Who doesn’t love a good non-Newtonian fluid – once they know what it is? For the uninitiated, you are encountering a non-Newtonian fluid when you shake a long-necked bottle of ketchup and find it impossible to get any to come out of the bottle.
Non-Newtonian Fluids in Action
When you pour from a container of milk or water, you tilt the container and watch the liquid flow out freely. It doesn’t matter whether the container has a narrow neck, like an older ketchup bottle, or no neck, like a gallon of milk. You tilt; the liquid flows. You’re not tempted to hit the bottom of the container to start or increase the flow because if you did, it wouldn’t make a difference in the rate of flow.
To start the flow from the long-necked bottle, you need to disturb the mass of the ketchup so air can flow in and displace the fluid; slightly tilting it or smacking it does nothing. Even with a plastic bottle, you need to squeeze the bottle to start the ketchup flowing. The ketchup just sits there in either bottle because it’s not the shape of the bottle that keeps it from flowing; it’s ketchup’s properties as a non-Newtonian fluid.
What’s Newton Got To Do With It?
In the video above, you can see that shaking the bottle does no good. If the girl were to smack the bottom of the bottle, that wouldn’t make the ketchup flow, either. It isn’t until the bottle is held at an angle that makes it possible for the air to displace the ketchup that the ketchup flows. This is because ketchup is a Non-Newtonian fluid – one in which adding pressure makes the fluid less viscose – less able to flow freely.
A Newtonian fluid is one in which adding pressure makes no difference; the fluid is still going to flow. Pressure does not make the liquid act as if it is a solid. That a liquid could act as a solid sounds impossible until you realize that fluid has layers and those layers move relative to one another due to shear stress.
In fact, shear rate is at the heart of it all. If a fluid is not affected by the shear rate – the rate at which the layers of a fluid move past each other – it is a Newtonian fluid. Case in point – you can’t walk across a swimming pool full of water because shear does not affect viscosity.
By Duk at the English-language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4168566
If a fluid is affected by the shear stress, you can make your way across the surface of the pool of liquid by applying force. You may need to jump or stamp your feet, depending upon what was used to create the mixture, but you can make your way across the liquid surface without sinking, just as you’ll see in the video below.
What Piqued My Interest
While writing a book about modeling ships and spacecraft for Springer, I came across a description of Non-Newtonian fluids. The idea that a fluid could be in a liquid state and act as both a liquid and a solid without modifying the fluid or the state of the fluid, was hard to believe. I read about mixing corn starch with water to achieve the optimal balance, but it didn’t occur to me to start small, and the thought of creating a pool of water/corn starch mix wasn’t practical. That didn’t keep me from being curious. And then, I found the video…
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