What is it about goo that is so fascinating to humans? Moreover, when we find out we can make goo in our own kitchens, why are we so compelled to do so?
Experiment: Make a liquid turn into a solid by tapping it.
Add 1/4 cup of dry cornstarch to the bowl. Add about 1/8 cup (2 tablespoons) of water to the corn starch and stir slowly. Add water slowly to the mixture, with stirring, until all of the powder is wet. Continue to add water until the cornstarch acts like a liquid when you stir it slowly. When you tap on the liquid with your finger, it shouldn’t splash, but rather will become hard. If your mixture is too liquid, add more cornstarch. Your goal is to create a mixture that feels like a stiff liquid when you stir it slowly, but feels like a solid when you tap on it with your finger or a spoon.
What Did We Observe?
Short Answer: I don’t know, but it is awesome!
Slightly Longer Answer: A non-Newtonian fluid
Scientific Answer: All fluids have a property called “viscosity” which dictates how well the flow. Water has a low viscosity, while honey (at room temperature) has a higher viscosity. If we were to heat the honey two things would happen: First, it would become even more delicious…
Second, the viscosity would drop, and it would flow easier than it did previously. These fluids, honey and water, are called Newtonian Fluids, because they behave in a manner that was first described by Sir Isaac Newton (when he wasn’t busy discovering gravity).
But while Newton was a genius and laid the foundation for much of modern physics, his method of describing viscosity does not apply to every fluid. Some fluids, like ketchup, silly-putty and our corn-starch/water mixture’s viscosity is dependant not only on temperature, but also on the force applied to it.
This is because at a microscopic level, these non-Newtonian fluids have liquids and granules. Think of a busy sidewalk. The best way to move through a crowded sidewalk is to slowly side-step the people in your path. If you were to try and run through the crowd, you would bump into someone and be stopped. You would also be a jerk.
Apply force to the non-Newtonian fluid is very much the same thing. If you apply too much force, the granules in the fluid will grind together and increase the viscosity of the fluid. This will make the area where force is being exerted more solid than liquid. This makes Isaac Newton cranky.
Andrew Baxter is a Secondary School Teacher and Curriculum Specialist with Enable Education. Like Isaac Newton, he is sometimes upset by unpredictable viscosity.
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