Suction Cup effect - theory explained?

Can someone explain me the suction cup effect?

I’ve printed already parts which were affected by the “suction cup” effect. I know how to arrange a part to avoid it.

But I don’t understand why it should be vacuum or a suction effect. From my point of view when a cup is moved straight in to a liquid with the opening faced down, the air in the cup is compressed and can’t escape. The entraped air should be compressed and create an overpressure and not vacuum. Or do I overlook something?

That’s a good question, and I don’t have a direct response, other than it has something to do with the liquid itself acting as “a seal”. Take a water glass and put it on a flat counter open end down. You can pick it up no problem, but put a layer of water and put the glass down, it instantly forms a seal and lifting the glass requires more force.

Maybe someone well versed in hydrodynamics can explain all this, because I’m curious too.

Sorry what is a flat counter, a flat surface?

I can only experience a resistance when lifting up a glass, if I put the glass angled into a liquid. This way I can see bubbles, the air is escaping from the glass. Now when I lift up the glass, water is also escaping due to gravity, the air is not compressed anymore and there is less air pressure inside of the glass compared to atmospheric pressure. But again this is only when the glass is moved angled into a liquid, not straight.

A smooth flat surface like the ones you find in your kitchen. When I say flat, I mean it should not be warped, and it should be horizontal. This way, when you put some water on it, it just pools, doesn’t run off.

I’ve tried that it really works, there is a noticeable pop off sound.

I would guess it’s because when lifting up the glass the sealing liquid is moved also a little up. This way there is more space for the air inside the glass, the air can expand, the pressure inside goes down causing a vacuum effect.

However I’m not sure that this is the same what is happening on the machine, since I cannot feel any force when lifting up a glass out of a silicone or other container, filled with water.

Don’t know. As I said, maybe someone who understands hydrodynamics can enlighten us.

My thoughts on the effect. Tl;dr: Surface tension.

When you move a glass into a body of water, the water forms a seal around the glass opening due to surface tension. It’s possible that some of the air inside of the glass is being compressed, but I think it’s a lot more likely that the volume of air is still the same. Maybe the water is able to generate a weak vacuum or weak pressure, but I’m not sure it actually matters.

Now, when you attempt to lift the glass, you have the additional force of surface tension pulling the glass down. When the glass is pulled far enough away, the seal rapidly breaks, and causes some fun fluild dynamics to occur with both the water and air. As for the popping sound, anything that moves fast enough in a fluid will release energy, sometimes as sound.

Anyways, that’s my $0.02 on the matter.

I agree that surface tension could play a role on sealing, but I don’t think that it generates a noticeable force against lifting.
I’ve tested lifting up a glass moved straight into water, there is as good as no resistance.

So I’m still wondering what causes the holes in printed parts with cupping effect, vacuum, pressure, surface tension?