What is quicksand and how to get out of it

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How often have you seen a movie character trapped in quicksand and helplessly reaching out to grab any branch or blade of grass until their head disappears under the surface? Quicksands are sands that have a lot of air or moisture from underground sources, causing them to literally suck up everything that gets into them. In this lesson of the Windy.app Meteorological Textbook (WMT) for better weather forecasting you will learn in detail what quicksand is, including what you should and shouldn't be afraid of about it, and how to get out of it, if you get trapped.

A fluid that doesn't act like one

As you know, everything around us is in one of three primary states: solid, gas or liquid. Different properties characterize different states of matter. One of the most important of these is viscosity.

Viscosity determines how much matter resists attempts to change its shape from external pressure. It is defined by the bond type between molecules, and their shape.

Almost all common fluids that we deal with in our everyday life–water, honey, oil–have a viscosity that doesn't change based on applied pressure: they follow Newton's Law of Viscosity. But some fluids can become more or less "liquid" under pressure. They are called non-Newtonian fluids.

The simplest non-newtonian fluid can be made by mixing cornstarch with a small amount of water (in a ratio of about 3/4 to 1/4, respectively). The mixture will look like a liquid, but if you squeeze it in your fist, the viscosity will rise, and part of the mixture will stay in your hands as a piece of wet clay, shaped in the mould of your hand.

At rest, it is an ordinary fluid, but when force is applied, starch particles push the water molecules out, creating a solid surface under your palm.

Another example of a non-newtonian fluid, as you already guessed, is quicksand. Similar to ketchup, quicksand becomes less, rather than more, viscous under pressure.

Non-Newtonian fluids. Valerya Milovanova / Windy.app

The house of cards made of sand

We do not fall beneath the surface of dry sand when we walk on it, because individual grains of sand are supported by each other. In the same way, the walls of a gothic cathedral don't fall on top of the unsuspecting worshippers because symmetrical halves of the structure meet in the middle and support one another. The same goes for a house of cards.

But everything changes if you add some water. If, as a result of an earthquake, high tide or rising groundwater, that water appears under the sand, in some specific proportions, this mixture can become a non-newtonian fluid.

It can look like perfectly solid sand, but when the pressure of a foot is applied, the system of support between grains of sand is destroyed, and now only small and "slimy" water molecules are left between them. A similar thing can happen, as experiments show, if "extra" air is added to sand instead of water.

The foot will sink in, and the more pressure that is applied, the more support is broken, and the deeper the foot will sink.

Kenny Eliason / unsplash

Instructions for dealing with quicksand

So, you sink in. First of all, try to calm down and avoid big and sharp movements.

Remember that people get fully covered and drown in quicksand only in movies; in reality, you are unlikely to sink any deeper than your waist. As in any liquid, your body has buoyancy, and when the weight of the replaced sand equals your own, you won't be able to sink much further.

But you can still die or get seriously hurt if stuck in a quicksand for long, primarily because of heat, hunger, thirst, and wild animals.

As with a swamp, you shouldn't try to push with your legs (as you don't have anything to push against anyway), but instead, try to lay down on your back and climb or crawl out of the dangerous area, distributing the weight of your body on as much surface as possible. It would be very helpful if you have something to grab on solid land.

Be ready for the process to be long and tiresome: sometimes pulling out one leg at 1 centimeter per second may need strength comparable to lifting a car. If you don't manage to lay on your back, try to find a stick of something similar that you can use to "loosen" the mixture behind your back. More water will rush there, and the sand will become less viscous.

After releasing yourself, remove and change your clothes: drying sand can scratch the skin and lead to an infection.

Text: Jason Bright, a journalist and a traveller

Cover photo: Brandon Kaida

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