This is a bit of an abstract question so I'll try explain this as best I can from the bottom up.

I would like to know if it is possible to observe a sheet of fluid, much like a sheet of glass, falling completely flat and level.

Imagine a volume of water that is at rest on some sort of very, very fast trap door, which once opened, would allow the volume of water to retain its shape and fall through the trap door.

Is this possible if it were done in a room containing a near-perfect vacuum to discount drag effects? would things like surface tension stop this happening? perhaps using a super-fluid?


  • $\begingroup$ If you could build an "ejector" similar to laminar flow nozzles, once a flat sheet of fluid is falling freely in a vaccuum, not touching any walls, there's no differential force to cause the sheet to change shape. $\endgroup$ – Carl Witthoft Jul 22 '14 at 11:29
  • $\begingroup$ Related physics.stackexchange.com/q/82009 $\endgroup$ – Bernhard Jul 22 '14 at 12:32
  • $\begingroup$ This would be a good experiment to try in micro-gravity. I think the only significant force would be surface tension, which you could minimize by a) making it large, or b) making it soapy :), or c) putting a solid ring around it. $\endgroup$ – Mike Dunlavey Jul 22 '14 at 18:16

EDIT: replaced "fluid" with "liquid", thanks to Kyle.

I am not aware of any material with a liquid phase in near-vacuum. Probably, the liquid would evaporate and maybe a part of it freezes solid due to evaporation cooling.

EDIT: NeuroFuzzy pointed to a youtube video containing an ionic liquid, which is able to retain liquidity in very near vacuum. What would happen, if you prepared such a liquid in sheet form and dropped it in vacuum? I think, that the strong cohesive forces would pull the liquid together, so that the sheet is transformed into a droplet. However, this would take some time, and it may be that the shape, which splashes on the ground, very well resembles the original sheet.

If you supply enough pressure for a liquid phase to exist, there will be two forces:

  1. Drag from the surrounding medium
  2. Cohesive force between fluid molecules

Together they will probably effect formation of isolated droplets.

  • 1
    $\begingroup$ Ahh, ok. So it was a misconception on my part, for which I want to apologise. I am a native German speaker and we only have the term "flüssig", which means both liquid and fluid. I was not aware of a distinction and assumed that fluid is the same as liquid. $\endgroup$ – M.Herzkamp Jul 22 '14 at 14:08
  • $\begingroup$ But, to answer the original question: can you agree that only liquids can form "sheets" in absence of confinement? $\endgroup$ – M.Herzkamp Jul 22 '14 at 14:10
  • $\begingroup$ In the absence of other forces, a fluid will retain its shape. $\endgroup$ – Kyle Kanos Jul 22 '14 at 14:21
  • $\begingroup$ A blob of gas in space will only retain it's shape when it has reached equilibrium, e.g. a radially symmetric density distribution such that the outward pressure is cancelled by gravitation. But I can't imagine a sheet-like density distribution in equilibrium. $\endgroup$ – M.Herzkamp Jul 22 '14 at 14:30
  • $\begingroup$ I should have added an extra condition: and it's not moving. In this case, then $\partial\rho/\partial t=0$ (i.e., the volume is constant so it retains its shape). If there are motions and forces, it will deform. $\endgroup$ – Kyle Kanos Jul 22 '14 at 14:45

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