An Oobleck Recipe and Fun Experiments With a Non-Newtonian Fluid
What Is Oobleck?
Oobleck is a strange and very entertaining liquid. It becomes a solid when pressed, hit, or shaken and then returns to its liquid state when the stress is removed. Playing with oobleck is great fun for both children and adults. It's quick, easy, and safe to make and contains only two ingredients: cornstarch and water. I've found that students in the youngest grade of high school right through to the oldest grade love making oobleck and exploring its behaviour.
Experimenting with oobleck is not only enjoyable but also illustrates some interesting science facts. The only disadvantage is that experimenting with the material can create a mess, but even this problem can be solved (or perhaps reduced) with some careful foresight and preparation.
Green Goo in a Doctor Seuss Story
Oobleck is named after the sticky green substance in the 1949 Doctor Seuss book Bartholomew and the Oobleck. In the book, King Derwen is bored with the weather in the Kingdom of Didd and wants to see something new fall from the sky. He asks his magicians to create a magical spell to solve the problem. The spell causes green oobleck to fall on the kingdom.
The goo creates multiple problems. It traps people and animals and clogs spaces. The king wants the magicians to stop the oobleck falling from the sky, but the cave where they live is covered by the goo. Bartholomew is a page boy in the kingdom. He suggests that the king says "I'm sorry" for his foolish request. Once the king does this, the oobleck disappears.
How to Make Oobleck
1 cup of water
1 1/2 to 2 cups of cornstarch or corn starch (which is called cornflour or corn flour in the UK)
(Any ratio of about 1 part water to 1.5 to 2 parts cornstarch will work.)
- Add the water to a bowl.
- Children may like to rub some cornstarch between their fingers before they add it to the water. The starch has an interesting, silky feel.
- Gradually add the cornstarch to the water and mix with a spoon (or your hand).
- Once you’ve added 1 cup of cornstarch, add some more slowly and start mixing with your hand so that you can feel when the oobleck is ready.
- Squeeze the oobleck as you add the cornstarch. If it forms a solid ball as you squeeze and then liquifies when you stop squeezing, it's ready to use.
- If you make a mistake during mixing, add extra water or cornstarch until the oobleck forms.
Some people like to add food colouring to the oobleck for fun, but don't add too much because it can stain skin and clothing. Add a small amount of the colouring to the water before you mix the water with the cornstarch. A green colour would be fun for children who like the Doctor Seuss book. Making green goo could also be a way to introduce children to the book.
Experiments for Children
You might want children to wear an apron during these potentially messy experiments. You might also want to think carefully about where the experiments should be performed and to cover the surface that is being used. In my experience, oobleck can spread widely when used by enthusiastic students.
- Squeeze some oobleck to make a solid, then open your hand and watch the solid turn back into a liquid.
- Roll some oobleck into a ball. Open the hand that is holding the ball and watch the solid change into a liquid.
- Create an oobleck ball and try to pass the ball to someone else before it liquifies. (This is potentially a very messy activity.)
- Try bouncing an oobleck ball on the rest of the oobleck.
- Measure how long it takes for strands of liquified goo to drip into a container from a specific height.
- Rest your fingers on the surface of the oobleck and let them sink, then try to pull your fingers out quickly.
- Find out how fast you can move your fingers through the goo.
- Try using a hand or fist to slap or hit some oobleck. A large aluminum tray is good for this experiment.
- Fill a large container (or two smaller containers) with oobleck. Try walking on the goo. You will have to move your feet rapidly to avoid sinking.
The Creeping or Dancing Oobleck Experiment
Creating creeping oobleck is one of the most interesting experiments with cornstarch goo. The goo seems to have a mind of its own during this activity. To perform the experiment, a speaker that produces sounds that are łoud enough to vibrate the speaker is needed. One with strong base sounds is best. Once a suitable speaker is obtained, the rest of the process is easy.
- Place the speaker on its side.
- Cover the speaker cone with strong plastic wrap (such as that used to make a garbage bag) or with a double layer of the wrap. Make sure that the edges of the wrap are securely attached to the speaker box with strong tape.
- Plug in the speaker.
- Place a light-weight metal tray containing oobleck on top of the plastic and the speaker cone. Some people place the material directly on the plastic to get a stronger effect. Don't use a large amount of oobleck in case its weight breaks the plastic.
