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We have a stack of thin (0.025in) stainless steel plates (round 2in)that seem to stick together. Surfaces are clean of oil/grease, and when we lift the top piece off the stack with a suction cup, the following plate, sometimes more, stick to the plate being lifted. Why does this happen?

Mark Valenti
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3 Answers3

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The phenomenon you speak of is actually routinely used to test the flatness of precision gauge blocks used in metrology: their ability to "wring" together (this is the term used) both confirms their flatness specifications and is useful for storage, when a very light film of oil enhances the wring - and thus keeps corrosive environments away from the calibrated flat surfaces when the latter are not in use.

According to the Wikipedia article on Gauge Blocks, under the heading "Wringing", the effect is thought to be a combination of

1. Molecular Attraction

as in ACuriousMind's comment:

You are perhaps observing accidental cold welding

(See the Wikipedia Page on "Cold Welding") or, as Richard Feynman put it, when the interfaces between like materials are very flat, there is "no way for each atom to know which block it is in", so the bond across the interface gets more and more like the normal molecular forces holding the lattice together the flatter the interface;

2 Suction

i.e. the blocks have expelled air from between them and air pressure helps force them together, as in BowlOfRed's Comment:

if it's very flat and very clean, you've created a suction cup. No air can get in, so air pressure holds them together. This is especially likely if you can slide them easily

and

3 Surface Tension

From any water vapor that gets caught between the blocks.

Selene Routley
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You may be observing van der Waals force, an electrostatic attraction between uncharged materials caused by temporary shifts of electrons on one surface which cause electron shifts on an adjacent surface.

These shifts of electron clouds to one side of their molecules are of extremely short duration (10^-14 seconds). They may be caused by vibrations in the molecular bonds, or simply by random fluctuations, but by creating electron-rich and electron-poor areas, the shifts may cause electrons on adjacent surfaces to oscillate in unison. Temporary dipoles align on the two surfaces, and you get electrostatic attraction. The closer the surfaces, the more effective the van der Waals force. (Here's a more complete explanation: http://antoine.frostburg.edu/chem/senese/101/liquids/faq/h-bonding-vs-london-forces.shtml)

Thus, you might be getting a temporary weak polarization of dipoles on the surface of each disk. Here is a link that includes a way to quantify the attraction per unit surface area: https://galileo.seas.harvard.edu/images/material/335/115/Israelachvili_ch11.pdf.

The question is whether two stainless steel disks are susceptible to this. Other materials adhere to stainless steel via the van der Waals force, so it may be possible. However, the van der Waals force is very weak, and the mass of the disks, even at .025 inch thickness, might be too great for the surface areas to adhere. It's been calculated that adhesive pressure between two planar surfaces 10nm apart in air or a vacuum is 0.05 atm. Closer distances result in much greater adhesion (see 2nd link above, p. 179).

Ernie
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  • Yeah, but Van der Waals is very weak. Geckos have an insane number of hairs on their feet to increase the surface area. Do you really think two pieces of smooth metal could be held together by this? (Honest question) Just seems like the surface area involved would be way too small to be noticeable. – CoilKid Jun 08 '15 at 23:06
  • @CoilKid: True, the van der Waals force is very weak. I'm going to do a calculation of adhesive force between the two surfaces, assuming there is a near vacuum between them (BowlofRed may have a point, too), and I'll edit the answer. – Ernie Jun 08 '15 at 23:17
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wow the thing is I am working currently on very smooth metal parts, very small and light (Ø12, as thin as paper, C75) not flat but stackable, which seem somehow to stick together to a degree that is a problem for my customer. this sticking is easily eliminated, just hitting slightly on the package seem to loosen the parts, but the customer doesn't want to have to do anything. I have checked several batches but can't find any item where I can see this effect. the parts are dryed and then fed through a vibrating bowl made of plastic to be stacked on rods. I was wondering if some of these phenomenon affect my parts and generate this attraction force ? or do my parts get magnetised through rubbing against plastic in the bowl?

XtL_BW
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