I love to use everyday familiar items to explore complex scientific ideas. If you tell your kids that non-Newtonian fluid is a liquid with non-constant viscosity, they will probably yawn and forget. As scientists have discovered, we only retain about 5% of what we hear. If you introduce a relevant demonstration, the retention jumps to 30%, but learning by doing has the most remarkable effect. It leads to 70% retention and recall.
I first thought of making homemade peanut butter with my kids as a way to explore the changing states of matter: solid to liquid. When you take a handful of peanuts and apply a force on them by mashing them in a blender, you get butter, which is a liquid, right? Or is it?
If you take a jar of freshly made peanut butter and flip it upside down, would it all flow out? Nope! But if you scoop up a bit of peanut butter with a spoon and put it on your cracker, does it flow? You bet! If you try to eat it, it might even drip on your shirt.
So, it turns out that peanut butter is a great example of non-Newtonian fluid. One minute it behaves like a solid, and the next it flows like a liquid. Non-Newtonian fluids can switch between a solid and liquid state depending on the forces acting upon them.
Do you know what peanut butter, toothpaste, quicksand, and blood have in common? You probably have guessed it by now; they are all examples of non-Newtonian fluids!
Do you want to join us in making peanut butter while learning about states of matter and non-Newtonian liquids? All you need are peanuts!
A Homemade Peanut Butter
What you need
Food processor (blender didn’t work as well for us)
Mason jars (I like lots of small ones so that we can make different flavors)
(Optional) Baking sheet (if you want to toast peanuts before blending)
What to do
If you don’t mind an extra step, toast your nuts, even if you use roasted peanuts. Toasting nuts gives peanut butter an incredibly deep flavor and even slightly changes its consistency. But you can skip this step. It will be tasty anyway.
- If you are toasting peanuts, heat the oven to 325F. Place the nuts on a baking sheet and toast for no more than 10 minutes. Be warned: peanuts burn fast, so don’t leave them in the oven too long, and keep an eye on the color. You want a slightly golden tint. The nuts will also begin to look wet and glossy with oil.
- Put peanuts in a food processor and run it for thirty seconds. Scoop out about half a cup of peanuts and save them. We’ll need them for our experiment later.
- Scrape down the bowl’s sides and process for an additional minute (or two or three). The longer you go, the smoother the consistency.
You might or might not have noticed how the quantity of peanuts is decreasing with each consecutive photo. No magic involved. Just lots of tasting…. You’ve gotta get it right, you know.
- Transfer half of your peanut butter into jar #1. Add the rest of the peanut butter to jar #2 along with the reserved half a cup of nut pieces (remember?). Cover. Refrigerate for Peanut Butter Investigation we will be doing below. We are done unless…..
- Unless you like to play with flavors as we do. You can take some of your peanut butter and blend it with maple syrup, another nut butter (hazelnut is my favorite), cocoa powder, cinnamon, or raisins. But what we love the most is something a little bit more decadent….
Yep, chocolate…. Just break it into pieces and drop it in the food processor along with your peanut butter.
And you end up with Chocolate Peanut Butter… the yummiest thing ever! (Piece of advice: hide it before it’s all gone)
Lunch break! We’ve got more tasting to do.
The Science Behind Peanut Butter
Before we move on to a hands-on peanut butter investigation, let’s talk about science.
Peanut butter is a fascinating substance. It tastes good (to most people), has great nutritional value, and stays fresh in your pantry for months without any refrigeration. What’s more, it has some amazing properties. It can act as a solid or as a liquid, depending on the external conditions. It has what scientists call variable viscosity.
Viscosity refers to fluid’s resistance to flow. Low viscosity means the fluid flows easily. High viscosity means the fluid resists the motion. Newtonian fluids, like water and oil, have a stable viscosity. Water flows out of a bottle the same way whether you stir it with a spoon beforehand or not. However, it’s a completely different story with honey, which is a non-Newtonian fluid. The more you stir the honey, the easier it is to get it out of a jar.
Non-Newtonian liquids, like ketchup, blood, and honey, have variable viscosity. To get ketchup to flow out of a jar, you need to shake it. When the force is removed, it returns back to its original state of high viscosity.
Types of non-Newtonian fluids
However, not all non-Newtonian liquids are created equal. Some fluids, like ketchup, blood, and motor oil, become less viscous. In other words, they start acting like a liquid when mechanical stress (or shear) is applied to them. They are called shear-thinning fluids. Some fluids, on the other hand, become more viscous. They start acting like a solid when agitated. A great example of a shear-thickening fluid is an oobleck.
Making oobleck is a must-do hands-on activity to learn about the properties of non-Newtonian fluids. Oobleck is ¼ cup of cornstarch mixed with ¼ cup of water. When you roll it into a ball in your hand, it takes the shape of a ball, but as soon as you release the pressure and relax your hand, it flows through your fingers like a liquid. In other words, when you apply pressure to oobleck, it mimics the properties of solids, and when you release the pressure, it mimics the properties of a liquid.
It’s time for our peanut butter investigation.
Warning: If you use store-bought peanut butter for this investigation (and not homemade butter), your results might vary from ours. The addition of salt, oil, and/or preservatives in many commercial brands changes the properties of butter in significant ways. Plus, many brands put their peanut butter through a process of hydrogenation (it prevents oil separation) and causes it to behave differently.
Peanut Butter Investigation
- Take a spoon of water and a spoon of peanut butter. Pour it out (or try to). Which one acts more like a liquid? (Answer: water)
- Put a spoonful of peanut butter on a plate. Put a peanut (or other nut) on another plate. Push on each with an index finger. Which one acts more like a solid? (Answer: nut)
- Scoop a spoonful of peanut butter out of a jar. Note the difficulty with which it was done. Now, with a spoon, vigorously stir the rest of the peanut butter in the jar for a minute. Scoop another spoonful of peanut butter. Did it become easier or harder to scoop it out? (Answer: easier)
- Simple Viscosity Experiment. Take two jars of peanut butter (we put them in the fridge after blending): one smooth, the other with chunks of nuts. Do a series of experiments to see which one flows more easily. You can scoop it with a spoon, spread it on toast, and flip a jar upside down. (Answer: the more chunks of nuts in the butter, the more like a solid it will act).
- Temperature experiment. Take three pieces of bread. Put one piece on a plate in a fridge. Toast the second piece. And leave the third one at room temperature. Spread the same peanut butter on all three pieces. Which toast has the runniest butter? (Answer: the toasted slice, if you applied butter while it was still hot).
- Comparison experiment. If you made more than one flavor with us, do this simple experiment. Scoop a spoon of each and turn them over your plate (or over your open mouth if you’re very patient). Which one takes longer to flow out? The more it resists the flowing motion, the higher the viscosity. (Note: If you didn’t do a different flavor, you can use a jar of store-bought peanut butter for comparison).
Are you interested in learning more about viscosity? We recently did a fun experiment to compare the viscosity of maple syrup, honey, ketchup, and thickened pie filling. Stay tuned!