- Turn the speaker and the source of the music or sounds on and enjoy the strange result.
- Don’t use an expensive speaker for this process in case the plastic breaks. Watching YouTube videos of the process may be safer for equipment than performing the experiment in real life. Many people find that the plastic protects the speaker, but there's no guarantee that this will be the case.
- Once the experiment is over, turn off the speaker and unplug it with dry hands.
The oobleck forms strange, changing tendrils as it solidifies and then liquifies in response to the vibrations coming from the speaker. Watching the tendrils can be fascinating. Two of my senior students demonstrated creeping oobleck during a project on non-Newtonian fluids. They used an iPod touch to drive the speaker.
How to Dispose of Oobleck
Never pour oobleck or cornstarch down the drain. The drain may block if liquid oobleck solidifies inside it. Instead, pour or scrape the oobleck into a garbage can. The dried material becomes a powder and is easy to brush away.
Wash your containers and hands (and any other body parts or clothing covered with oobleck) only when most of the material has been removed and put in the garbage container. Warm water will help to remove oobleck remnants from hands.
What Are Newtonian Fluids?
Most fluids are classified as “Newtonian” fluids. They’re named after Isaac Newton, the famous scientist who lived from 1643 to 1726. He made many extremely valuable contributions to our present knowledge of science. Newton stated that fluids have a constant viscosity (ability to flow) if the temperature is kept constant. Applying a force or stress to the fluid doesn’t change its viscosity.
An example of a Newtonian fluid is water. If you press your hand on water in a container, the water doesn’t resist the force that you’re creating or change its viscosity and your hand falls into the water. It you try to walk on the water, you sink.
What Are Non-Newtonian Fluids?
Non-Newtonian fluids behave differently from Newtonian ones when a force or stress is applied to them. If you press, hit, or shake a non-Newtonian fluid, its viscosity changes. In some fluids the viscosity increases while in others it decreases. In oobleck, the viscosity increases with stress as the oobleck resists the applied force and the liquid becomes a solid.
Four Types of Non-Newtonian Fluids
Shear Thickening or Dilatant
Viscosity increases as stress increases
Squeezing or hitting oobleck causes it to solidify.
Shear Thinning or Pseudoplastic
Viscosity decreases as stress increases
Tomato sauce or ketchup
Shaking a bottle of thick ketchup causes the ketchup to become more liquid.
Viscosity decreases as a stress is applied over time
Continually stirring solid honey causes it to liquify.
Viscosity increases as a stress is applied over time.
Continually whipping cream causes it to become thicker.
Why Does Oobleck Solidify Under Stress?
Oobleck is a colloid, which is a mixture consisting of large but still microscopic particles suspended in another substance. Cornstarch particles are spread through the water in liquid oobleck but aren't dissolved in it. The starch particles exist as long chains.
When the oobleck is not under pressure, the cornstarch chains and water molecules slide past each other and the oobleck is a liquid. When pressure is applied, the cornstarch molecules are pushed together and water molecules are pushed out of the way. Friction increases as the cornstarch molecules come into contact. The starch molecules can no longer slide over one another and the oobleck appears to be a solid. When the pressure is removed, water moves in between the starch molecules again and the oobleck returns to its liquid form.
It should be noted that some scientists believe that there are additional processes at work in oobleck solidification. It may not be a simple process. Researchers at Georgetown University think that the study of oobleck's behaviour could be useful in the creation of soft robots and a new type of body armour.
Walking on Oobleck or Custard
Traditional custard is a mixture of egg yolks and milk which is heated until it thickens. While I was growing up, however, “custard” meant Bird’s Custard to me. This is sold as a powder containing cornstarch mixed with artificial flavour and colour.
If custard powder is mixed with water in the right proportion, oobleck forms. If you had enough of the custard oobleck you could put it in a pool, as shown in the video below. Then you could walk on custard, as the demonstration shows.
An Educational and Fun Activity
Don't let the potential messiness of playing with oobleck discourage you from performing the activity. It can certainly be a very messy activity, especially when performed by enthusiastic children (or even by eager adults). It's wonderful to have a fun and safe experiment for youngsters that can also teach them about the science of fluids, though. With some guidance from a parent or teacher, playing with oobleck can be not only fun but also educational.
© 2011 Linda Crampton