Oobleck

While playing with Oobleck students should identify that normally solids have a definite shape whereas a fluid can change shapes because it flows.

Oobleck doesn't quite follow the rules, this suspension only behaves as a fluid some of the time. For this reason oobleck is known as a non-Newtonian Fluid. If you apply a force to it by smacking or squeezing it this fluid will become a solid.

A non-Newtonian fluid is a fluid that does not follow Newton's law of viscosity. In non-Newtonian fluids, viscosity can change when under force to either more liquid or more solid. Ketchup, for example, becomes runnier when shaken and is thus a non-Newtonian fluid.

The explanation for the strange behaviour of Oobleck lies in the shape of cornstarch particles, which are long and thin. When cornstarch mixes with water, the starch does not dissolve, but remains in suspension. Move the mixture slowly, and the particles slide past each other. Move it quickly, and the particles tangle with each other so that the mixture hardens.

Using a material that doesn't behave according to the 'rules' is a great way to get students to explore what they already know about the difference between solids and fluids.

Objectives

• Investigate the properties of a non-Newtonian fluid.
• Describe the properties of a solid and a liquid.

Materials

• Per Pair or Group:
  cornstarch
  water
  tub
  mallet
  food colouring (optional)

Key Questions

• What do fluids and solids do, what determines their shape, and can they be compressed?
• How is Oobleck like a fluid?
• How is Oobleck like a solid?

What To Do

1. Dump 3 or 4 boxes of cornstarch into a shallow container.
2. Add about a litre of water, slowly, and mix thoroughly. At just the right consistency, a handful of fluid will dribble out of your hand and solidify if you squeeze it. It will flow again soon after being released.
3. Let students play with the Oobleck — punch it or hit it with a mallet, it doesn’t splash. Slowly sink your hand into it and try to pull your hand out quickly.

Teacher tip: For coloured Oobleck, add food colouring to the water before you mix it with the cornstarch.

Extensions

• Can you think of any fluids that flow better when pressure is applied? What advantages does this type of liquid have? (Latex paint and ketchup are non-Newtonian fluids of the opposite variety to oobleck. Paint will flow off a brush onto a wall but won't easily drip or run. Ketchup won't pour out of containers unless smacked, squeezed or shaken).
• Oobleck is a non-Newtonian fluids of the opposite variety ketchup, and in a bottle it should behave opposite to ketchup- it will flow out easily when inverted but won't flow when you smack, squeeze or shake it. Give it a go!
• Try to paint with oobleck, it will be difficult to smear and will then run away.