on its surface, making it more likely to adhere to another surface. This is especially true for older, more fragile glass.

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