What will happen if I try to pour a cup of water in zero gravity, into another empty cup? Will the water come out of the cup? The adhesive force between the water molecules and the interior of the cup should prevent the water from coming out. Is it correct? Or is there something more to to it?
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3$\begingroup$ check this video out youtube.com/watch?v=o8TssbmY-GM $\endgroup$– pentaneMar 14, 2015 at 15:54
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2$\begingroup$ This question is another reminder that this (excellent) site "is cut in two". Two stunningly different types of QA appear here. (About half each, I'd say.) The first half is truly difficult questions "inside" modern physics (whether about elementary particles, mathematical physics, or whatever). The second half are purely pedagogical QA regarding extremely basic high-school level Newtonian 'physics'. Of course, obviously and trivially, every expert on this site completely understands every aspect of (say) this question: the trick to answer quality is purely pedagogic excellence. $\endgroup$– FattieMar 15, 2015 at 3:16
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$\begingroup$ @JoeBlow "CompletelY" is such a strong word :-). Depending on what it comes in contact with, and various secondary factors, water in zero or "micro" gravity may be drawn around a contacting object by "surface tension" and form an outer skin. Not all "experts" commenting on this are going to be aware of this, and none of those commenting or answering here so far have noted it. jdlugosz came closest. $\endgroup$– Russell McMahonMar 15, 2015 at 15:34
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5 Answers
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If you simply held a cup upside down in zero gravity, the liquid ought not to pour out. However, things in zero gravity still obey Newton's laws. If you pull away the cup, the water ought to stay behind. In reality, a sudden move of the cup would create a lower pressure behind the water than in front so the air pressure would try to keep it in the cup, but the acceleration that this can achieve is finite so a rapid motion could "liberate" the water from the cup.
Alternately, if your cup is hemispherical, you could take the water out with a quick flick of the wrist - rotating the container with the liquid staying behind. You could then "scoop up" the liquid ball in space with the other cup.
A potential problem with doing this in a zero gravity environment: this is likely to get messy, and the one thing you prefer not to have in satellites (like the ISS) is "stuff" (little drops of water, conducting bits of broken pencils etc) floating around and getting into electronics. That said - electronics is pretty well protected these days, and the filtration system inside the ISS takes care of most what floats around; but when the filters get wet there is always a risk of mold growth...
In short- while gravity is absent, liquids still obey laws of physics. Inertia, surface tension, and atmospheric pressure continue to operate normally. The force of surface tension on a liquid in a cup is very small compared to the force of cabin pressure - for a given surface tension $\sigma$, radius $r$ and contact angle $\theta$ you would compute the force as
$$F = 2\pi r \sigma \sin\theta$$
With $\sigma$ = 0.07 N/m, $r$ = 5 cm and $\theta$ = 45° (ball park figures), you would have a force of < 0.02 N. That will hold the liquid in place only if no other forces act. If there is 200 ml of liquid in the cup (0.2 kg), - an acceleration of just 0.1 m/s$^2$ would be sufficient to "shake it out".
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$\begingroup$ It's easy to create a zero-gravity environment. Just toss the cup of water in the air. Then while it's in the air, just grab the cup away from the water. My son once asked me how things can float in space. I just demonstrated with car keys. I tossed them up and followed them with my hand. They "floated" above my hand :) $\endgroup$ Mar 14, 2015 at 14:33
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2$\begingroup$ @user3932000: No, there's really no such thing as zero gravity, unless you go far out in the universe, far away from any galaxies, etc. Aboard the ISS, there's plenty of gravity. The whole structure and everything in it is falling toward the earth. It just happens to be going so fast horizontally that as fast as it falls, the surface of the earth bends away because it's curved, so the ISS never gets any closer to the ground. They call it "micro gravity", but all that means is everything is falling together at the same time. $\endgroup$ Mar 14, 2015 at 16:39
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2$\begingroup$ @user3932000 Mike was responding to your objection to his free fall analogy; in fact, all "micro gravity" situations (ISS as well as parabolic flight) really only simulate zero gravity since everything is falling at the same rate. I think we are all agreeing here... $\endgroup$– FlorisMar 14, 2015 at 16:56
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1$\begingroup$ Mike -- a good way to explain that, is that, satellites are indeed precisely just like the "vomit comet" .. but they just go for longer. $\endgroup$– FattieMar 14, 2015 at 17:09
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2$\begingroup$ In regards to the 'messiness' of loose water droplets, there are several videos, including the one pentane linked to, of astronauts doing various things which scatter a few water droplets, and they do not seem concerned. $\endgroup$ Mar 14, 2015 at 20:59
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Pour? No such thing without gravity.
In NASA TV (see video), I saw the prototype coffee cups. They are shaped with a sharp crease, to allow liquid to ride up the groove. More advanced product would also mix waxy and wettable surfaces to keep it stuck to the inside of the cup but not crawl over the brim, except at the sip line.
The pictures are hard to figure out; watch the video or read an article that shows a series of pictures and diagrams.
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1$\begingroup$ This is the best and clearest answer. You simply cannot pour without gravity. The question is not unlike asking "What if I 'fall' in zero gravity ..." well, there is no falling. $\endgroup$– FattieMar 14, 2015 at 17:10
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1$\begingroup$ @JoeBlow Or falling is exactly what you're doing all the time. $\endgroup$– userMar 14, 2015 at 20:03
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$\begingroup$ @MichaelKjörling depending on where you are. If you are halfway between the Milky way and Andromeda, you are not falling at all. There is simply not enough gravity acting on you for you to feel anything. $\endgroup$– k_gMar 14, 2015 at 23:37
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$\begingroup$ Hi Michael -- that's true, but don't forget this QA is entirely pedagogical. Any expert here completely understands all the subtleties involved and insights like "you're falling even when standing on a planet!" are uninteresting. The entire pedagogical crux of the QA is for the OP (or other absolute-beginner readers) to realise that pouring entirely relates to "gravity" and if you're in a "no" gravity situation, no pouring. of course, you and I know that "zero gravity in orbit" is a simplification - but the lesson at hand is at that level. $\endgroup$– FattieMar 15, 2015 at 3:11
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Remember the laws of Newton. In this case the water will only accelerate with the forces you apply when tilting the cup. Assuming not fierce tilting of the cup: By the hydrogen interactions the water will therefore most like just float around shaped as one or more slightly deform bubbles in mid-air, or inside the cup depending on the "tilting forces".
answered Mar 14, 2015 at 13:44
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The adhesive force between the water molecules and the interior of the cup should...
Even in absence of adhesive force, the water will never move in 0-gravity, because there is no up nor down, no force is acting on it.
You can clearly see in this video at 1:15 that in order to get the water out of a plastic cup you have to tap it on the bottom
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Water couldnt stay in liquid form, because without gravity there wouldnt be pressure, and a liquid needs pressure to stay in liquid form
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1$\begingroup$ And how do you explain this: youtube.com/watch?v=s63JXdsL5LU ? $\endgroup$– MartinApr 1, 2016 at 12:17
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2$\begingroup$ That is not at all true for a closed container like a spacecraft. $\endgroup$– AsherApr 1, 2016 at 12:18