Is Oobleck a Solid or a Liquid?

Table of Contents

If you filled a swimming pool with a substance called oobleck, you could easily run across the pool as if it were a solid surface. But if you tried to stand in the oobleck pool you would slowly start to sink. So, what is this strange substance that is both solid and liquid at the same time? To this, Dr. Seuss would like to say, “Won’t look like rain. Won’t look like snow. Won’t look like fog. That’s all we know. We just can’t tell you anymore. We’ve never made oobleck before.”

Different bright slimes on white marble background
Different bright slimes on white marble background/Credit: 123rf

So, let’s make some oobleck and unravel its strange properties. It holds the answer to some age-old questions, like: Why is ketchup so hard to pour? and, Can you walk on chocolate pudding?

What is oobleck? How is it made?

First, let’s address the fact that “oobleck” sounds like a made-up word that means nothing, because it is. The term “oobleck” first appeared in Dr. Seuss’s book Bartholomew and the Oobleck, an excerpt from which is mentioned above. In Dr. Seuss’s world oobleck is described as a green gooey substance that falls from the sky. In reality, it doesn’t fall from the sky, but worry not—you can easily make some in your kitchen.

Green goo, not from the sky but from your kitchen/Credit: Andrew Curran , Lisence: CC BY-ND 2.0

To make oobleck at home you will need:

 

a) 2 cups of corn starch,

b) 1 cup water, and

c) a few drops of food coloring (if you want your oobleck colored, or else white works just fine).

 

Now, put your corn starch in a bowl and slowly add the water (or colored water) until the mixture becomes a thick paste. If the mixture becomes too runny add more corn starch, and if it becomes too thick adjust it with more water. Your oobleck is now ready!

 

Now, if you pick up some of it from the bowl and let it rest on your palm, you’ll observe that, just like any other liquid, it drips back into the bowl. On the other hand, if you pick up the oobleck and squeeze with your hand, you’ll observe that it turns into a solid lump. But the moment you let it rest on your palm it turns into a liquid again. And just like any other liquid, you can easily pour it into another container and the oobleck will take its shape. But if someone tried to punch the oobleck with their fist, it would feel like punching a solid wall (please don’t try this at home).

 

So, how is it that the same substance sometimes acts like a solid and sometimes as a liquid?

Green goo, not from the sky but from your kitchen
Walking on oobleck turns it from liquid to solid/Credit:Kelly Hofer, Lisence: CC BY-ND 2.0

Non-Newtonian Fluids: To Flow or Not to Flow

Before we get into non-Newtonian fluids, we need to get familiar with a very important property of liquids—viscosity.

 

If you look around in your home you will find a wide range of fluids, like water, oil, dish soap, or honey. From our day-to-day experience, we know that each of these liquids flows at a different rate. It is much easier and quicker to empty a bottle of water and oil than dish soap and honey. That is because soap and honey have a higher viscosity than the other two liquids. In scientific lingo, the term “viscosity” stands for the measure of a fluid’s resistance to flow.

Viscosities
Left: Low Viscosity; Right: High Viscosity/Credit: Wikicommon/Synapticrelay/Lisence: CC

But where does this resistance come from?

 

In solids, the molecules are packed together in a fixed structure which makes them rigid. But in fluids like honey or water, the molecules are not packed in a fixed manner. This allows the different layers of fluid molecules to slide off each other, allowing them to flow. Even though loosely packed, these molecules still have to face intermolecular attraction. The attractive forces then create friction between different fluid molecules, which gives rise to resistance. So, the stronger the intermolecular attraction, the greater the friction, and the slower and stickier the liquid.

 

Now we come to Sir Isaac Newton and his law of viscosity. He stated that at a given temperature a fluid has a constant viscosity and is independent of shear rate . In other words, water’s viscosity will not change even if you try to create friction by stirring or shaking it. Most liquids around us, like oil, milk, alcohol, and of course water, follow this rule and are therefore known as Newtonian fluids.

 

But then there are fluids like oobleck that change their viscosity when a force is applied but return to their original state as soon as the force is removed. This is why squeezing oobleck turns it into a solid ball but it then goes back to liquid when you open your palm. Rather than increasing with force applied, the rate of flow decreases instead. This behavior goes against Newton’s law of viscosity, which makes oobleck… non-Newtonian fluid!

 

The unpredictable nature of non-Newtonian fluids isn’t just fun, but can also be put to good use. For instance, oobleck can be used to temporarily fix potholes. Fill the pothole with oobleck and as soon as a car passes over it, it turns solid, making car rides much smoother. However, these weird properties also give rise to our ketchup and chocolate pudding dilemma.

