Why do wet plates stick together with a relatively high force?

Question

When I wash up plates and don't dry them right away and place on top of each other, they seem to stick together with a relatively high "force": i.e. when I try to pick the top plate hours later, the bottom plate sticks to the plate that I try to pick up to the point that it "beats" gravity.

• What is the name of this effect?

• Is it possible to calculate the strength of the binding force (i.e. per area of the plates?)

• Is there any research on how various types of liquids (not just water) affect this sticking force? (would oil provide better or worse sticking?)

• Is there an "optimal" thickness of the liquid between the two plates that maximizes the sticking?

Edit: As per the request in the comments, I attach a picture of the plates (regular porcelain plates).

PS: I did an experiment with cold water (as suggested in one of the answers below) and the plates still stick together very quickly. So it must be the water viscosity & surface tension & adhesion of water molecules (as per Niels's answer below), rather than "vacuum" seal created by cooling "hot" water).

Answer 1

This has turned out to be a very interesting question indeed. Based on all the good comments I will amend my answer here. There are (at least) two parts to this problem. One is why the plates stick together in the first place and the other is the exact mechanism by which the adhesion between them is broken when you pry them apart. My original answer dealt with what makes them stick together (the adhesion mechanism) but there's a lot of interesting physics having to do with what happens when you do pull hard enough to separate the plates. There are a number of simple and fun experiments that could be done to put some light on what's going on here. First, you could repeat the experiment with the plates immersed in water so any effects having to do with wet/dewet dynamics are eliminated. Second, you can do the experiment in air but with detergent-spiked water, to greatly reduce surface tension effects. Third, you could perform the experiment with transparent glass plates or better yet sheets of flat glass and record what happens with a digital camera as they are separated. I now return you to the regularly-scheduled program.

When a set of identical plates is stacked together, their adjacent top and bottom surfaces fit together well with very little gap space between them. If we try to pull them apart, it is easy for air to flow into the gap and hence allow that gap to grow, and for the plates to come apart.

If there is water in that gap, and we try to pull them apart, several things happen: first, we have to retract the thin film of water into a glob in the center of the plate stack, and the viscosity of the water resists that deformation.

Second, the adhesion that the water molecules have for the surfaces of the plates makes the water want to stay in contact with those surfaces and not get sucked back into a glob. It takes work to de-wet the plate surfaces, and so work must be applied in order to pull the plates apart.

In essence, the water acts as a (lousy) glue, but a good enough glue to illustrate several of the properties of a good glue: 1) it has to develop a high viscosity after being put into a gap, and 2) it has to completely wet the surfaces of the gap before its viscosity starts climbing.

Answer 2

The other answers here offer a lot of explanations for general cases in physics, but there is another factor that should be considered in your specific example: suction.

If you wash your dishes with hot water, then you will naturally heat up the plates in the process. After stacking your plates, the air trapped between them will absorb some of that heat as the plates settle, expand as per the ideal gas law, and push some of the air out into the surrounding atmosphere.

After several hours of cooling, the air trapped between the plates will shrink, exerting less pressure on the plates. At that point, the surrounding air will tend to push the plates together, similar to a suction cup.

The 'release' of the two plates is the same effect you see when opening a new jar of jam; the "popping" of the lid is caused by the spring action being released as air is able to re-pressurize the container. The low-pressure state is caused by packing the contents while hot, and allowing the cooling to "suck" the lid down, forming an air-tight seal which keeps the food fresh for years.

Notably, this is an easily testable theory: next time you wash your plates, run them under cold water for a short time before drying. Leave them for the same amount of time, then attempt to separate them. If the plates are easier to separate, then the suction from the shrinking air is a significant factor. If they are not, then the suction is not a significant factor (or the plates weren't cooled enough).

Answer 3

They don't stick together for all liquids

If you were to put a liquid with a large wetting angle (such as Hg which "beads" up), the plates will be pushed apart. For a liquid such as water that tends to spread on a plate, the forces become very large at small separations.

The force depends on the surface tension of the liquid, the wetting angle at the plate, the volume of the liquid, and the separation of the plates.

This can be worked out from capillary theory with the calculus of variations:

Here is a paper that shows the capillary force between two plates as a function of how well the liquid "wets" the plates: https://www.sciencedirect.com/science/article/abs/pii/0001616088903288

Answer 4

There is a very nice answer by Niels Nielsen, I feel I need to add some detail about the sticking mechanism:

1. The Van der Waals force is not only responsible for giving water molecules the ability to stick together, it is also responsible for giving water molecules the ability to stick to other surfaces. Contrary to popular belief, there are multiple types of Van der Waals forces. Now in your case the Van der Waals force (one type) gives the ability on one side to water molecules to stick to one of the plate's surfaces, and for water molecules to stick together (another type of the Van der Waals), and then on the other side to stick to the other plate's surface. And there you have it, the effect is like the two plates stick. The Van der Waals force is commonly accepted to be based on electromagnetism, so we are back to the EM force dominating over gravity at short distances. If you have a single wet plate, and hold it up, you can see that not all water falls off of it, the Van der Waals force keeps some of the water stick to the plate (you can see it commonly referred to as surface tension). The EM force dominates over gravity in this case.

of the molecule will orient itself with the extremely negative side of another molecule.

Explanation of van der Waals force

2. When you have two wet plates, and push them together, the water gets inbetween squished and the surface area increases extremely, and the Van der Waals force seems to be extremely strong against gravity, if you have two plastic plates, they might not even separate if you only hold the top one. Of course in your case, if you use heavy ceramic plates, the Van der Waals force becomes inferior and gravity takes over and the bottom plate separates from the top. By the way, this is how geckos stick to surfaces (including other electrostatic forces).

the gecko's amazing climbing ability depends on weak molecular attractive forces called van der Waals forces,

https://www.eurekalert.org/pub_releases/2002-08/lcc-sph082202.php

Although direct charge measurements clearly demonstrate that the contribution of CE-driven electrostatic interactions in gecko adhesion is dominant, it should be noted that, along with electrostatic forces, van der Waals (vdW) and capillary forces could also contribute to the measured adhesion forces [7,9,24];

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4233685/

You can see phrases where they use surface tension and it is correct. I just wanted to make sure that the underlying mechanism (Van der Waals) and the fundamental force (EM) at work is clear.

Answer 5

This process is apparently called wringing. From Wikipedia:

Because of their ultraflat surfaces, when wrung, gauge blocks adhere to each other tightly. Properly wrung blocks may withstand a 300 N (67 lbf) pull. While the exact mechanism that causes wringing is unknown, it is believed to be a combination of:

• 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 that 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. There is no magnetism involved.

Note that this theory applies to extremely flat surfaces ("The minimum conditions for wringability are a surface finish of 1 microinch (0.025 μm)").

In case of less perfect objects (e.g. dinner plates), the two first sources (air pressure & surface tension) will be the most significant.