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This Goop Is Made For Walking

July 19, 2012

Eva Gusnowski

Have you ever seen the cornstarch monster? It grows out of your stereo speakers when your parents tell you to turn the music down, but you don’t. Is it magic? No...it’s science!

 



Water mixed with an equal or greater amount of cornstarch makes what is called a “non-Newtonian fluid.” To understand this a little better, first let’s explore what a Newtonian fluid is. A Newtonian fluid is one like water, in that its viscosity does not depend on external stress; rather, its viscosity is always constant. The best explanation I’ve come across is a drinking straw: the harder you suck on the straw (i.e. increasing stress), the faster the water will move because its viscosity is constant. For the most part, non-Newtonian fluids are different though, in that their viscosity does depend on external stress. So, using the straw example, if you were to suck harder on a straw that was immersed in a non-Newtonian fluid, its viscosity would increase, making it harder to draw it up the straw. Even more force results in a further increase of viscosity, and so on and so forth.  These are known as "shear-thickening" non-Newtonian fluids, because a shear force (such as stirring), will make their viscosity increase (i.e. thicken).  As a side note there are non-Newtonian fluids, such as ketchup, that work the opposite way and actually decrease their viscosity when a force is applied to them, called "shear-thinning" non-Newtonian fluids. This is why smackin’ a ketchup bottle on its arse will make the ketchup flow faster from the bottle.

The reason this occurs is that non-Newtonian fluids are colloids, meaning that the particles of cornstarch do not dissolve in the water (this would be a solution), but rather stay suspended in the fluid. When at rest, the particles of cornstarch will keep their space from one another (if only this were true on the Skytrain), and the mixture will behave as a liquid. However, if you apply a force to the non-Newtonian fluid, the particles move closer together and will behave more like a solid.

New research has used X-rays to examine this phenomenon in order to explain exactly why this happens. The authors of this study used a steel rod to smack the surface of a cornstarch/water mixture in order to apply an external stress to the fluid. The X-ray machine was used to track particles and see how they moved in response to this stress. What they found is that when stress is applied to the fluid, water is essentially squeezed out from between the particles. This means that the cornstarch molecules are close enough together to generate solid-like properties because they jam onto one another. Leave it alone and water weasels its way back in between, and the mixture regains its fluid-like status.

And now for the fun part...why do we care? Aside from being an award-winning science fair project in many a high school, cornstarch + water is fast becoming a staple party trick. If you spread the mixture on a speaker you can generate the dreaded cornstarch monster. Make a giant pool of it and you can run across the top. Just don’t stop or fall, because now you know that if you stop moving and let the fluid rest it will become liquid-like again and you’ll sink, just like in quicksand.

 

 

So let me ask a question then.  Was the mimetic polyalloy that the T-1000 was made from actually a shear thinning non-Newtonian fluid?  Nevermind, I'll just ask John Connor the next time I see him.

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