Chocolate pudding
Chocolate pudding/Credit: 123rf.com
ketchup show strange fluid properties
ketchup show strange fluid properties/Credit: 123rf.com

The Delicious Dilemma: Pudding and Ketchup

When you jump into a pool, you apply force to the water, and as a reaction, the water splashes back at you. This is due to Newton’s third law of motion: every action has an equal and opposite reaction. If you replace the water in the pool with chocolate pudding and jump on it, you might not get the splash you expect. That’s because the main ingredients of chocolate pudding are corn starch, milk, cocoa powder, and sugar. The first two ingredients make it a non-Newtonian fluid like oobleck. As soon as you jump into a pudding pool it turns solid due to a process called shear thickening.

 

Hence, the answer is: Yes, you could walk on a chocolate pudding!

 

Now, moving on… why is ketchup so hard to pour?

You have a hot plate of fries, so you open your ketchup bottle to pour some. You could end up in these two scenarios: 1) You have no ketchup on your plate because it won’t leave the bottle; 2) You have way more ketchup on your plate than you need. There is no in-between. These two problems arise because our beloved condiment is also a non-Newtonian fluid.

 

Ketchup is made up of tomatoes that have been turned into paste. When you tip the bottle and tap it a little bit, this small force causes tiny tomato particles in the paste to shift a little; not enough to leave the bottle but enough to get in each other’s way to create a traffic jam of tomato particles. This causes the ketchup to act like a solid, and therefore it can’t leave the bottle.

 

But the moment you shake the bottle with more force, the ketchup just flows out like any other liquid. This happens because ketchup, as a non-Newtonian fluid, undergoes a process called “shear thickening,” which is the opposite of what happens in oobleck. The more force you apply the less viscous, or the thinner, the fluid gets. Hence, the uncontrolled flowing of ketchup.

 

So, how do we deal with this problem? First of all, instead of turning the bottle completely upside down, hold it at an angle. Then, slightly tap on the sides of the bottle to prevent both the traffic jam and the surprise ketchup flow. Or, you could just use one of the greatest inventions of all time: a reusable squeeze bottle!

Transfer your ketchup into a squeeze bottle and you are good to go
Transfer your ketchup into a squeeze bottle and you are good to go/Credit:123rf

Glossary

Shear: the deformation caused in a substance when two parallel internal layers of the substance slide past each other.

 

Shear rate: the rate at which shearing deformation occurs in a substance. It is also defined as the difference in velocity between two fluid surfaces divided by the distance between them.

Flesch Kincaid Grade Level: 6.7

 

Reading Ease: 74.6

Oozing oobleck activity – Science Museum Group. Learning. https://learning.sciencemuseumgroup.org.uk/resources/oozing-oobleck/ (Accessed September 9, 2022)

 

Oobleck’s weird behavior is now predictable. MIT News | Massachusetts Institute of Technology. https://news.mit.edu/2019/oobleck-behavior-predict-cornstarch-1006 (Accessed September 9, 2022)

 

Evans, J. (2021, July 23). Why can we walk on custard? Chemistry World. https://www.chemistryworld.com/news/why-can-we-walk-on-custard/5212.article (Accessed September 9, 2022)

 

No hit wonder. Acs.org—Chemmatters. https://www.acs.org/content/dam/acsorg/education/resources/highschool/chemmatters/issues/2016-2017/February%202017/chemmatters-feb2017-d3o.pdf (Accessed September 9, 2022)

 

Ouellette, J. (2012, September 5). Walking on. . . Custard? Fun with Non-Newtonian Fluids. Scientific American Blog Network. https://blogs.scientificamerican.com/cocktail-party-physics/walking-on-custard-fun-with-non-newtonian-fluids/ (Accessed September 9, 2022)

 

Zaidan, G. (2014, April 8). Why is ketchup so hard to pour? TED-Ed. https://www.youtube.com/watch?v=KB43fM_ozKQ (Accessed September 9, 2022)

Contributors

  • Sanjukta Mondal
    : Author
    Sanjukta Mondal (she/her) has a Master's degree in Industrial Chemistry and loves to explore the world through the lens of chemistry and her trusty camera. During postgraduate research, she worked on the development of functional materials for battery technology and water treatment. Currently, she is trying to navigate through the labyrinth of scientific writing and science writing, fuelled by coffee and her fascination for the extraordinary science and stories hidden behind everyday objects. Writing for SmoreScience will allow her to explore the enthralling world of science and share it with other curious minds.

Copyright @smorescience. All rights reserved. Do not copy, cite, publish, or distribute this content without permission.


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