The students of Duke University filled a pool with a mix of cornstarch and water to create a non-Newtonian fluid known as"oobleck". When stress is applied to the liquid it exhibits properties of a solid. Watch as they walk, run and jump on this amazing fluid! But don't slow down!
Fluids Behaving Strangely
Key Concept: Non-Newtonian fluids are so named because their properties cannot be described in terms of the concepts of classical fluids. Unlike normal Newtonian fluids, these materials possess properties that depend on how gently or strongly they are stirred or pulled. The study of the flow of materials that behave in this unusual manner is known as rheology.
Quicksand is a common example of a non-Newtonian substance that tends to solidify when placed under stress. The harder a person thrashes around to get out, the worse matters become. Ketchup, on the other hand, behaves in the opposite way. The more it's shaken, the more readily it flows.
Oobleck Station Reading is FUNdamental Pittsburgh, Junior League Member VolunteersWhen some non-Newtonian fluids experience a sideways force known as shear, they tend to solidify. A mixture of cornstarch and water is such a fluid. Known to many as Oobleck, this strange substance offers students an opportunity to become amateur rheologists. To make Oobleck, students will need 1 cup of cornstarch, 1/2 cup of water and, if desired, food coloring. Instruct them to:
- Put cornstarch in bowl.
- Slowly and while stirring (hands are fine) add the water.
- Add food coloring as desired.
Once your students have made their Oobleck, you may wish for them to try the following experiments.
- Have them test the Oobleck by hitting it hard, then softly. They should then stir it quickly, then very slowly.
- After pouring some of the Oobleck on the table, have students push on the puddle with the side of their hand. The Oobleck will become solid with the application of a force, but will return to its liquid state as soon as the force is removed.
- Have students attempt to pick up some Oobleck. Once they have it in their hands, ask them to try to keep it in solid form by continually kneading it.
- Have students play catch with Oobleck. They will notice that as soon as they stop kneading the Oobleck it will return to its liquid state. This is very obvious as it flies through the air and is caught.
For something that is sure to delight your students you may wish to have them…
Shake it Up: Animating Oobleck with Sound Waves
As you have seen, when stress is applied to a mixture of cornstarch and water it exhibits properties of a solid. Especially interesting is the effect produced when the mixture is disturbed at certain frequencies.
To produce effects that have to be witnessed to be believed, you will need a function generator, an amplifier, a subwoofer and a dish or pie tin containing a mixture of cornstarch and water (Oobleck). You may wish to begin with a mixture of two parts cornstarch and one part water.
Support the container containing the Oobleck over the top of the subwoofer. Begin your experimentation with a 50 Hz signal and adjust until fingers of Oobleck begin to rise from the surface of the liquid. Here are some video examples of some incredible phenomena resulting from the acceleration of Oobleck with sound.