Fun With Physics: Non-Newtonian Fluids

Galleons, many of you may have had really fun, engaging science courses in your primary school days. And you may have mixed together a little cornstarch and water for a fun day with non-Newtonian fluids. I, sadly, had really crappy science classes. And so, to rectify what I believe to be a gaping hole in my early science education (and certainly not because it sounds like boatloads of fun), we’re going to begin today’s post with an experiment.

9:35 Gathered all ingredients together.

9:36 Mixed about 1/4 the box of cornstarch with 1/2 cup water. This shit does not want to mix. The cornstarch is forming large, gloppy clumps in the bowl. Stirring is a chore.

9:38 The loud squelching sounds emanating from the bowl are the stuff of nightmares. This must be what R’lyeh sounds like. It’s disgusting.

9:39 Cornstarch finally yields to water, forming a very thin syrup. MOAR CORNSTARCH.

9:40 What have I done?

9:41 New cornstarch is floating in a strange island on top of previous mixture. The bottom has solidified, while the unexposed-to-liquid top half remains innocuous and powdery.

9:42 It tried to eat my spoon.

9:43 The squelching sounds are now interlaced with vaguely styrofoamy rasping noises as I scrape through the dry cornstarch in an attempt to mix it. Adding a bit more water.

9:47 What have I created? I am afraid it is about to leap out of the bowl and suction to my face. I would die writhing about on my kitchen floor, this horrible, slightly plasticine smelling substance muffling my screams.

9:48 Oh hey, I think it’s working.

9:49 Adding final bit of cornstarch. Yes, I used the whole box. BECAUSE I CAN.

9:50 And more water. It looks like I’m going to end up using about 1 3/4 cups of water to one 16 oz box of cornstarch.

9:51 Realize I could have just said 1 lb box of cornstarch. Feel like a twat.

9:56 SUCCESS!

9:58 This is the most amazing thing I’ve ever made. I just punched a bowl of liquid and it didn’t splatter everywhere!

10:03 Yeah, I’m just going to keep playing with this gunk. *squee*

10:07 I really wish I had access to a subwoofer…

It’s taken me two years since our last post on non-Newtonian fluids (only the bizarrely devoted among you, dear galleons, would remember that we’ve discussed this subject before) to finally perform this experiment, but BY FEYNMAN I have. Huzzah.

And there’s actually a reason I’m cycling back to the subject of non-Newtonian fluids. My previous post gives a pretty good description (if I do say so myself) of shearing forces in regards to fluids that exhibit apparent viscosity changes with the duration of stress, like canned whipped cream and hair mousse. Suffice to say that shearing forces are forces applied parallel to the surface of a liquid. Which is why forcing whipped cream through a little nozzle changes its viscosity.

However, cornstarch and water is not the same as whipped cream. The cornstarch/water mixture doesn’t change viscosity based on duration of stress- its viscosity simply increases with increased stress.

Despite this, shearing was the widely believed culprit behind the cornstarch/water mixture’s viscous insanity. I say widely believed because, while scientists have been puzzling over the stuff since the 1930s, there’s never been a complete description of why this stuff behaves so strangely.

Until now.

Scott Waitukaitis and Heinrich Jaeger of the University of Chicago have finally found the solution to the old problem. With some fancy instruments… and a cement mixer full of your everyday cornstarch and water.

Now, the aforementioned shearing forces have been the primary focus of small experiments using the cornstarch and water substance. Unfortunately, shearing forces cannot explain the large forces necessary to keep a fully-grown adult aloft while they run across the stuff.

Oh yeah, that happened:

So, the UChicago team decided to think bigger. By using very large quantities of the cornstarch/water suspension and equipment (like force sensors and high-speed cameras) to capture measurements and images of those instants of viscosity change, the scientists found that compressive forces are actually the culprits behind the strange behavior.

“We found that when you hit the suspension, a solid-like column grows below the impact site,” said Waitukaitis.

It’s like shoveling snow. If you just keep on pushing your snow shovel through loose snow, a large mass of compacted snow builds up in front of the shovel, making it increasingly difficult to push that damn shovel. The cornstarch suspension behaves similarly. As an object impacts the suspension, cornstarch grains pile up in front of it (or, you know, underneath it… semantics), becoming jammed as compression eventually halts all movement. The scientists have started calling that “jamming” moment that turns the liquid into a solid impact-activated solidification (real clever, right?).

Note that all of this “jamming” and solidification takes place in an instant. Pretty cool, no?

When laid out like this, it all seems fairly obvious. How could it have taken us 80 years to figure this out? Well, this was actually the first experiment to look at direct compression in these non-Newtonian fluids. And a good thing they finally did, too- turns out driving a rod into such a suspension yields stresses 100 times greater than any shearing stress.

Interesting as all this is, it could have some massive practical applications. The properties of these suspensions are being examined for use in new body armors. Imagine a liquid substance that would easily conform to a certain shape while maintaining flexibility for ease of mobility, a substance that would become hard the moment it was struck. Smart armor. A fascinating idea, and one that could become a reality thanks to the knowledge of these suspensions the UChicago team has given us.

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