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In movies or on TV usually for entertainment reasons, quicksand is quite often and inaccurately portrayed as a kind of super fast acting, almost inescapable and extremely deadly trap capable of swallowing entire humans and animals whole.

So in my world there's this weird grenade-sized projectile that essentially does this very same thing. Upon activating it causes the ground underneath its victims feet and within a certain radius to turn into a strange sticky tar-pit like substance (basically fast acting quicksand but much harder to get out of, which is why I've decided on making it sticky substance instead)

This causes victims to rapidly sink inside the ground until the chemical reaction or strange phenomenon eventually ceases, re-solidifying the ground underneath. Which either leaves the victims trapped or in the worst case scenario encased/swallowed whole into the ground.

I've was already planning on handwaving this with magic but I ask this question more-so out of curiosity that anything.

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    $\begingroup$ Made on Gallifrey. But, serious comment: How would you judge the answers? $\endgroup$ Jul 29 '20 at 9:51
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    $\begingroup$ Acid resistant humans. But not acid resistant surface. $\endgroup$ Jul 29 '20 at 9:57
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    $\begingroup$ Does it have to work on any terrain or is it OK to work on a limited range of natural surfaces? (presumably you can have the planet/battlegrounds frequently occur on that type of ground) $\endgroup$
    – VLAZ
    Jul 29 '20 at 13:21
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    $\begingroup$ Have you ever seen the videos of people stomping/jumping on the beach on dry sand? Suddenly it turns semi-liquid and moves, after enough agitation (though not enough for people to sink into it really)? $\endgroup$
    – John O
    Jul 29 '20 at 14:19
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    $\begingroup$ Is there any way to eliminate electrostatic cohesion between molecules with a field effect? I can't think of one, but if there was, I see some potential. Alternatively, some means to break interactions between atoms in molecules. Again, not sure how you could without more energy than is worth discussing. Across the board, a big bomb is simpler, easier, cheaper, etc. $\endgroup$
    – DWKraus
    Jul 29 '20 at 21:31
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It’s highly unlikely that something the size of a grenade would contain enough material to effect a large enough area.

What might work but would need to be much larger would be some large underground air pump that could turn the surface (sand, mud or dust?) into a fluid bed of low density. Anyone standing on it falls right through (proper fictional quick sand style) as they are denser than the fluidized bed around them. And when it’s switched off they would become trapped there.

A similar mechanism of gas in water has sunk ships and oil rigs.

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Simple: Have it release a liquid that soaks into the soil and reduces its density to below 1010 kg/m^3 (the average density of the human body). It compresses the liquid via extreme pressure, or if you wanna go there, warping space so it's bigger on the inside. If we work with a soil of an average density of 2650 kg/m^3, and using gasoline, or a liquid with a similar density, that liquid has a density of 700 kg/m^3. If we go with an equal volume of gasoline and soil, the density should be 1675 kg/m^3, which is too dense. But if we were to go with 3 parts gasoline for every part soil, we have a density of 1188 kg/m^3. 7 parts gasoline, 1 part soil, and the density is 944 kg/m^3.

And as a bonus, the slurry is also flammable.

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    $\begingroup$ Liquids generally fill in the vacant pores that already exist in soil, so wetting soil typically makes it more dense, not less. For density to decrease, you'd need to actively move some of the soil elsewhere, and replace it with a lower-density fluid. Dumping liquid on the ground doesn't displace anything but air, you're basically putting more weight into the same volume. $\endgroup$ Jan 14 at 19:29
  • $\begingroup$ Nuclear Hoagie, what if it's a special liquid that absorbs into the ground, and expands to reduce its density until the mixture is lighter than water? $\endgroup$ Jan 14 at 23:37
  • $\begingroup$ I suppose that would work, but the expanding material would need to displace soil - you could add low-density material, but it would have to push high-density material out of the way to have the intended effect. This will take a lot of energy - imagine being buried in dirt up to your neck, and trying to push dirt either out or up to dig yourself out. The energy needed to move the dirt would be like a hand grenade in the first place, rather eliminating the need for the more creative aspects of the weapon. $\endgroup$ Jan 15 at 15:54

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