Oobleck is a commonly utilized fluid in demonstrations of non-Newtonian behavior. Rather than being linearly viscous with respect to shear, oobleck is shear thickening, meaning that it becomes more viscous the more that it is sheared. This is what causes crazy formations when it’s vibrated, makes it useful as liquid armor, and enables people to run across pools full of it. Yet it flows readily when undisturbed. #
Tag: shear-thickening
Cornstarch Monsters
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Shaking a fluid surface often results in standing waves known as Faraday waves, but with a non-Newtonian fluid like oobleck, at some frequencies it’s possible to incite other behaviors. Oobleck is shear-thickening, meaning that its viscosity increases when force is applied. This is what allows it to develop finger-like protrusions under high frequency vibrations.
Cornstarch Monsters
The patterns formed when vibrating a liquid on a speaker cone are standing waves known as Faraday waves. With a large enough amplitude, this produces some very cool effects with a shear-thickening non-Newtonian fluid like oobleck. (It would actually be interesting to see what happens when you vibrate a shear-thinning liquid like shampoo…) This video also details how you can set up this demonstration yourself at home.
Reader Question: Oswald de Waele
fyeahhexagons-deactivated201103 asks:
Could you do a quick post explaining the Oswald de Waele relationship please? Thanks!
Sure! The Oswald-de Waele relationship (a.k.a. a power-law fluid) is an attempt to generalize the relationship between shear stress and shear rate in fluids. For a Newtonian fluid, that relationship is linear:
This relationship describes many fluids–like air or water–very well. But there are plenty of non-Newtonian fluids as well, both shear-thinning (paint, shampoo, ketchup) and shear-thickening (oobleck). The Oswald-de Waele relationship approximates the behavior of these fluids using:
Values of n less than one correspond to shear-thinning (or pseudoplastic) fluids; a value greater than one is a shear-thickening (or dilatant) fluid. And n = 1 corresponds to a regular Newtonian fluid. #
Vibrating Oobleck
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This video explores some of the non-Newtonian behaviors of oobleck when shaken. The pattern across the surface once the vibrations start is called Faraday waves, a type of nonlinear standing wave that forms once a critical vibrational frequency is passed and the flat surface of the fluid becomes unstable. Toward the end of the video, the frequency of the vibrations is increased until “finger-like protrusions” form. This is a behavior exhibited by shear-thickening non-Newtonian fluids.
Protecting an Egg with Oobleck
Using non-Newtonian fluids as “liquid armor” is an active area of research and development. Here students demonstrate the efficacy of shear-thickening as a defense against sudden impact by dropping a bag of oobleck containing a raw egg from different heights.
Non-Newtonian Fun
Non-Newtonian fluids are a favorite for displaying odd behaviors. High-speed video simply improves the experience.
Remember, though, that non-Newtonian fluids don’t actually become solids when you hit them. They just react similarly to a solid because they exhibit a nonlinear response to deformation.
Cornstarch Monsters
In shear-thickening, non-Newtonian fluids, disturbances make the fluid more viscous (“thicker”, more resistant to motion). This is well-demonstrated by putting some on a speaker and turning it on! #
