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For a story I am drafting I would like to have droplets of fluid that form another shape than the regular domed droplets we usually see. Is there any other shape such as e.g. a doughnut-shape that droplets could naturally form given the right circumstances?

Answers are rated by the coolnessTM of the achieved shape, as well as by the simplicity of achieving that shape:

  • The more different the achieved shape is from the standard domed droplets, the cooler
  • The fewer conditions that need to be fulfilled for a fluid to form into that shape, the better
  • The less hurtful the conditions under which a fluid forms into that shape is for humans, the better
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  • $\begingroup$ Had you ever played with the hot wax droplets of a candle? $\endgroup$ – jean May 30 '18 at 19:16
  • $\begingroup$ How long do you need them to maintain their cool shape? Ordinary water droplets actually don't form the classic "teardrop" shape; if they fall long enough they eventually settle down to spheres, but on the way there they oscillate back and forth in some very complex shapes. $\endgroup$ – Securiger Aug 16 '18 at 11:58
  • $\begingroup$ @Securiger long enough so I can somehow use it in a story $\endgroup$ – dot_Sp0T Aug 16 '18 at 12:00
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Perhaps ferrofluids in a magnetic field could achieve something close to what you're looking for?

A ferrofluid is a liquid that becomes strongly magnetized in the presence of a magnetic field. In such a state, it tends to shape itself to the magnetic field; extruding small spike-like shapes all along its surface. A single droplet in the presence of a strong magnetic can be split into smaller droplets that are arranged in a pattern.

Check out this video (or others, there's plenty) for a sense of how drops of ferrofluid can behave. No torus-shapes, unfortunately. https://www.youtube.com/watch?v=G5wtdb174eg

Magnetic fields are not harmful to humans, barring interactions with pacemakers/hearing implants, etc.

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    $\begingroup$ Can you edit this to contain an explanation of what ferofluids are, instead of relying on a link. We expect answers to stand on their own without needing to rely on external sources to remain unchanged. $\endgroup$ – sphennings May 30 '18 at 18:32
  • $\begingroup$ I've added an explanation of why ferrofluids are relevant to the question. I think the link is still helpful, though. Should I remove it anyway? $\endgroup$ – Qami May 30 '18 at 18:53
  • $\begingroup$ There's nothing wrong with having links. It's just that links should be to supplemental material and not essential to the question. $\endgroup$ – sphennings May 30 '18 at 18:56
  • $\begingroup$ @qami copying the data in helps in case the links go stale. But keeping the link is fine. $\endgroup$ – SRM - Reinstate Monica May 30 '18 at 19:40
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Droplets of liquid-phase long-chain polymer with unusually strong Hydrogen Bonding behaviour could form almost any shape you wanted given the right molecular base shape. They should tend towards angular polyhedral forms though.

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    $\begingroup$ Wouldn't long polymers form "droplets" like threads of viscous goo? I am thinking of drooly dogs, or maple syrup, or drooly dogs with mouths full of maple syrup. $\endgroup$ – Willk May 30 '18 at 19:05
  • $\begingroup$ @Willk Not if the hydrogen bonds gave them enough surface tension, they'd wrap around each other and form all sorts of odd shapes as they pseudo-bonded together. $\endgroup$ – Ash May 30 '18 at 19:10
  • $\begingroup$ Sounds cool, do you have any photos or links to examples? $\endgroup$ – realityChemist Aug 14 '18 at 0:11
  • $\begingroup$ @realityChemist A couple of months ago when I put this answer together maybe but nothing I can point you at off the top of my head now no sorry. $\endgroup$ – Ash Aug 14 '18 at 14:43
  • $\begingroup$ @Ash Ah well, that's ok! When I've got some free time maybe I'll see what I can dig up $\endgroup$ – realityChemist Aug 14 '18 at 14:45
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What about star-shaped droplets?

Leidenfrost drops in water

These are called Leidenfrost droplets, and they form under certain conditions when you drip a liquid onto a hot surface. The droplets pictured above - star shapes with between 2 and 13 points - are water drops on a small, 350 Celsius metal dish. The hot surface forms a thin layer of vapor under the droplet, and instabilities in the film are amplified to form these patterns.

For more details about how this works, see the paper by X. Ma and J.C. Burton on the topic (that's where I got this image). If you want to see some videos, check out the lab's YouTube channel.

Safe for Humans?

You bet! You could almost make these guys on an electric stove, although it probably wouldn't get quite hot enough. A propane flame should work though. Even if you had one of these going and accidentally spilled it on yourself, it probably$*$ wouldn't burn you, as the vapor layer insulates the droplet and keeps it from getting hot enough to boil. And you can do it with pretty much any liquid, given the right conditions, so water is a good, non-toxic bet.


$*$The hot metal dish would totally burn you, though. If anyone is planning to try this at home make sure you conduct the experiment safely, and take precautions to avoid burn and ignition hazards.

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This is not particularly cool, but say your droplet is somewhat electrically conductive, and each molecule is polar or can have a charge.

You put it in a vertical electric field, say with the negative charge on top.

Then the droplet will get a positive charge on top and a negative charge on the bottom. It ought to stretch to an elliptical shape. And if it's then subject to some shear forces it might split into two charged droplets. The positive one will head upward and the negative one will head downward, unless they attract each other enough before they get some distance away.

Say you have a lot of these droplets. Look at one of them in the middle. It gets a net attraction by all those above it, and a net attraction by all those below. A droplet on top gets a net attraction downward, and one on the bottom gets attracted up. So they tend to stay together and not just spread out indefinitely.

Something vaguely along these lines might happen in actual water clouds on earth. They say that for thunderstorms it takes as lot of vertical movement of ice downward and water droplets upward, but still electrical effects could change the shape of water droplets as part of it.

