How can this guy walk across this pool of “water?” Find out how this amazing demonstration works and then try it at home!
It’s a liquid. No, it’s a solid! Actually, what you have seen in this video is a Non-Newtonian Fluid. This particular Non-Newtonian Fluid is a mixture of cornstarch and water sometimes called Oobleck after the Dr. Seuss book Bartholomew and the Oobleck. You can try making it at home, just mix 1 part cornstarch to 2 parts water and have fun!
But, what is a Non-Newtonian Fluid and how does it behave like a solid and a liquid at the same time?
What is a Non-Newtonian Fluid?
To answer this question, it is best to first understand what a Newtonian Fluid is. We all know Isaac Newton for his work with gravity, however, he practically wrote the book (well actually books) on science. He made observations and derived conclusions on basically everything. One of the areas of science he studied is called fluid mechanics which is an area of physics that studies fluids and the forces that act on them. Essentially, Newton proposed that the viscosity of a fluid will not change with the amount of stress placed on that fluid. So that means that if I was pouring water out of a container, the water could not suddenly pour with the viscosity of honey if I started to squeezing it.
How does a Non-Newtonian Fluid behave like a Solid and a Liquid?
A Non-Newtonian fluid on the other hand, does change its viscosity with the amount of stress placed on that fluid. The viscosity of a fluid is a measure of the resistance of a fluid to deform its shape. Generally, this deformation is quantified by the strain rate of a fluid, which measures the rate of deformation (how quickly the fluid flows) over time.
There is much more we can discuss about fluids to better understand why Non-Newtonian fluids behave the way they do. Like the viscous stress tensor, which is unique coefficient that will describe the strain rate of a fluid. But we will keep it simple.
So here is the bottom line. A Newtonian fluid will not change its ability to pour, however, a Non-Newtonian fluid will change its ability to pour if you “stress it out.” Think of it as if you were running a 400 m race. If you behaved like a Newtonian fluid, you would run at the same rate no matter how many people were screaming at you to run faster. On the other hand, If you behaved like a Non-Newtonian fluid, the more people screaming at you to run faster the slower you would actually run.
The Oobleck in the video is Non-Newtonian because its viscosity greatly increases as stress is applied. The Oobleck behaves almost as if it was a solid as people run and jump on it. Essentially, the harder you could impact the Oobleck, the more solid it should become while if you do not “stress it out” it will remain a liquid with a relatively low viscosity. Make sure you try it at home. All you need is corn starch and water!