The Science and Math of Oobleck

Source: MIT News, Oct 2019

When you mix cornstarch and water, weird things happen. Swish it gently in a bowl, and the mixture sloshes around like a liquid. Squeeze it, and it starts to feel like paste. Roll it between your hands, and it solidifies into a rubbery ball. Try to hold that ball in the palm of your hand, and it will dribble away as a liquid.

Most of us who have played with this stuff know it as “oobleck,” named after a sticky green goo in Dr. Seuss’ “Bartholomew and the Oobleck.” Scientists, on the other hand, refer to cornstarch and water as a “non-Newtonian fluid” — a material that appears thicker or thinner depending on how it is physically manipulated.

Now MIT engineers have developed a mathematical model that predicts oobleck’s weird behavior. Using their model, the researchers accurately simulated how oobleck turns from a liquid to a solid and back again, under various conditions.

In all scenarios, the simulations matched the experimental data and reproduced the motion of the oobleck, replicating the regions where it morphed from liquid to solid, and back again.

To see how their model could predict oobleck’s behavior in more complex conditions, the team simulated a pronged wheel driving at different speeds over a deep bed of the slurry. They found the faster the wheel spun, the more the mixture formed what Baumgarten calls a “solidification front” in the oobleck, that momentarily supports the wheel so that it can roll across without sinking.

Kamrin and Baumgarten say the new model can be used to explore how various ultrafine-particle solutions such as oobleck behave when put to use as, for instance, fillings for potholes, or bulletproof vests. They say the model could also help to identify ways to redirect slurries through systems such as industrial plants.

“With industrial waste products, you could get fine particle suspensions that don’t flow the way you expect, and you have to move them from this vat to that vat, and there may be best practices that people don’t know yet, because there’s no model for it,” Kamrin says. “Maybe now there is.”

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