And shear forces might give some interesting temporary shapes.

Say that you had loud sound waves coming from different directions. Each sound wave is a sequence of moving high-pressure and low-pressure areas. With one of them you could get a spherical droplet getting squeezed and stretched in one axis, at some frequency. With another sound from another direction the droplet gets squeezed and stretched in a different direction, at different frequency. At resonant frequencies for the droplet, energy builds up....

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Turns out all you need is water and an atmosphere full of some interesting nanoparticles. Apparently, self-assembling nanoparticles can be applied to the surface of a droplet of water. If the droplet's shape then changes, additional nanoparticles affix themselves to the new surface area, preventing the droplet from returning to its ordinary form. This currently has only been proven to work when the water droplet is suspended in silicone oil containing the nanoparticles, but with some tweaks, an entire atmosphere could be filled with similar nanoparticles, suspended in the air (I have no idea how such an atmosphere would form, but it is undoubtedly possible).

An external field is applied to a nanoparticle coated water droplet, allowing additional nanoparticles to coat the new surface area, which prevents the water from returning to its ordinary shape

Apparently, this can even result in the formation of tubes, meaning that toruses are almost certainly possible. An atmosphere full of such nanoparticles1 would cause water droplets to retain whatever shape they originally formed with, and any changes to the shape would be retained (within reason). Water would behave in this way over the entire planet, and this behaviour would make the planet a rather... interesting... place to live (assuming one could live there, see footnote). I could imagine water-sculpting contests involving magnifying glasses.


1: Please note that the effects of such nanoparticles in an atmosphere on the survival of life has not been tested in any way and that there could be detrimental effects, up to or including death.

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Phase change

You need a special fluid that changes phase at droplet-forming conditions (so, if you're at one Earth atmosphere and zero degrees Celsius, water would be good).

Next, you need a second mechanism whereby the phase change is slowed down something fierce; an organic compound with very high latent heat would be good. Water already has a high latent heat of liquefaction, but the more the better.

The fluid must have a very rigid crystalline form. Not mandatory but this allows more coolness to form.

Actually, any liquid-to-solid phase change will do, not just freezing. Maybe air exposure is enough.

However, the fluid deposits on a surface in domed drops and starts cooling. Phase transition kicks in, gradually from the outside inwards, creating a core of liquid under greater pressure. As a result, the fluid starts spurting out.

It can do so gradually, and your drops will sport a single spike, or it fits and starts - and you get a sort of snowman, with a domed drop with another drop on it, and another, and another, maybe stacked vertically or wobbling every which way.

Coagulation

Same as above, but you get a higher gross factor. The "solid" coagulated phase undergoes a partial colliquation some time later, so the drop:

  • starts as a normal liquid drop
  • becomes opaque
  • after some minutes, it starts shrinking and splits open in two to six symmetrical segments and sprouts as many smaller bubbles. The original drop is now a shriveled "hub" hidden by two to six smaller droplets. With large enough drops to begin with, the process can iterate fractally.
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The shape of a droplet is dictated by the equilibrium between the surface tension and the surface adhesion forces.

You can use a suitable frame to "trick" surface forces into a different equilibrium, like you can do with soap bubbles.

cubic soap bubble

or you can change the surface properties to interact differently with the liquid.

superhydrophobic surface.

In particular, you can generate suitable patterns of hydrophobic and hydrophilic areas to generate complex shapes

droplet pattern

Toroid droplets seems to show shrinking instability, so maybe they won't last enough for your usage.

But if you put a hydrophobic coated object into a suitable volume of liquid, you may get something resembling a donut

enter image description here

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Use a tone generator and a wave driver.

Substances such as sand and water can be placed on top of a plate attached to a wave driver and a tone generator. The frequency of the tone will shape the substance into a series of unique geometric patterns, becoming more complex as the tone increases.

Photo

I've included a video for an example. https://www.youtube.com/watch?v=wvJAgrUBF4w

Another video but weird and possibly fake. https://www.youtube.com/watch?v=6y5jRRPprgE

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  • $\begingroup$ Answers that are mostly or entirely links to other websites are generally discouraged here as links can become invalid. If you included some pictures of the shapes of droplets you can achieve with a tone generator and wave driver, your answer would be much improved. $\endgroup$ – walrus Aug 16 '18 at 11:18
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    $\begingroup$ Updated per recommendations. $\endgroup$ – Clay Deitas Aug 16 '18 at 11:45
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    $\begingroup$ Now that i see what you mean, two problems come to mind: 1) Sand is not a fluid 2) The patterns generated by the driver cannot really be called droplets As such, I'm not really sure that this actually answers the question. $\endgroup$ – walrus Aug 16 '18 at 12:00
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    $\begingroup$ I think it would still work on water, but sand is a lot more common for these displays. You could produce some effects with a single droplet, but it wouldn't be as pronounced. I'm not sure if the question is defining droplet as a small amount of liquid, or a liquid that is bound within a field do to viscosity, but the latter definition would work a lot better for my answer. $\endgroup$ – Clay Deitas Aug 16 '18 at 12:57
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Antibubbles.

falling bubble http://www.pulsedburst.com

A rising air bubble in water is a familiar but still very cool thing. The forces governing the changing shape of a rising bubble are complicated. As seen here a rising bubble can take on a large, medusoid bowl-like shape, with the trailing edge shedding small bubble-lets. They grow as they rise and can split into multiples of similar smaller bubbles.

Your fictional droplet will move like a bubble, but downwards. The droplets will fall at the same speed bubbles rise, wobbling and shedding tiny droplets as they do.

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