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Help! I’m Trapped in a Drop of Water

I am looking at a blob of water. Not pond water. Not pool water. Just ordinary H2O floating about in what appears to be zero gravity. And inside its wetness there’s a passenger, a goldfish, a very alive goldfish …


… that is trying desperately to escape—or so it seems. It flings itself at the blob’s edge, pushing it outward.


Then it tries to get out the back. That’s its head peeping through.


Then it charges and stretches the skin of the bubble almost to the breaking point.


But try as it might, it can’t get free. Poor little fish, I thought. It’s a prisoner.

I found it, the fish and its floating prison, on my favorite fluid dynamics blog, FYFD, which is curated by aerospace engineer Nicole Sharp. She had posted the video version produced by Professor Mark Weislogel and his students at Portland State University in Oregon. They used a “drop tower”— basically, a free-falling elevator—to create near-weightless conditions. So they’re the ones who packed the goldfish into its droplet-y cage. Here it is in full “get me out of here” motion.

What a struggle! As Nicole put it on her blog:

For years, I have wondered what a fish swimming in microgravity would look like. Finally, my curiosity has been rewarded. Here is a sphere of water in microgravity, complete with a fish. Personally, I am impressed that, despite the fish’s best efforts, the surface tension of the water is strong enough to keep it confined. This may not bode well for microgravity swimming pools at space hotels.

Got it. Fifty years from now, when I book my room at the Hotel Galactica a hundred miles from Earth, I’m not bringing a swimsuit. And anyway, who flies to the edge of space to go for a swim? Not me. Not you. We’d go for the view—obviously. So this video shouldn’t have bothered me. But after I saw it once, then again, then yet again, something about it didn’t seem … um … quite right.

EXCUSE ME, but …

What, exactly, is holding that fish inside the blob? Could it be the water? Here on Earth, fish have no trouble breaking through a pond surface to snatch a fly or a bug. Yes, the pond surface is a little resistant due to molecular or surface tension, but a goldfish is stronger than water. It whips its tail, propels itself up, and grabs lunch. No problem.

Is there something about zero gravity that changes that? I called my friend Henry Reich, author/illustrator and physics explainer over at Minute Physics. I showed him the video and asked, Is this fish trying to escape? And if it is, why can’t it get free?

“This fish is thrashing, yes,” he told me on the phone, “but I have a hunch it isn’t trying to escape. I think it’s testing its surroundings …”

Me: What do you mean “testing”?
Henry Reich: Well, I can’t think of any good reason why the fish couldn’t break through, even though it’s at zero gravity.
Me: So the surface isn’t holding it in?
HR: No, I don’t think so, and if you look at its fins and tail, you can see what’s really going on. Up close, you’ll see it’s paddling up to the edge, pushing forward and then, just when it could escape, its peddling back.
Me: You can see that?
HR: Yeah, look back at the video, and watch the fins. And the curled back tail. That’s what you’ll see …

I did. And I’m not sure if I saw what Reich saw. He sent me a video of goldfish swimming backward, and I couldn’t quite tell if my fish was doing what that fish did.

Drawing by Robert Krulwich
Drawing by Robert Krulwich

But just to go along with his notion, I asked Reich: If our goldfish was able to burst out of the blob—if physics didn’t prevent it—what’s making it stay? Henry said, OK, remember that a) I am not a goldfish, and b) This is just a wild guess, but …

HR: I think it’s scared.
Me: Scared?
HR: Yes.
Me: Of what? It’s a goldfish.
HR: Of the strangeness of being in a bubble of water.

For millennia, he went on to say, fish have evolved in rivers, lakes, ponds, seas—places where the surface was always “up,” or above them. Fish have no experience with surfaces that are underneath them or to the left or right.

So here’s this poor fish that finds edges in all the wrong places. It’s encountering a world that’s totally strange, and it’s poking about, testing, and discovering—uh oh—an edge here, and, oh my, an edge here too?

Pardon the unpardonable anthropomorphism, but this fish is freaking out. “It’s thinking, This is weird,” says Reich.

Drawing by Robert Krulwich
Drawing by Robert Krulwich

So maybe the bubble is not caging our fish? I have another physicist pal, Aatish Bhatia (whom I play with over at Noticing.co, where we solve puzzles together). He suggested that it’s possible the blob of water is pushing back on our fish—at least a little. Water at microgravity likes to be sphere-shaped. I might resist being splatted and stretched because, says Aatish, “The ideal shape of a water drop would be round … It’s the most compact shape possible.” So the water might be pressing back at the fish’s thrashes, but, like Reich, Aatish says do not pity this fish.

It’s no prisoner. It can break free. And then, lo and behold, Aatish proved it!

“Born Free, Free as the Wind Blows”

On October 23, 2014, Weislogel published a YouTube video from Portland State University. It was a lecture he gave, and I don’t know how Aatish found this, but 27 minutes in, up there on the big screen, is our entombed fish, the very one I saw—but with a different ending!

Apparently, the video on the fluid dynamics site was chopped, and in real life, our fish escapes! It flings itself out of the water blob, and breaks through! Here’s the moment:


But Wait …

My heart leapt at the sight, until I thought, Wait a second, where did the fish go? It’s in an elevator dropping six floors at 55 miles an hour in a near weightless state heading to the ground floor of the engineering building. When the elevator lands this fish is going to, um, land with it. Did they retrieve it? Give it a medal? A martyr’s burial?

