Image Source (top): Zach Dischner via Flickr
Inside: What is quicksand? It was a favorite B-movie ploy – death by quicksand. But can it really suck you under? Discover that quicksand is not such a fearsome thing.
Death by quicksand! It is a favorite B-movie death scene. The more the unfortunate victim struggles, the faster they sink. Until all that’s left is a solitary cap sitting on top of the sand. It is a scary notion, getting sucked quickly into muck. And if you’ve watched too many B-movies like I have, panic would set in quickly. More struggling, faster sinking and no rational thought would surface to help you think your way out of your predicament.
What is quicksand anyway? Is it a harbinger of death as the B-movies would like us to think? Let’s look at the science of quicksand and find out shall we.
What is Quicksand?
Quicksand is a non-newtonian fluid. It can behave both like a solid and a liquid and its viscosity or thickness depends upon the level of force exerted on it. It is a colloidal hydrogel meaning it is made up of fine granular material like sand, silt or clay suspended in water. The quick in quicksand refers to how quickly the sand shifts when disturbed. Quicksand occurs anywhere under the right conditions but is most common along:
- lake shorelines
- near underground springs
You note that these areas have sand and a lot of water. Quicksand is merely ordinary sand that is so saturated with water that the friction between sand particles is greatly reduced.Non-newtonian fluids behave like both a solid and a liquid at the same time. Click To Tweet
What Happens When you Step on Quicksand?
When you come upon quicksand, it looks just like sand. But that’s where the similarity ends.
- The ground that looked solid begins to liquefy when you step on it because sand particles are surrounded by water.
- With little friction between the sand particles, they cannot support your weight.
- You begin to sink.
- However, as you struggle, the pressure forces water out from between the particles.
- Sand particles now touch each other and the friction between them causes the quicksand to behave more like a solid.
- Your struggles increase pressure, then decrease it causing the quicksand to alternate between a solid and liquid state.
- The suction created makes it extremely difficult to climb out of quicksand.
Can a Person Really Sink Under Quicksand (like in those B-movies)?
In a September 2005 volume of the journal, Nature, it was revealed that it is impossible for a person to sink completely in quicksand. Well I don’t know about you but I am sighing in relief! In reality, humans will float in quicksand. The following should elaborate:
- At rest quicksand acts like a liquid.
- The granules of starch are surrounded by water.
- Surface tension of the water keeps the water from completely flowing out of the spaces between the granules.
- Thus, the granules move freely. But, if the movement is abrupt, the water is squeezed out from between the granules and the friction between them increases rather dramatically.
- When the sand is lightly stepped on by an unaware adventurer, the quicksand acts like a liquid and the victim begins to sink.
- The more a person struggles, the faster the victim sinks but……only so far.
- Movement like struggling is abrupt.
- Thus, the water is squeezed out from between the granules and the friction between them increases rather dramatically.
- This heavy pressure causes the quicksand to form a solid.
- It encases her and she is stuck but never fear. Contrary to popular belief she will not sink.
This is because people aren’t dense enough. Do I mean they’re not stupid enough…..of course not. I’m referring to their mass verses their volume kind of density.
Human Density and Quicksand
Quicksand has a density of about 2g/mL. Human density is about 1 g/mL. Because people density is lower than quicksand density a person won’t sink completely under. In fact, you’ll sink up to your waist and then float. So, good to know if you’re ever trapped in quicksand. You won’t sink under.
Can you still die when trapped in quicksand…..sure you can. Because quicksand is often near shorelines, a tide moving in can result in drowning. Starvation or predatory animals could also be a problem. You still have to get out of the mess you’re in. That is not so easy. Again, let’s look at the science.
Image by: Dplanet via Flickr
Why is it So Difficult to get out of Quicksand?
The difficulty in escaping quicksand lies in the changes in viscosity that take place while you’re getting stuck. When you lightly step on quicksand, it liquifies. Dry or damp sand can support a lot of weight because of friction between the grains of sand. A force chain is created by that friction and the load is distributed across a large area. However, when you add a whole lot of water, the sand and water becomes a suspension meaning the sand particles are floating within the water. Friction between the grains of sand is greatly reduced and so is the sand’s ability to support weight. Because the water often seeps in from the bottom, the top layer of sand may appear dry, You think its normal sand…..until you step on it and start sinking.
