Your Ooze is made up of tiny, solid particles of cornstarch suspended in water. Chemists call this type of mixture a colloid. As you found out when you experimented with your Ooze, this colloid behaves strangely. When you bang on it with a spoon or quickly squeeze a handful of Ooze, it freezes in place, acting like a solid.
The harder you push, the thicker the Ooze becomes. But when you open your hand and let your Ooze ooze, it drips like a liquid. Try to stir the Ooze quickly with a finger, and it will resist your movement. Stir it slowly, and it will flow around your finger easily. Smack water with a spoon and it splashes. Smack Ooze with a spoon and it acts like a solid.
Most liquids don't act like that. If you stir a cup of water with your finger, the water moves out of the way easily--and it doesn't matter whether you stir it quickly or slowly. Your finger is applying what a physicist would call a sideways shearing force to the water.
In response, the water shears , or moves out of the way. The behavior of Ooze relates to its viscosity , or resistance to flow. Water's viscosity doesn't change when you apply a shearing force--but the viscosity of your Ooze does. Back in the s, Isaac Newton identified the properties of an ideal liquid. Water and other liquids that have the properties that Newton identifies are call Newtonian fluids. Your Ooze doesn't act like Newton's ideal fluid.
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. In , 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.
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. But cornstarch is different, he said, largely because the particles are so tiny. Cornstarch particles are a micron to 10 microns in size, smaller than the diameter of a human hair. At this size, particles are susceptible to the tiniest of thermal and electric forces, Kamrin said.
As a result, cornstarch particles in water actually repel each other slightly, held apart by forces too weak to impact something as large as a grain of sand. This repulsive force helps the slurry flow, as the particles prefer a layer of fluid between then. But when squeezed together, friction takes over and the particles move like a solid. Kamrin and his team started with a computer model of wet sand that they'd already developed, making adjustments to better mimic wet cornstarch.
Most importantly, they added an extra variable to predict how many grains of cornstarch touch one another in a given region of the fluid. This variable, which Kamrin jokingly refers to as "clumpiness," allows the model to determine how solid-like or liquid-like the oobleck will be.
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