Oobleck Can't Make Up Its Mind

Try this: mix one cup of cornstarch with half a cup of water. Stir and stir and stir until all the cornstarch is dissolved. (You can add food coloring, too, if you want.) Take one finger and push on the mixture slowly. Your finger will sink right in. As long as you keep moving the mixture slowly, you can play with it like it’s mud.

Now try pushing on the mixture again, but this time push really hard and fast. It feels like you’re pushing on concrete! The same mixture changed depending on how fast you pushed on it. This crazy substance is called oobleck, and it can’t decide if it’s a solid or a liquid.

Oobleck is a funny word to say, but it’s an even funnier substance to play with. Usually materials are one of the three states of matter: solid, liquid, or gas. But oobleck can seem like both a liquid and a solid. How does this happen? Well rate — how fast or slowly a force is applied — makes a big difference for a substance like oobleck. This is because it’s what’s known as a non-Newtonian fluid.

Newtonian fluids get their name from Sir Isaac Newton, who was a well-known mathematician and scientist. He was the first person to describe how the liquids behave using math. These are ordinary types of fluids, like water and air. (Scientists use the word “fluid” for both liquids and gasses.) No matter how you push on them, these fluids always act the same way.

Non-Newtonian fluids, on the other hand, are weird. These are the types of fluids like oobleck. They change depending on how they’re being pushed. All non-Newtonian fluids change with rate.

But non-Newtonian fluids don’t just exist in science experiments. They’re more common than you might realize. Non-Newtonian fluids are all around us.

The next time you squirt ketchup from a bottle, watch the last drop. If it were a bottle of water, the last drop would fall down. But the last drop of ketchup pulls back into the bottle! It’s almost as if the ketchup was acting a little bit like a rubber band. It pulls up and into the bottle, because ketchup is a non-Newtonian fluid, just like oobleck.

Another frustrating thing about ketchup is getting it out of the bottle. You can tip the bottle upside down, but for a long time, nothing happens. Then, it seems like a whole bunch of ketchup comes out at once! What is going on?

It’s all a matter of what ketchup is made of. Ketchup is made by mixing tomato paste and sugar with some other ingredients. When these are mixed together, microscopic molecules form long chains called polymers. Usually these polymer chains are tangled together. They stay tangled until enough force is applied to stretch them out.

When the bottle is upside down but no ketchup is coming out, the long chains of molecules are tangled together. When there’s enough force from gravity after the bottle is upside down, the chains become organized. Unfortunately, all the molecule chains get organized at once. Lots of ketchup comes out at the same time!

Your body is full of non-Newtonian fluids. Have you ever spit out toothpaste and found a line of sticky saliva stretching down with it? That wouldn’t happen if saliva acted like water. Saliva is a non-Newtonian fluid. Blood is also a non-Newtonian fluid. Red blood cells, white blood cells, and platelets float in plasma. This combination of parts makes blood act differently than a normal fluid. The liquid inside your joints that helps cushion motion is also a non-Newtonian fluid. These fluids all change how they behave depending on how they’re being pushed. It’s part of what makes our bodies work well.

There are lots of weird and wacky ways that fluids behave all around us.