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General Science => General Science => Topic started by: paul.fr on 21/04/2007 19:04:49

Title: quicksand
Post by: paul.fr on 21/04/2007 19:04:49

not sure where to put this!

what is quicksand, and how does it work..ie, why do you get stuck in it, once stuck how do you get out? or don't you, like in those old gangster films where they bury bodies in quicksand.

Title: quicksand
Post by: JimBob on 21/04/2007 20:45:41

Hey! I know this one!

Quicksand is sand with a high enough water content that separates the sand grains enough to allow shear modulus¹ to become quite low. (That means the force necessary to slide the sand grains against each other becomes very low.) The sand and water mixture is thixotropic. it looks like a solid but acts like a liquid when stress is applied. The amount of water in the sand determines how much like a liquid the sand-water mixture will act.

A beach near the water is thixotropic. If you stand with your feet planted in one spot near the surf zone and move your body around the change in the center of gravity of your body's mass will apply shear force to the seemingly solid sand and your feet will sink a small amount into the sand. Quicksand exhibits the same property but to a much more pronounced extent. Weight alone applied to the surface of the sand is enough to turn an apparent solid into a liquid with a certain viscosity that is dependent on the water content. It becomes a non-Newtonian fluid².

So, the more you struggle while standing, the deeper you sink. The way out of this trap is to reduce the unit stress per area. In plain English, get horizontal. The more horizontal you become, the easier it is to move as if you were swimming and slowly get to solid ground. in fact, the faster you sink the easier it is to extract yourself IF you don't loose your head and just flail around.
 
The clay content will also effect the viscosity of the quicksand but the math becomes much more complicated. The formulas below assume a sand without clay content and of a small range of size of sand grains.


¹ Shear modulus is defined as S. It is the tangential force applied between solid particles per unit of area divided by the angle of the tangential forces or S=(F_tanΦ/A)/ Φ.   

² A non-Newtonian fluid a fluid where the viscosity changes proportional to the applied strain or η= dτ/dD

Title: quicksand
Post by: lyner on 21/04/2007 23:27:53

I used to love the quicksand in all the old cowboy films - it looked just like porridge!
To get out of a quicksand you always had to whistle for your horse and he would lassoo you and pull you out at the last minute.

Title: quicksand
Post by: ukmicky on 21/04/2007 23:51:00

Quote from: sophiecentaur on 21/04/2007 23:27:53

To get out of a quicksand you always had to whistle for your horse and he would lassoo you and pull you out at the last minute.

Thats not fair, i want to see a horse lassoo a man and pull him out of the quick sand :)

Title: quicksand
Post by: lightarrow on 22/04/2007 11:13:10

Quote from: paul.fr on 21/04/2007 19:04:49

not sure where to put this!
what is quicksand, and how does it work..ie, why do you get stuck in it, once stuck how do you get out? or don't you, like in those old gangster films where they bury bodies in quicksand.

If I remember correctly, there has been a thread on this with an interesting answer given by Chris. Ask him if you don't find the thread.

Title: quicksand
Post by: lyner on 22/04/2007 11:23:23

lightarrow is clearly not old enough to have seen the films that I saw. ( a compliment, I think)

Title: quicksand
Post by: DrDick on 22/04/2007 13:29:39

Quote from: JimBob on 21/04/2007 20:45:41

² A non-Newtonian fluid a fluid where the viscosity changes proportional to the applied strain or η= dτ/dD

There are even different types of non-Newtonian fluid.  A Newtonian fluid is one that retains the same viscosity, regardless of the amount of force applied and the amount of time over which that force is applied.

Therefore, a non-Newtonian fluid is one that doesn't follow those rules.  Some non-Newtonian fluids get thicker or thinner with more force or with with the same force, but over time.  Corn starch/water is one of the coolest of these.  It's fun to make a hard ball of this mixture by squeezing it hard, then stop squeezing and let it run over your fingers.  [:o]

Dick

Title: quicksand
Post by: lightarrow on 22/04/2007 18:34:10

Quote from: paul.fr on 21/04/2007 19:04:49

not sure where to put this!
what is quicksand, and how does it work..ie, why do you get stuck in it, once stuck how do you get out? or don't you, like in those old gangster films where they bury bodies in quicksand.

I found the thread I was talking about:
http://www.thenakedscientists.com/forum/index.php?topic=5502.0

Title: quicksand
Post by: lightarrow on 22/04/2007 18:39:50

Quote from: sophiecentaur on 22/04/2007 11:23:23

lightarrow is clearly not old enough to have seen the films that I saw. ( a compliment, I think)

Thank you very much for your compliment.
I'm not so young however (45).
Bye!