Question about quicksand

[Hypercube]

I recently got into a debate with someone on YouTube after I had commented on a quicksand video where someone drowned in it. I had made the point that the idea of someone drowning in quicksand is pure Hollywood nonsense, and he contradicted me rather rudely I might add.

 

Correct me if I'm wrong. Whatever special properties quicksand (I'm talking about classical quicksand, not dry quicksand) might have as a non-Newtonian fluid, it is still a fluid, and one that is more than twice as dense as the human body. And as everyone learns in high school physics, if an object/fluid of density X is placed into a fluid of density nX where n>1, than that object/fluid will just float on/in the denser fluid. Which would seem to imply that, barring somehow going into the quicksand head first, in which case drowning is probable, it would be almost impossible for a human to sink under the surface of the quicksand (their whole body I mean, feet to face).

 

Don't misunderstand, I'm not trying to suggest that I don't think quicksand is dangerous, because I know that it is. I'm only saying that the typical Hollywood depiction of quicksand slowly dragging a person under and drowning them is complete nonsense overruled by the laws of physics. Now if I'm wrong, or at least have overlooked something, please let me know.

[michel123456]

I recently got into a debate with someone on YouTube after I had commented on a quicksand video where someone drowned in it. I had made the point that the idea of someone drowning in quicksand is pure Hollywood nonsense, and he contradicted me rather rudely I might add.

 

Correct me if I'm wrong. Whatever special properties quicksand (I'm talking about classical quicksand, not dry quicksand) might have as a non-Newtonian fluid, it is still a fluid, and one that is more than twice as dense as the human body. And as everyone learns in high school physics, if an object/fluid of density X is placed into a fluid of density nX where n>1, than that object/fluid will just float on/in the denser fluid. Which would seem to imply that, barring somehow going into the quicksand head first, in which case drowning is probable, it would be almost impossible for a human to sink under the surface of the quicksand (their whole body I mean, feet to face).

 

Don't misunderstand, I'm not trying to suggest that I don't think quicksand is dangerous, because I know that it is. I'm only saying that the typical Hollywood depiction of quicksand slowly dragging a person under and drowning them is complete nonsense overruled by the laws of physics. Now if I'm wrong, or at least have overlooked something, please let me know.

 

That seems correct to me. I remember as a child a book about common nonsenses with a small article about that. The book is lost I am afraid and I can't recall the title, it was 40 years ago.

[John Cuthber]

If you were stuck in quicksand on the beach and the tide came in you might find that reality was even more rude than your friend.

Strictly speaking you wouldn't drown in quicksand but the effect would be the same.

[michel123456]

the problem with quicksand is that you stuck in it. In the French Wiki article it is described as acting as cement. In the english article:

Quicksand is a non-Newtonian fluid: when undisturbed, it often appears to be solid ("gel" form), but a minor (less than 1%) change in the stress on the quicksand will cause a sudden decrease in its viscosity ("sol" form). After an initial disturbance such as a person attempting to walk on it the water and sand in the quicksand separate and dense regions of sand sediment form; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to increase suddenly. Someone stepping on it will start to sink. To move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it. The forces required to do this are quite large: to remove a foot from quicksand at a speed of .01 m/s would require the same amount of force as "that needed to lift a medium-sized car."

bolded mine.

 

Which makes me think that once you have put something in quicksand, it will be very difficult to get it out, no matter buoyancy.

Edited April 23, 2012 by michel123456

[Hypercube]

OK. It looks like I need to make my question more specific. When I say 'drown in quicksand' I'm not referring to incoming tides, or anything like that. And I'm not questioning that it is extremely difficult to get out of quicksand once you're stuck in it. All I want to know is whether there is ANY way, barring the person wearing heavy weights or something, that a person can continue to sink further and further into the quicksand until they are completely submerged in it and drown, leaving only their hat floating on the surface of the quicksand; like in the old Tarzan movies. Or will the laws of fluid dynamics still apply to quicksand, in which case the person would be buoyant in the quicksand.

[mathematic]

From Wikipedia:

 

 

 

Quicksand is a non-Newtonian fluid: when undisturbed, it often appears to be solid ("gel" form), but a minor (less than 1%) change in the stress on the quicksand will cause a sudden decrease in its viscosity ("sol" form). After an initial disturbance — such as a person attempting to walk on it — the water and sand in the quicksand separate and dense regions of sand sediment form; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to increase suddenly. Someone stepping on it will start to sink. To move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it. The forces required to do this are quite large: to remove a foot from quicksand at a speed of .01 m/s would require the same amount of force as "that needed to lift a medium-sized car."^[1]

 

Because of the higher density of the quicksand, it would be impossible for a human or animal to completely sink in the quicksand, though natural hazards present around the quicksand would lead people to believe that quicksand is dangerous. In actuality the quicksand is harmless on its own, but because it greatly impedes human locomotion, the quicksand would allow harsher elements like solar radiation, dehydration, hypothermia or tides to harm a trapped person.^[2] Pulling a person from quicksand with too much force could be fatal.^{[citation needed]}

 

The way to escape is to wiggle the legs as slowly as possible in order to reduce viscosity, to try spreading your arms and legs far apart and lying prone to increase your surface area, which should allow you to float.^[3]

 

 

[Moontanman]

This actually depends on how you encounter the quicksand, such "sand" is common where I live. If the quick sand is shallow and you are able to walk out into it you will not drown if you don't panic but if the quicksand is very deep and you step out onto it your momentum will cause you to go under much like stepping out onto water so you go under even though you can float, the problem with quick sand is that it impedes your rising back to the surface. You can go under but not come back up.

 

Around here in many places is a thick layer of organic silt, it looks solid, plants can root and grow in it but it is not much more dense that water and is often very deep, step out onto it and you disappear but it's viscosity is enough to prevent you from coming back up.

 

Sometimes this solid layer will be covered over by fine sand. the sand appears to be solid when you step out onto it but as you walk it starts to shake like jello and down through the layer of sand you go into the silt. Coming back up is more difficult than going under, going under you have gravity pulling you down, coming up your buoyancy is not enough to propel you back up.