Mylar balloon midair

October 1, 2024 4:11 PM

Is it possible to have a mylar balloon, pillow shaped, stay absolutely still about 5 feet off the ground in a closed room, with no air drafts. I don't want it floating around. A proper mix of helium and regular air should work, right.

Not long term either, say an hour at most.

posted by Czjewel to Science & Nature (16 answers total)

Yes in theory of course, but in practice it's very tough. Doing it by mixing air and helium is the super hard way. How are you planning on mixing them or adjusting? Best way is to fill and float it, then attach a weight that barely sinks it, then trim some weight until it floats how you want. True neutral is hard to get, the tiniest fraction off will tend to send it up or down over the course of an hour. As will the smallest puffs of air jostle it. Eg if you open a door into that room, the eddy currents propagating around will move the balloon. So really you're just going for it moving as slowly as possible; true "motionless" is nearly impossible in the real world.
posted by SaltySalticid at 4:16 PM on October 1

Is it possible to have a mylar balloon, pillow shaped, stay absolutely still about 5 feet off the ground in a closed room[?]

No, not in a regular room. There are too many complex factors in play.

You might get it to work in a custom built air tight room in a lab, but it's not going to happen anywhere else.
posted by Tell Me No Lies at 4:43 PM on October 1

You specifically stated you don't want it "floating around." So, just tether the balloon to the floor and two walls (three degrees of freedom, three tethers). Piece o' cake!
posted by SPrintF at 4:54 PM on October 1

Jeff Koons's One Ball Total Equilibrium is kind of like that, but I've heard Koons used a trick: salt water is heavier than pure water, so if you fill a tank halfway with salt water, then gently add pure water to fill it up, you can float something between the layers.
posted by ectabo at 6:57 PM on October 1

ectabo has the right idea. Fill the room 50% with Sulfur Hexafluoride, and a Mylar pillow filled with air will float nicely at the boundary between SF6 and the air above.

Now you have to figure out how to keep it moving side-to-side.
posted by Hatashran at 7:05 PM on October 1

Absolutely still? Not likely. But there was (still is?) and exhibit at the Warhol museum in Pittsburgh that's a room full of pillow-shaped mylar balloons floating all over the place, between the floor and the ceiling. They did get moved around by gusts of HVAC, people walking around, etc.
posted by cooker girl at 7:23 PM on October 1

This should be easily accomplished if the balloon can have a ribbon, the kind you use to hold onto the balloon. This is based on my experience of having old, partially deflated Mylar balloons float around at chest level for days, dragging their ribbons on the ground. You should be able to achieve the effect easily enough by partially inflating the balloon, attaching some ribbons, and then trimming until you’ve achieved the proper weight.
posted by Winnie the Proust at 7:42 PM on October 1

The ribbon trick should work particularly well if at least one ribbon is floor length. If the balloon sinks a bit, more of the ribbon will start resting on the floor and taking weight off the balloon till everything finds its equilibrium. If it rises, the reverse will happen.
posted by How much is that froggie in the window at 8:11 PM on October 1

Maybe by using sound, perhaps ultrasonics, you can actively position or trap the balloon in one spot. Sort of the way laser tweezers work.
posted by Sophont at 1:40 AM on October 2

Maybe by using sound

Nope. Sound disperses in air almost totally unlike laser light, and while you can kind of focus the sound front to hit the balloon, there will be sound waves that go elsewhere to bounce off walls, after which some of that will hit the balloon a bit later, with different delays. So your control system has to be able to deal with that.
posted by Stoneshop at 2:45 AM on October 2

The trick described by ectablo might be done with a layer of warm air over a layer of cold air.
posted by SemiSalt at 4:35 AM on October 2

Back in the day, places that sold inflated mylar balloons featured a product that was a pillow-shaped mylar balloon with a set of small removable adhesive squares attached. The squares worked as ballast to keep the balloon from drifting to the ceiling. You simply removed enough squares to get the balloon hovering at whatever height you wished. Like five feet, for instance.

The holding still part will be difficult. The whole point of the balloon I mentioned was to wander around the place during a party as if it were a guest.

Even an empty room will have slight currents brought on by temperature variations.
posted by Thorzdad at 5:10 AM on October 2

just tether the balloon to the floor and two walls (three degrees of freedom, three tethers).

A balloon filled with ordinary air, no helium, tethered to the ceiling with a single line of spider silk might achieve the visual effect you're after.
posted by flabdablet at 6:50 AM on October 2

flabdablet: tethered to the ceiling with a single line of spider silk

That would still allow the balloon to swing on its tether with any disturbance; it would take three tethers attached to the ceiling, and one to the floor, to keep it positioned. Theoretically you would only need three tethers, not four, but that would involve spider silk with a zero stretch coefficient and a totally rigid balloon.

(mumble mumble spherical cow of uniform density mumble)
posted by Stoneshop at 9:28 AM on October 2

It would indeed, but the conditions described in the question specifically posited no drafts. I would expect the equilibrium between the weight of the mylar and the tension in the spider silk to do an adequate job of keeping the balloon in place once the sloshing air currents caused by the last human to leave the room had thermally degraded.

Of course I could be wrong and of course more spider silk would be the obvious fix. Pretty sure you would in fact need only three, though; there would be no need for a downward-pulling tether as the weight of the mylar would keep all three upward-oriented strands in tension. The balloon would then come to rest at a position that put the horizontal components of those tensions in equilibrium.
posted by flabdablet at 5:53 PM on October 2

On further reflection, a balloon suspended off a single spider silk in the middle of a closed room would act as a pretty sensitive detector for the presence of thermal gradients across that room, as any such gradient would inevitably create small convective air currents. So if the aim is imperceptible balloon motion, you'd probably want the room to be part of a structure with really well controlled interior temperatures.

I would expect single-thread suspension to make for a much more sensitive thermal air current detector than triple-thread.
posted by flabdablet at 6:45 PM on October 2