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Superfluid helium-4 is an extremely slippery substance, which is why I came upon the idea of weaponizing it. One of the planets in my setting is rich in helium, so the weaponization of the element would make sense realistically. The grenade would either leak the superfluid out, or it would detonate and spread it all over the nearby area.

Oncoming enemies would have to concentrate on not slipping, which would leave them vulnerable to oncoming fire from the person and his allies who tossed the grenade. It could also be used to stop individuals who are attempting to flee, by tossing it in their path so they'll slip and fall once they step in the puddle.

My setting has advanced technology like combat exoskeletons and such, but on-the-ground troops who move on foot are still commonplace, so the act of using a superfluid grenade to impair their movement would be useful.

EDIT: My story takes place on Earth for the most-part, so the physics of said grenade would apply to Earth's physics/temperatures.

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    $\begingroup$ Obvious question is obvious, but what temperature is this planet? Helium is only a superfluid at 2K. It wouldn't be particularly slippery when it vaporizes instantly when released from its container. $\endgroup$ – jdunlop Jul 3 '20 at 23:59
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    $\begingroup$ As a different sort of frame challenge than presented by @TheDaleks, helium is hard to contain. If your planet is good enough at it to weaponize it, there are lots of more dangerous things they could put in a casing. $\endgroup$ – jdunlop Jul 4 '20 at 0:50
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    $\begingroup$ Cool thought, but you're probably better off spraying the floor with a more conventional lubricant. $\endgroup$ – BBeast Jul 4 '20 at 4:49
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    $\begingroup$ "Cool thought" - I see what you did there. $\endgroup$ – jdunlop Jul 4 '20 at 5:17
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    $\begingroup$ @BBeast: go green, go banana peels ;D $\endgroup$ – user6760 Jul 4 '20 at 6:32
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No

Given the edit, the helium would vaporize so quickly it wouldn't have time to disperse. It would likely explode like a container full of liquid nitrogen.

It wouldn't be a superfluid; it wouldn't be a fluid. It wouldn't even stick around for long. It would definitely chill the area, you might give some people frostbite, but a hand-grenade sized container of liquid helium would never spread anywhere significant.

Edit: to be clear, liquid nitrogen is 75 degrees warmer than liquid helium, and you cannot meaningfully pour liquid nitrogen onto the floor without having a lot of it.

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    $\begingroup$ I once had a lecturer who would pour a thermos of liquid nitrogen over his own hand. Not only did it not reach the floor but body heat would make a layer of gaseous nitrogen next to his skin that then trapped the heat while the rest of the liquid nitrogen just poured off. It’s the reason rocket coolant is usually a slush rather than a liquid. Not sure if the same would happen with liquid helium droplets though. $\endgroup$ – Joe Bloggs Jul 4 '20 at 6:15
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    $\begingroup$ @JoeBloggs While I am sure there are ways to safely do such a trick with liquid nitrogen (my guess is to start with only a tiny amount of liquid nitrogen), I think that comment should be accompanied by a "DO NOT TRY THIS AT HOME" warning, as doing it wrong could give a severe case of frostbite. $\endgroup$ – BBeast Jul 4 '20 at 7:13
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    $\begingroup$ Gases are fluids... $\endgroup$ – AlexP Jul 4 '20 at 7:26
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    $\begingroup$ In the do not try this at home, or indeed anywhere else department, people at the Imperial College Astrophysics group in the 80s may remember a postdoc whose party piece was gargling liquid nitrogen, then gobbing it down the stairwell (Astro being on the 10th floor) to evaporate in a lovely cloud. Worked brilliantly until he overdid the swig-gob delay and got frostbitten tonsils. $\endgroup$ – MadHatter Jul 4 '20 at 9:34
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    $\begingroup$ @Joe Bloggs: It works for liquid nitrogen due to the Leidenfrost Effekt. With helium that kind of works too, but on a much smaller time frame (maybe 10x less time) because helium evaporates much much faster, so the by the time you can react your skin already got a frostbite burn, unless you can pull your hand back out in milliseconds... $\endgroup$ – schlenk Jul 4 '20 at 10:18
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No. Even with the obvious problems with Helium-4 noted by a other posters, extensive research has discredited the notion of both sticky and slippery substances as nonlethals. The US Army carried out quite a lot of work in this area -- the most promising was sticky foam but even that did not prove effective.

Your best case -- and you might want to use something like graphite rather than Helium-4 - would be that you create an area of slipperiness, assuming you have a flat smooth surface. In normal ground, carpeted surfaces etc the stuff will just be soaked up. This may be useful for preventing rioters from accessing an area, but it not useful in a combat situation, especially when your enemies have real weapons.

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No, but not in the way you are thinking.

As @jdunlop pointed out, the Helium would immediately boil. However, this in and of itself is extremely useful.

You see, while jdunlop is right that it will (more or less) instantly boil, it will still be extremely cold. "Extremely cold" is an understatement.

With that in mind, I would like to do a frame-challenge and take a step-by-step look at the effects of detonating one of these.

  1. Sublimation. When the liquid helium is released from its containment it is going to boil off as gaseous helium.

  2. Expansion. Unlike liquid nitrogen, all of the liquid helium is going to become gas more or less at once. Initially all of this gas is going to be in one spot; however, nature abhors unequal pressures, so the helium is going to explosively expand to equalize the pressure. While it won't be anywhere near as powerful as a hand grenade, it will still do the job of getting helium all over the place.

  3. Temperature Equalization. While the helium may have boiled off, it is still extremely cold. In addition, when a gas expands, the gas does work to overcome the intermolecular forces of attraction (it implies that the gas is spending it's own energy). This results in a decrease in internal energy of the system. Since internal energy is a function of temperature, expansion of gas decreases the temperature of the gas. The end result is a rather... drastic decrease in temperature. Brrr.

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    $\begingroup$ I absolutely agree with this. But while this makes a stellar "cryo grenade" per any number of video games, it doesn't make much of an area denial weapon. If you throw a hand-grenade's worth of liquid nitrogen (don't, people will be upset), you'd have a similar if less pronounced effect, and it wouldn't meaningfully slow pursuing enemies. $\endgroup$ – jdunlop Jul 4 '20 at 0:30
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    $\begingroup$ @jdunlop I realize that; this is meant to be a frame challenge. $\endgroup$ – The Daleks Jul 4 '20 at 0:34
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    $\begingroup$ 1 kg of helium occupies circa 8L of volume as liquid and 6 m^3 as a gas. If we assume a tennis ball-sized volume of liquid helium, that's ~158ml, or about 0.118 m^3 of gas. It would be like stepping into a walk-in refrigerator. $\endgroup$ – jdunlop Jul 4 '20 at 0:37
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    $\begingroup$ So @Niobium_Sage, the former. Uncomfortable cold, no Mr. Freeze effects unless someone were unlucky enough to be struck directly by the burst of supercooled gas as the grenade burst. (And even then, only if they were hit somewhere unprotected - the thermal capacity of helium is tiny.) $\endgroup$ – jdunlop Jul 4 '20 at 0:38
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    $\begingroup$ @Niobium_Sage According to my calculations, the temperature averages out to about -100º Fahrenheit, but only for the first few milliseconds. That being said, I'm not a physicist, so I could be wrong. $\endgroup$ – The Daleks Jul 4 '20 at 0:40

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