In this demo experiment from our school workshop series, you can show your students the weird and wonderful physics of corn starch particles when mixed with water, and how they defy the normal laws of states of matter.
Read on to find out more!
You will need
Corn flour/corn starch, water, a bowl, food colouring (optional for fun!)
Watch the video and try this at home!
(Video made by Tara Ryan)
Here’s the science…
Think of the an Olympic stadium, full of visitors anxious to get to the next event. If you try to run through the crowd, you’ll quickly find yourself bumping into people and causing a pile-up. The most efficient way to get to the event is to move slowly through the crowd, twisting and turning past people. Cornstarch molecules are big and act like a crowd of people: they move slowly round each other in water. If the movement is too fast the big
starch molecules bunch together giving the solution ‘solid’ properties.
Corn flour or corn starch is made of lots of long, stringy particles. When water is added they do not dissolve in water, but they do spread themselves out. This allows the Oobleck or Gloop to act both like a solid and a liquid. When you roll the mixture in your hands or apply pressure to it, the starch grains or particles rub against each other and lock into position, and the mixture feels solid. But if it is left to rest or is held up and allowed to dribble, the particles slide over each other and it feels like a liquid. The phenomenon that lets oobleck do what it does is called “shear thickening,” a process that occurs in materials made up of microscopic solid particles suspended in a fluid. This is because the high surface tension of water causes water droplets to surround the starch granules. Water acts as a liquid cushion or lubricant, allowing the grains to flow freely.
When you dispose of the Oobleck or gloop, don’t just pour it down the drain! Dilute the mixture with lots of water first and then pour it away.
This will prevent it clogging our pipes somewhere along the way. Or put the mixture in a bag and put it straight in the bin.
Did you know?
Oobleck and other pressure-dependent substances (such as Silly Putty and quicksand) are not liquids such as water or oil. They are known as non-Newtonian fluids. This substance’s funny name comes from a Dr. Seuss book called Bartholomew and the Oobleck!