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Hank: We've all been there. You're walking down the street, going to your favorite ice cream shop when BAM! You fall into quicksand. Within seconds you've faded from sight never to be seen again.

Kidding. Of course, quicksand might be a vicious killer in the movies but in real life, you wouldn't even sink all the way in. But that doesn't mean that quicksand isn't dangerous or that you can't get stuck in it because you totally can. But as long as you know a little bit about the strange physics of quicksand, you can probably wiggle yourself free.

Quicksand is created by water seeping into sand from underground springs or standing pools. You often find it at the bottom of lake beds, swamps, marshes, near rives or on beaches at low tide. It's made up of about 30 to 70 percent water which completely changes the way it behaves.

So when you walk in through regular sand, like at the beach, your feet will sink in a little bit but then quickly stopped. That's because normal sand is packed so tightly together that each sand grain is jammed into place. The friction and pressure created by this compaction known as a force chain creates a firm surface.

In quicksand, instead of being crammed together, the sand grains are suspended or loosely packed. They're basically floating around in the water. The friction and pressure is reduced and the sand can't handle the same weight load. And that is what makes a non-Newtonian fluid which we've talked about before in our first video ever.

Essentially, this is a fluid that doesn't act like your everyday run-of-the-mill Newtonian fluid, water for example. No matter how much force you apply to water, say by stirring it really fast, it's viscosity or the measure of how it flows remains the same.
If you got more viscous, it would be gooier and if it became less viscous it would be runnier. But if you apply a moderate force to quicksand, say by kicking your legs around it, it's viscosity does change. When quicksand is left alone for a while, the water fills in the gaps between each sand grain and the two materials become evenly distributed in what's known as a colloidal gel.

But it's a very delicate balance like a house of card, with water filling the empty space. When stress and vibration are applied, say stepping on the quicksand, that house of cards collapses and the sand liquefies and you leg sinks.

Then, and here's the weird part, the quicksand becomes more viscous. Once you fall in, the water which is less dense rises to the top and the sand which is more dense falls to the bottom.
As the water rises, it leaves a vacuum in the dense sand which compacts around your leg making it harder to move.

So hard in fact that the force you'd need to suddenly remove your leg from that quicksand is the same force you'd need to lift a medium sized car. You aren't gonna drown but if there's no one nearby to help you, you're still in trouble. So how do you get out of such a sticky situation? Well at most you'd only sink about half-way because you are less dense that the sand.

Humans have a density around 1 g/ml whereas quicksand has a density around 2 g/ml. So you wiggle your legs just a little bit. That will create some space between the sand and your legs where the water can flow back in and mix with the sand, making it less viscous and therefore loosening it's grip on you. Keep doing this and you can eventually free yourself from a quicksand pits. But it could take a long time. And if you're stuck on the beach when high tide comes in, that can be dangerous. 

That's why rescuers often use an injection pump that fires water into the pit, liquefying it quickly so that they can pull out the person and save them and give them a really awesome story to tell their grandkids.

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