Non-Newtonian fluids

It helps to know what a Newtonian fluid is before you start thinking about non-Newtonian fluids.  Conveniently, just about every fluid you can think of is Newtonian.  When you push on a fluid, it moves out of the way.  Some move fast, some move slow (this is quantified as viscosity), but when a fluid seizes up and stops,,when it becomes like a solid in response… That goes against Newton!

Obleck bounding on a speaker
Non-Newtowning fluids are better with food coloring

Perhaps the most common non Newtonian fluid people run across [foreshadowing] is cornstarch and water. It’s used to thicken gravy and other sauces (because of viscosity). But when making gravy, you are cautioned to mix the corn starch in cold water first and then add that to your hot liquid that needs to be thickened to avoid clumps. This is because the corn starch on the outside will mix with water first and be agitated (pressed on) by the hot liquid and turn solid, preventing even distribution of the inner cornstarch. But if you mix it with child water first all of the corn starch will be able to mix with the hot water.

5 kids mixing conrstarch and water by hand in a 15 gallon bucket
A hands-on experience

So we mixed it 50lbs bag of corn starch with water, and learned that if you put water into cornstarch it will mix on the surface, and further attempts to mix in more water will cause the outer cornstarch solution to turn solid. Preventing further mixing…. Much like cornstarch dumped directly into hot water. The right way to mix large batches is to add the corn starch slowly into water until the desired thickness is achieved.

You can then put that on a speaker and play a time of just the right frequency to see the corn starch turn solid, be thrown in the air and turn liquid again on the way down

Or you can jump in a big tub of it and see if you can start running before you begin to sink. It’s harder than it looks.

Or you can cut to the chase and just build a trough that you can run across… Or stop and sink into.

Making liquid dance on a speaker is pretty cool, and visually memorizing, but hands on (or feet in) is really the best way to experience and believe in the properties of Non Newtonian fluids.

kids gather around and playing with obleck in a speaker

No matter what you do though, make sure you leave lots of time for cleanup.