When I was a kid, my mother would sometimes give my younger brothers and me a large tub of oobleck, telling us to go play outside and make a mess.
Oobleck is a milky-white, shiny substance known as a non-Newtonian fluid. It flows like thick paint when you pour it, but mash your hand onto its surface and it forms a hard skin. Squeeze some in your palm and it will form a tough glob. But the second you release it, oobleck trickles down over your fingers in a slurry. It’s gross, it’s fun, and any kid will be caught up in its magical ability to switch back and forth between a solid and a liquid.
Oobleck is actually a pretty simple mixture of cornstarch and water. Its common name (which I later learned is not what all kids call it) comes from a Dr. Seuss story, Bartholomew and the Oobleck, where a young boy’s wish for something other than rain or snow to fall from the sky is granted. The characters in Seuss’ book soon need to be saved from this sticky new form of precipitation, but real-life oobleck is far more benign, and scientifically interesting.
On their own, boring old water and cornstarch don’t seem like they’d be able to create such an interesting product. But get them together and they form a non-Newtonian fluid. To really understand these materials, we need some knowledge of their opposite – that is, a Newtonian fluid.
British polymath and Enlightenment hero Isaac Newton studied lots of things: optics, gravity, waves, mathematics, astronomy, history, religion and alchemy and so on. Then in his spare time, he investigated how liquids flow and thus got a whole branch of fluid dynamics named for him. Newton observed how common liquids, such as water, flow the same regardless of how much stress you subject them to. Push a stirring stick into a cup of water and swish it around. The water’s viscosity – how smooth or sticky its consistency is – stays the same.
Pretty simple, yeah? Many liquids that we interact with on a regular basis work this way: things like water, milk, oil, or juice. But there are also a lot of common fluids that don’t. These are non-Newtonian fluids; substances whose viscosity changes based on how much pressure you apply to them.
For instance, tip a tub of thick yogurt upside-down and it will slowly ooze out. But shake the yogurt for a while first and its viscosity will decrease, allowing it to pour much more easily. Things like toothpaste, conditioner, and ketchup are another example. They usually sit like a custardy solid at the bottom of their container. They are sticky, their viscosity is high. But such substances actually just need a bit of a kick to get flowing. You must provide enough force to overcome the internal friction that keeps them stationary, such as by giving them a squeeze, which lowers their viscosity. They can then be easily extruded from a tube, bottle, or packet.
When you mix cornstarch into water, the starchy grains become suspended in the liquid, creating a substance with weird non-Newtonian abilities. When you apply pressure to oobleck, it works the opposite of the previous examples: The liquid becomes more viscous, not less. At the places you apply force, the cornstarch particles get mashed together, trapping water molecules between them, and oobleck temporarily turns into a semi-solid material. This force can be anything, including the sound vibrations from music speakers or a rapidly shaking container, as in the video at the top of this post.
That particular experiment really highlights oobleck's strangeness. The vibrating dish creates bumpy Faraday waves in the liquid. A puff of air introduced into this system creates a hole in the oobleck that just hangs out, not disappearing like you would expect. Speed up the vibrations and the hole will turn into a writhing mass that slowly takes over the entire surface of the oobleck. I don't know about you, but I can't watch that video without some internal WTF alarms going off.
Of course, the most famous force applied to oobleck is the weight of a person slamming their foot down as they run over a vat filled with the stuff. You can find plenty of videos on Youtube of people repeating this amazing feat, including the one above. It’s not just little children, college students, and Ellen viewers who are impressed. Explaining all of oobleck’s properties is actually the subject of serious scientific investigations.
In 2012, researchers at the University of Chicago published a paper where they described the battery of experiments they performed on oobleck (you can watch a video of their tests below). It’s hard not to be impressed by all the science these guys are doing on some bizarre stuff I used to play with as a kid: Lasers! High-speed cameras! X-ray machines! Their lab has got it all.
After measuring all the forces and deformations involved inside of oobleck, the researchers think they know how it is able to generate the support for messiah-like party tricks. If you hit oobleck hard and fast, the cornstarch particles get shoved together, bunching up like snow in front of a snowplow. This creates a quasi-solid column just below your foot, which can support your weight. But if you stop moving, you stop applying force and the oobleck returns to a liquid state.
That’s fairly intuitive. But many mysteries actually remain with oobleck. The researchers still don’t know all the details, such as whether or not the cornstarch particles actually touch one another, what causes them to move apart after the pressure is taken off, and how different grain sizes would affect what the oobleck does. This last point is important because some engineers would like to create new substances with oobleck-like characteristics. These could be good at absorbing tremendous impacts, finding uses in bulletproof vests or cushions that inflate during a car crash.
I think that ultimately, the moral of the story is this: Parents, let your kids play with non-Newtonian fluids. They’ll grow up to create life-saving devices, or at least write about them on the internet.