I called Weislogel.

Not to worry, he told me. The fish (not a goldfish, actually, but a neon tetra) belonged to his son’s friend. This was a high school experiment done on campus, and there were a number of fish involved, all of them on loan “from somebody’s aquarium.”

The one in that video, he said, was the “most curious” of the bunch. Many stayed stock still during weightlessness. Some curled. This one poked, probed, and, because it was so lively, it probably made several trips. It was his favorite.

OK, but—what about the landing?

“No fish was harmed in the making of this video,” Weislogel said in an announcer’s voice. When gravity kicked back in, both the blob of water and the fish settled softly (“like a baby in the womb kind of thing”) into a receiving bowl placed at the base of the chamber. Nobody died. All were, gently, returned to the aquarium.

Of course. I completely forgot this experiment was conducted in Portland, Oregon, the town that’s made unpleasantness illegal. If this story ever becomes a plotline on the TV show Portlandia, each fish will have its own monogrammed landing pillow.

I should never have worried.

Special thanks to Henry Reich, who was coming off a plane from Cambodia when he got my “Help, what’s this fish doing?” message and, before getting on his next plane, called to tell me his nutty—but, as it turned out, totally accurate – opinion of what was going on. And thanks also to Aatish Bhatia, who’s my partner over at the other blog, and the guy I go to to when I can’t figure someting out, because he always can (and would have found the Higgs boson in two minutes somewhere on the Internet, no need for an atom smasher, if he’d only been asked). He can find anything.

15 thoughts on “Help! I’m Trapped in a Drop of Water

  1. I wasn’t nearly as concerned about a fish being trapped as I was about his ability to breathe. What happens to dissolved oxygen in the water in zero-G … and since it wouldn’t take long to use up what little bit there is in a droplet, how does “new” 02 replenish that which is used up?

    1. Water absorbs oxygen and other gases even through the surface tension. Additionally the fish does not use the oxygen up that fast and fish such as gold fish and carp, etc. do not die from lack of oxygen as fast as some others. And finally, the fish in th bubble of water is not in long enough before it lands back in the catch basin of water.

  2. For some reason this article is missing the byline. I can tell by both style and drawing image credits who wrote it, but wanted to point out that it is missing.

    (And great story!)

  3. Beyond hydrogen bonding that keeps the water coalesced into a drop, could it also be the case that the zero G effect on the drop is as important as the behavior of the fish? In normal conditions water has weight, and the effort of a fish is rewarded by escaping the surface of a pond in part because gravity is “holding” the water down even as the fish moves against and out of it – in other words, the weight of water assists in breaking surface tension. Without gravity, the water would stick not only to itself but to the fish – following it and deforming (not breaking the surface tension which would quickly happen in normal gravity – you can see the water initially deform around the nose of the fish rather than break), which might help explain the initial lack of success. To me it looks like a Newtonian thing where the fish has to generate enough force to break the surface tension of the zero-G water. I think the delay is just the fish discovering/learning that it has to try really hard in order to do that.

  4. all the people calling it ‘zero-g’, gravity is still there, the elevator is in free fall. it’s like if you jumped off a plane and opened a bottle of water, a blob of water will fall with you to the ground.

    i’m curious about how they avoided harming the fish. did they gradually slow down the elevator? how did they keep a bowl of water at the bottom for the fish to land in? where does the fish go when it bursts out of the bubble (it would momentarily asphyxiate when it’s out of water)?

    1. By that definition, there is no such thing as “zero-g” anywhere in the universe. The term refers to zero effective gravitational force, which is the case here.
      There would be no need to “gradually slow down” the elevator. The bowl simply needs to sit on the floor of the moving car, and when it comes to rest, the fish will “fall” at exactly the same speed it would if it had been dropped from its current height above the floor of the elevator.

  5. But after “escaping” the water bubble, did the fish “swim” in the air due to the lack of gravity? Could he float like the astronauts or he just stood still until the experiment ended?

  6. How much emotions I had to control, reading this ariticle. I am glad, that the story ended well and nobody was hurt. Thank you very much for this new insights.
    There is no math nor physics who could have forseen these results. Experiments are still the most important part of sciences. That’s why I honor Raymond Davis jr. the most for his measurements concerning neutrinos. They oscillate !!! They have mass and they travel almost at the speed of light. They know time intervals. They are not trapped in their “neutrino-cloud”.
    Why did the goldfish not turn into a prince as he was leaving his water bubble ? What a surprise, If he could oscillat ! Everybody would be perplexed.

  7. Interesting, and I’m very glad if no fish were not harmed for the production. However, noting that a fish is “freaking out” is not at all anthropomorphic. Science has shown that fish suffer fear and pain. They are known to panic in extremely stressful situations. Rather, denying emotions to nonhuman animals is anthropocentric.

  8. Seriously? No animals were harmed in this experiment? What do you call a fish who is stressed out beyond belief? Oh. Excuse me, it’s just a goldfish. I’ve never read a more speciesist article than this one. You should be ashamed of yourselves for this dribble.

    1. I speak on behalf of the fish, and want you to know that the neon tetra (not a goldfish as you claimed, you speciesist) was very excited to have done its part to act in the interests of science and the furthering of universal knowledge. The fish also wants you to know that a little stress never hurt anyone, and that swimming through micro-gravity was easier than its clarinet recital last week.

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