Move like crazy - or sink!
Recently at Science Delivered, we created an Oobleck Pool. Never heard of this before? Watch the video below and see if you see anything unexpected.
wHAT'S GOING ON?
"Oobleck" is the popular name given to a corn starch and water mixture. This mixture has the fascinating property of being a solid or a liquid depending on the pressure, or shearing force, exerted upon it. More on this down below, but in practical terms, it means that smacking the Oobleck Pool with your feet temporarily turns the surface into solid. But if you place you feet (or hands, or elbow etc) into the Oobleck slowly, you'll get sucked right in.
Where does the name "Oobleck" come from?
The term "Oobleck" comes from an early work of Dr. Seuss, Bartholomew and the Oobleck. Magicians make a green gooey substance come down from the sky; it creates havoc in the land and look quite similar to the cornstarch and water mixture when you play with small amounts of it and let it drip. Our Oobleck can create havoc too - if you get stuck in it!
What is a non-Newtonian Fluid?
This might be best answered by defining a "Newtonian" fluid, which is an ideal liquid. In lay terms, the flow of a Newtonian fluid will not change no matter what you do (assuming constant temperature). If you consider water, you can pour water our slowly, or stir it really fast, or shake it up in a bottle and flows remains the same in all conditions. The viscosity does not change.
In contrast, consider trying to get ketchup out of a glass bottle. The ketchup will sit, clumped, requiring you to bang the bottle to get it out . . . at which point it would all come out at once. The applied force (hitting the bottle) causes the viscosity of the ketchup to change. The Oobleck is a mixture of corn starch and water. When you apply force to this mixture, the mixture hardens, enough so that you can walk on a pool of it. But if the force is too weak, the substance will act as a liquid and you will sink right in.
Are all non-newtonian fluids the same?
No! Non-Newtonian fluids can be classified as shear-thinning or shear-thickening. Meaning, fluids can either decrease or increase their viscosity in response to force. Remember how the ketchup flowed quickly out of the bottle once you hit it? That means it is sheer-thinning, it flows more easily in response to force. Other shear-thinning fluids are paint, blood, whipped cream and lava. The Oobleck corn starch and water mixture, on the other hand, is shear-thickening. This mixture becomes a solid with increased force, and acts as a liquid otherwise, as you can see from the video above and this one here.
It is interesting to note that the Oobleck is often compared to quicksand, however, according to this article, quicksand is shear thinning, and therefore behaves in an opposing manner to the Oobleck. In quicksand, it is the force of walking on it that liquifies the substance, which is why frantic struggling will only get you deeper. See the powerful effects of quicksand, and how to escape here.
So How do you make an oobleck pool?
Oobleck is easy to make in small quantities - just add a cup of water to ~ 3/4 cups of cornstarch and kneading it with your hands until the cornstarch is mixed in and not clumpy. More detailed instructions can be found here or here.
Making a pool is a whole other story. It's messy and expensive, and a difficult clean up, but if you are really hankering to do it we will give you instructions!
You will need:
200 pounds corn starch (can be modified for different size pools)
20 gallons of water
Tarp to protect the ground
~15 heavy duty trashbags for disposal.
We started by using this kiddie pool and finding a wholesale food company who generously allowed us to open an account even though we wouldn't be making regular purchases. 500 pounds of cornstarch delivery later and we were ready to go! You can order large quantities of corn starch off Amazon, although it's not cheap.
We then rented a cement mixer. Do not even attempt to mix this by hand! Ours fit one 50 pound bag of cornstarch at a time. We used roughly 5 gallons of water for every 50 pound bag. Even with the cement mixture we would get clumps of unmixed cornstarch stuck to the sides and have to scrape them off. Test the mixture with your hands, if you move your hand slowly through the material (with the mixer off of course) it should be relatively clump free.
We ended up getting a pretty decent pool with "only" 200 pounds of cornstarch, so it took four cycles in the cement mixture. I'd recommend leaving at least an hour for set up assuming you have several strong adults.
If the pool is left untouched for a few hours the cornstarch will separate from the water and settle at the bottom - this pool is good for one-day use only.
Put down a tarp! This is an extremely messy project. The cornstarch billows everywhere and it does not come off the ground easily. The ground protection we used was inadequate and after several sweeping and hitting our patio with a hose there is still cornstarch stuck in cervices.
Disposal is another issue. We let the cornstarch sit for a couple days in an attempt to let it dry out, but it will start to rot so you can't let this go on for too long. We then double bagged heavy duty trash bags and picked it up in clumps into the bag. The trash can was extremely heavy but luckily the city still took it away.
There you have it! Everything you wanted to know about Oobleck Pools. For the unique chance to try this yourself, come to Science Fest 5K. [edit: Science Fest 5K has passed]