Summary: Students mix cornstarch and water to create "oobleck". Oobleck has unusual physical characteristics, that of a non-Newtonian fluid, and through exploration students better understand properties of polymers.

Estimated Time: 30 - 45 minutes [Depending on number and age of participants]

Materials Needed:

• 1 plastic bin or dishpan
• 1 clear disposable 8 ounce cup OR 1 clear disposable cup marked at 1 cup
• 1 box of cornstarch
• 1 spoon
• water
• paper towels (for cleanup)

Safety Notes: Students should not ingest any lab materials or oobleck.

Introduction:
You have been learning a lot about polymers. What polymers do you see in front of you? The plastic bin, plastic cup and paper towels are all made of polymers. The cornstarch is also a polymer. This is a polymer found in nature. It comes from corn. We are going to experiment with this to see if we can learn some more about polymers.

Procedure:

• Place the bin in front of you. Open up the cornstarch. Using the spoon, fill up the cup with corn starch (to the line if bigger than 8 oz) and dump it in the bin. Do this a second time. [So you should have 2 cups of corn starch in the bin.]
• Now fill up the cup with water and dump it into the bin. Only put one cup of water into the bin.
• Mix up the cornstarch and water using your hands. Make sure to get all of the cornstarch wet. Your oobleck should not be runny or powdery.
  • What is strange about the oobleck?
  • Can you make a ball out of oobleck? What happens if you let go of the ball?
  • What happens if you hit the oobleck fast and hard? What does it feel like? What happens if you just set your hand on top?
  • When does it act like a solid and when does it act like a liquid?

Think About It:

Elementary Level:

When does the oobleck act like a liquid and when does it act like a solid? It acted like a solid when you rolled it, pushed it, or hit it. It acted like a liquid when you moved slowly through it, held it or set your hand on top. Oobleck acts like a solid when enough pressure is applied and acts like a liquid when little or no pressure is applied.

Why do you think it acts this way? What do polymers look like? The look like long chains. Is this big or small? It's small to us but it is big to atoms. It's big if you are standing inside of the oobleck. So there are these long chains trying to move. If I press them hard and fast what happens? They get tangled up and in the way so the oobleck can't move. If I press on the chains slow, they have time to move.

Can you think of anything else that acts like oobleck? Quicksand does! So how do you think you should move through quicksand?

Any time we add a polymer to a liquid it makes it harder for the liquid to move. Usually we can't tell that it made it harder because it still moves very fast. Cornstarch is special because it slows the movement a lot! We don't know why cornstarch is so special. Maybe one day you can figure that out!

Middle/High School Level:

What caused the oobleck to act like a solid or like a liquid? Pressure. When enough pressure was applied, the oobleck acts like a solid. When little or no pressure is applied, it acts like a liquid. Why do you think oobleck acts like this? What size are polymers in the world of atoms and molecules? Polymers are very long chains. How might that influence the behavior of oobleck? When pressure is quickly or heavily applied the chains have less time to move and get tangled up. When little pressure is applied, the chains have time to slide past each other.

Adding the cornstarch to the water changed the reaction time of the liquid. Usually liquids move so quickly we do not notice how long it takes them to move. Oobleck has a finite reaction time. We can see how long it takes for it to react. We can make oobleck act like both a solid and a liquid. A liquid like this is called a non-Newtonian fluid because it does not follow the rules that Newton discovered most liquids follow. A non-Newtonian fluid has properties of both a solid and a liquid and reacts to stress with increased viscosity. Can you think of other non-Newtonian fluids? Quicksand is one example. How do you think you should you move in quicksand?

Adding any polymer to a liquid will increase its reaction time. However, different polymers will increase the reaction time different amounts. Most of the time, we will still not be able to tell the reaction time changed because it will still be very fast. We do not know why cornstarch increase the reaction time so much. Cornstarch is often appears as a special polymer. This is an area that you could research as you learn more science.

Was the creation of oobleck a physical or chemical reaction? It was a physical change. Cornstarch (amylose) and water can be considered a colloidal suspension. A colloidal suspension is a two-phase system in which the starch and water are not dissolved but simply mixed into a permanent suspension that will not settle on standing. Other examples of colloids are blood, fog, whipped cream, foams, Jell-O®, and styling gel.

Teaching Tips:

• Typically we have students complete this activity in groups of 4.
• The amounts of cornstarch and water are approximate and depend on the container used. If a larger container is used, increase the amounts of cornstarch and water used. Always maintain a 2:1 ration of cornstarch:water. Experiment with oobleck ahead of time so that you know how it should act. Sometimes students will need a little more of one ingredient. Over time the water will evaporate from oobleck, so students may need to add small amounts of water as they play.
• Oobleck can be washed down the drain. If oobleck is on tables or carpet simply let it dry. The water will evaporate and you will just be left with cornstarch that you can vacuum or wipe clean. You can let the water evaporate from the containers and reuse the cornstarch later.
• Do not stack bins or leave oobleck in closed container. Oobleck will grow mold under these conditions.

This work by The Science House is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.