Oh, the Places You’ll Flow!

Dr. Seuss made oobleck famous in his 1949 classic Bartholomew and the Oobleck in which the sticky substance oobleck threatens to destroy a whole kingdom. More recently, oobleck made an appearance on this very blog, where it decided to take a break from its world-domination prospects to star in a homemade music video.

Disclaimer: what follows seeks to de-mystify the mysterious substance known as oobleck. To add some distance between this disclaimer and what follows, please enjoy another video starring the famed oobleck, made once again in collaboration with filmmaker Chelsey Blackmon.

Oobleck is easily synthesized by combining about three parts corn starch in one part water. When sitting innocently in a bowl, it looks much like any normal fluid; however, do not be fooled. Dr. Seuss, though technically not a doctor, was rightfully wary of oobleck’s monster-like properties, for oobleck is no ordinary fluid like water or wine.

Oobleck is what is known as a *non-Newtonian fluid,* meaning, as its name implies, that this unruly concoction does not obey newtonian fluid dynamics. Specifically, its resistance to flow—viscosity—does not remain constant. If you apply a small amount of force, say by slowly and harmlessly dipping your hand into a bowl of oobleck, the mixture acts like a liquid. If instead you charge your hand into the mixture, it will literally push back, resisting deformation by acting as a solid. Its viscosity actually increases in response to increased pressure.

It is this increase in oobleck’s viscosity in response to the application of a stronger force that caused the finger-like monsters to emerge from the mixture in the video above. Oobleck is technically a suspension of cornstarch particles surrounded by water molecules; the cornstarch is not dissolved, but simply distributed evenly in water when at rest. Within the suspension, two forces compete. On the one hand, you have the intermolecular cornstarch-cornstarch // cornstarch-water forces holding the particles in an ordered liquid-like suspension. On the other hand, you have the external shear forces acting on these cornstarch particles, which push them apart into a disordered state.

When you slowly emerge your hand in the suspension, the cornstarch particles have the time and space to redistribute themselves evenly, allowing your hand to glide through. We can see these more gentle forces at work in the smaller ripples that form in the cornstarch//water solution when the bass is softer and the soundwaves more gentle. Once the soundwaves coming from the subwoofer under the bowl cross a certain force threshold, however, oobleck stops behaving like a liquid. At this point, the shear forces supplied by the bass overwhelm the intermolecular forces keeping oobleck in its orderly state. In response to the increased pressure, the water flows away at the site of impact allowing the cornstarch to lock up and build in a densely packed patch of particles—a fortified army of particles revealed. Once the pressure is removed, the cornstarch settles back into the suspension.

War metaphors aside, non-Newtonian fluids like oobleck—known as shear thickening or dilatant materials—are actually being used in conjunction with kevlar to improve body armor. Kevlar-dilatant armor is much lighter than pure-kevlar and has been found to perform better, as it provides additional strength by hardening when struck. Seems bartholomew was quite the little hero after all.

2 thoughts on “Oh, the Places You’ll Flow!

  1. Pingback: BMSA @ Go Green! Greenpoint Festival: Science in Action | ArtLab

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