Newtonian vs. Non-Newtonian Liquids

By Megan Ray Nichols

If you’ve seen any viral videos in the last few years, you’re probably familiar with the concept of non-Newtonian fluids — liquids that are fluid when moving slowly but when struck with force, they take on a solid consistency. Videos have gone viral of people filling entire swimming pools with a mixture of water and cornstarch, allowing them to literally run across the surface of the water. What is the difference between a Newtonian fluid and its non-Newtonian counterpart, and where might you encounter these fluids in your daily life?’

Newtonian vs. Non-Newtonian Liquids

First, what is the difference between Newtonian and non-Newtonian fluids?

Newtonian fluids have a constant viscosity that doesn’t change, no matter the pressure being applied to the fluid. This also means they don’t compress.

Non-Newtonian fluids are just the opposite — if enough force is applied to these fluids, their viscosity will change. These fluids are broken up into two categories — dilatants, which get thicker when force is applied, and pseudoplastics, which get thinner under the same circumstances.

These can be further broken down into rheopectic and thixotropic categories. Rheopectics work like dilatants in that they get thicker when force is applied. Thixotropic materials get thinner, like pseudoplastics do. The difference here is that the latter two categories are time dependant. The viscosity doesn’t change immediately but changes slowly over time as more and more force is applied.

Newtonian Fluids in Daily Life

These fancy names might sound like something out of a science fiction novel, but they’re really just the scientific names for things you encounter in your daily life. What Newtonian fluids have you encountered today?

If you took a shower this morning or had a drink, then you’ve already encountered the most common Newtonian fluid — water! Water does not change viscosity no matter how much pressure you put on it — it also cannot be compressed, so the amount of pressure you can put on water as a Newtonian fluid is negligible.

Other common Newtonian fluids include mineral oil, alcohol and gasoline.

Non-Newtonian Fluids in Daily Life

For this section, we’re going to break it down into the four categories of non-Newtonian liquid that we listed above.

Dilatants are probably the most well known nonnewtonian fluids. They become thick or almost solid when force is applied to them and are made up of water mixed with other materials. Oobleck, the colloquial name for a mixture of water and cornstarch, is probably the most well-known, but quicksand and silly putty also fall into this category.

Pseudoplastics might not sound very appetizing, but you probably have a bottle of one in your fridge right now. That’s right — ketchup is a non-Newtonian fluid. The fact that the viscosity changes as each new ingredient is added to the mix makes it tricky to mix ketchup on a large scale.

Now we get into the weird non-Newtonian fluids.

Rheopectic fluids get thicker in relation to the pressure being applied to them and the time that the pressure is being applied. The best example of a rheopectic fluid is cream. With enough time and pressure, cream becomes butter.

Thixotropic fluids are similar to pseudoplastics in that they get thinner as pressure is applied to them, but it’s also dependant on the time that the pressure is being applied. Things like cosmetics, asphalt and glue all fall into the thixotropic category.

It might seem like this is useless information, but it can actually be very useful, especially if you’re ever in a restaurant that still uses glass ketchup bottles. Simply remember that ketchup is a non-Newtonian pseudoplastic and will get thinner as more force is applied to it. Give that bottle a couple of good thumps, and you’ll be in French fry heaven.

Sources:

https://www.youtube.com/watch?v=RIUEZ3AhrVE

https://blog.craneengineering.net/what-are-newtonian-and-non-newtonian-fluids

https://www.philamixers.com/news/how-condiments-are-made/

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