When you mix regular off-the-shelf cornstarch with regular from-the-tap water you get yourself a physical anomaly. A substance that can be both a liquid and a solid at the same temperature.
It’s called a non-Newtonian fluid because it doesn’t behave according to the what Sir Isaac Newton discovered about the viscosity (flow) of liquids. In this case the corsntarch-water goo, often called “oobleck”, will slowly pour like a thick fluid under normal circumstances, but if stress is added, the fluid will firm up and break like a solid.
If you have the budget to fill a swimming pool with it, you can easily walk straight across the top as each footstep applies force to the oobleck, firming it up under your foot. However, if you stop, you will sink in, and have a very difficult time getting out. Any attempt to pull yourself up will again firm up the fluid in your path.
You can make some for yourself. Put some cornstarch in a bowl, and slowly stir in water. Keep adding water and stirring slowly until you feel it thickening up. The approximate recipe is 2 parts cornstarch to 1 part water, and just enough food colouring to make it more exciting than mathematics. You’ll know when you get it just right as you’ll be able to pick it up, and roll it into a rubbery ball in your hands. But the moment you stop rolling it will melt and drip back into the bowl. Or, more likely, all over the floor. This is messy science.
The reason this happens is that the structure of cornstarch comes in long chains of atoms bonded together. This is called a polymer. When things are flowing slowly, the chains can slip past each other, but as things speed up the chains get tangled and stuck making the structure more solid. It’s like trying to run through a crowd of people. You have to move slow if you want to get anywhere.
Watch this demonstration of some oobleck placed on a speaker cone. A low-frequency hum is put through the speaker, causing vibrations. As the stress is applied to the fluid it begins to come to life, building solid structures.
Other everyday non-Newtonian fluids included silly putty, which can stretch or snap depending on how fast you pull it.
Also, ketchup… which explains why it’s so slow to come out of the bottle. It requires a certain amount of force (usually gravity) before it will start flowing, but once it starts it moves easier. Whenever you drown your french fries in a sudden splurge of ketchup, it’s not Newton’s fault.
- Source: Goo Recipe – Colorado State Physics