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In the book "Munson and others -Fundamentals of Fluid Mechanics" (at least in my version translated to portuguese) it says that quicksand is a shear thickening fluid but in wikipedia and other sources it says that it is a shear thinning fluid. The lack of understanding here is how does it behaves?

In the book it says that is a shear thickening fluid because when a force is applied it becomes more rigid and thus being harder to pull something (or yourself) of it.

In wikipedia it says that is a shear thinning fluid because when a force is applied it becomes more fluid and so it is easier to sink in it.

How does quicksand behave?

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Here is how quicksand behaves, based on my own experiments.

It is a slurry of liquid clay with sand suspended in it. There is enough clay in the slurry to make the sand almost buoyant, and enough viscosity in the slurry to prevent the sand particles from settling out on timescales of order ~weeks or more.

When agitated, the sand particles decouple and fluidize, so the mix cannot sustain shear. If you wiggle your foot while applying your weight to it, your foot will slowly sink into the mix.

Then if you suddenly pull your foot upwards, you create a suction under your foot which draws the clay slurry from around your foot out of the sand, and the space under your foot becomes filled with de-sanded clay slurry. Meanwhile, the mix above your foot gets dewatered as well, and the sand particles there come into contact and lock. Your leg is now trapped, and if you put your weight on it, your foot sinks deeper into the liquid slurry under it. If you repeat this cycle enough times (wiggle and sink, then pull up), you can pull yourself down and bury yourself in the mix, even though your body is buoyant in it.

However, if you wiggle your foot while pulling up on it, you can unlock the sand particles and your foot will slowly come free.

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  • $\begingroup$ Does the very last sentence means that it will help to wiggle if you have help (rope or whatever to pull) from out of quicksand? I am not in quicksand area but it could be useful :) I don't understand the pull part, if I pull out my own foot, shouldn't the other one sink? Or it is not as we are buoyant? $\endgroup$
    – Alchimista
    Commented Apr 15, 2019 at 10:04
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    $\begingroup$ Yes, wiggle while someone else is pulling on you. andn note that if you push hard and steady with one foot, the goop locks and as long as you are wiggling your other foot, you can take turns wiggling and pulling and eventually free both of your feet. $\endgroup$ Commented Apr 15, 2019 at 20:28

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