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Consider this soy sauce bottle:

common soy sauce bottle, tipped and pouring soy sauce freely

The cap has two holes that are directly opposite to one another. Covering one hole, I find the soy sauce almost doesn't pour at all.

However, consider a bottle with a single hole. Wouldn't the contents easily pour out? A teapot has a single hole and it pours easily.

It seems there's a contradiction. How does this really work?

Do teapots have holes within them already to have smooth flow?

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  • $\begingroup$ I don't understand your question. Gravity pulls the sauce out. It seems obvious that if the sauce can flow in the direction of gravity, then it would pour easily. What exactly are you asking? $\endgroup$
    – Allure
    Commented Apr 13, 2023 at 6:37
  • $\begingroup$ When I block the top hole the sauce doesn't flow out easily. $\endgroup$
    – James Chadwick
    Commented Apr 13, 2023 at 7:20
  • $\begingroup$ Sounds like an air pressure effect, then. $\endgroup$
    – Allure
    Commented Apr 13, 2023 at 7:27
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    $\begingroup$ A tea pot does not have a single hole. The lid is not a perfect seal. $\endgroup$
    – Mooing Duck
    Commented Apr 13, 2023 at 19:42
  • $\begingroup$ @MooingDuck: I've encountered some teapot lids that actually do make a good seal, but those ones also have a small hole in the lid. $\endgroup$
    – Michael Seifert
    Commented Apr 14, 2023 at 11:29

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You need an air intake to level the pressure, which would otherwise drop and stop the outflow if you don't replace the lost volume of fluid in the container by air. (And btw. the tea pot sitting on my desk actually has a second hole, a small air intake in its lid.)

If there is only a single hole, for example with a bottle, you can still pour it out. Either you tilt it only so much that above the flowing liquid there is still some free space for the air to get in: https://www.youtube.com/watch?v=cCPoPqEcK5Y or instead of a continuous stream you get one where from time to time air bubbles have to get through, sort of interrupting the flow: https://www.youtube.com/shorts/EQ0HE1dA3So (Try this at home with a clear bottle.)

With your soy sauce bottle I recon that the holes are smaller than in a regular bottle, and maybe surface forces/capillary forces will start to play a role. Which will make it harder for air bubbles to get in. And then if you cover one hole and don't tilt the bottle too much, the hydrostatic pressure may be too small to overcome this effect, nothing comes out...

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    $\begingroup$ youtu.be/2IKZIWVHXvo?t=256 shows the air bubble thing when pouring $\endgroup$
    – Mooing Duck
    Commented Apr 13, 2023 at 19:39
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    $\begingroup$ Also, some bottles (e.g., the ones used for Tabasco sauce) are designed with a single tiny hole that does not let anything out at all, until you give the bottle a shake, which allows a bit of air into the bottle so a bit of liquid can get out. $\endgroup$
    – plasticinsect
    Commented Apr 13, 2023 at 20:29
  • $\begingroup$ The pressure in the bottle drops, but that is not what is keeping the liquid in. It can not get out because it can not push against the outside air pressure. If there is lower pressure inside the bottle touching the surface, it is clearly easier to go there. The air hole is there so the outside air pressure helps push the liquid out - it is actually pretty high, compared to the pressure from the mass of the liquid. $\endgroup$
    – Volker Siegel
    Commented Apr 14, 2023 at 5:03
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    $\begingroup$ @Volker, well, saying that the pressure on the inside is too low or the pressure on the outside is too high is really just two sides of the same medal, right? It's the same pressure difference counteracting the hydrostatic pressure in the fluid column caused by the height difference from the free surface inside to the opening where stuff pours out... $\endgroup$
    – kricheli
    Commented Apr 14, 2023 at 7:24
  • $\begingroup$ @kricheli In terms of actual physics, I think it is important that the low pressure inside does not somehow keep the liquid in, like trying to suck it in. The air inside tries to push the liquid out pretty hard actually. Just not as hard as the external pressure pushes back. The pressures on both sides are very high compared to the difference that is relevant for the balance. $\endgroup$
    – Volker Siegel
    Commented Apr 14, 2023 at 20:10

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