However, you will get tired of flailing around and start to look around for help. But why, if hight pressure turns quicksand into a liquid, is it so hard to escape? Your body weight and intial struggles turn create liquified sand. However, quickly after the quicksand’s viscosity (thickness) increases. The more solid feeling quicksand is due to the formation of sand sediment, which is very thick. It is extremely difficult to move in this stuff. To release just one foot would require 100,000 Newtons of force – the same force required to lift a medium sized car.
Your relaxed body will remain floating but trying to get out is a problem. Don’t ask your friend to pull you out. The suction force will likely rip you in two. Now that would be a scene for a horror movie!
So, How do you Escape from Quicksand?
So, how do you escape this horrifying predicament? The following steps should get you out of your jam:
- Wiggle your legs around. This creates space between your legs and the sediment allowing water to flow down and loosen the sand.
- Move your legs up slowly and repeat the above process.
- At the same time, lean back and spread out your arms to distribute your weight more evenly.
- Continue to wiggle those legs and slowly pull them up.
- Eventually, you should float back to the surface and that friend will be there to gently pull you along the surface of the quicksand to safety.
- And please, don’t forget your hat!
If you move into the quicksand this loose packing will collapse. We then have densely packed sand at the bottom, and water floating on top of it. It’s the difficulty of getting water into this very densely packed sand that makes it difficult for you to pull your foot out. — Daniel Bonn, Prof., Institute of Physics, University of Amsterdam
- 1 box of cornstarch
- mixing bowl
- ¾ cup water
- In a mixing bowl, mix small amounts of cornstarch at a time with cold water.
- Continue to add cornstarch and cold water to the bowl mixing well.
- When you are finished you want the mixture to look like whipping cream but have the consistency of thick honey.
- You’ll know you have the recipe right when stirring gently feels like a thick fluid but stirring quickly begins to create solid clumps.
Make sure the water is cold and slowly mix in the cornstarch. You’ll be gradually adding a lot of cornstarch. You’ll need a strong stirring arm.
Try these cornstarch quicksand activities with your kids at home or students at school. Introduce them to the wonder of quicksand and dispel some of those Hollywood myths. First you need to go back and make up a big batch of the quicksand recipe from the included instructions in this post. Young or older, kids will be intrigued by these easy experiments.
- Kids love getting messy. Have them put their hand in a bowl of “quicksand”. Tell them to stir slowly with their hand. How does the mixture feel? Older kids can record their observations in a chart. Younger ones can squeal in delight and tell you what they feel. You should get some of the following responses: cool, liquidy, easy to move, slimy, gooy, goopy.
- Then have them stir the mixture more quickly. How does the goop feel now? Responses might include: really thick, hard to stir, feels more solid, goopy.
- Have the students pick up some “quicksand” and squeeze it in their hands. They should be able to shape it into a solid clump which will quickly liquify again once they stop squeezing.
- Let them sink their entire hand into the goo, grab the mixture and pull it up. The sensation of sucking mud is the same as that felt when trying to escape quicksand.
- Tell the kids to make a fist and hit the “quicksand” hard. They will be expecting a big splash but should be surprised when the quicksand feels solid and no splashing occurs.
Some Less Messy Easy Quicksand Experiments
- Give each student a straw. Have them try to blow bubbles. Have them record and tell you their observations. It is harder to blow bubbles in this mixture. If you get it just right, a big bubble tends to form over the surface of the mixture. Ask the kids to give reasons why the bubbles in this non-newtonian fluid differ from those formed in a newtonian fluid like water. A further experiment for older kids would be to compare bubble formation between newtonian fluids (water, milk, honey) and non-newtonian fluids (cornstarch mixture, ketchup). Have them research explanations for the different bubble formations.
- Drop a plastic soldier or figure on top of quicksand that’s been sitting still for a while. It should rest on top of the mixture. Have the kids shake their container vigorously. The soldier should start sinking into the goo much like a person does after stepping on quicksand.
- Next have them pull the figure out of the goo. They should feel a pull or resistance and find it more difficult to pull out the toy.
- Try dropping objects of varying mass into the mix and have them record their observations. Did any of the objects sink immediately? Perhaps some only sunk part of the way? Did any remain on top? Again, shake the container vigorously. What happened to the objects still on top of the “quicksand”?
Homemade quicksand is easy to make and is messy. A surefire recipe to capture a child’s imagination.
If you loved this activity then try this super easy experiment to investigate the effect of soap on surface tension.