What Kind of Material is Oobleck?

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Oobleck is most commonly known as a non-Newtonian fluid, i.e., it thickens when a force is applied. More accurately, whenever there is a shear rate imposed upon a non-Newtonian fluid, or a rheopectic/dilatant material, it thickens.

What’s shear rate?

shear stressLet’s take a look at what shear rate means. Shear stress is most easily visualized as being internal. As the diagram shows, it can be seen as two parts of one object sliding against each other. The rate portion just means that it is either changing with time or is applied for a period of time, e.g., stirring. So a rheopectic fluid will thicken with stirring instead of thinning, like “usual” fluids do. Hence the nickname, “shear thickening.” The impulse of a bullet would impose a very high shear rate upon a mass of oobleck, thus causing the bullet to impact a fluid as if it were a solid.

What is Viscosity?

The thickness of this fluid is reflected in its resistance to flow. This resistance is called viscosity. Every fluid has viscosity, although a fascinating phenomenon called superfluidity allows a viscosity of 0 under certain conditions for certain fluids. For rheopectic materials, viscosity increases due to an imposed shear rate.

Other examples of materials which experience shear thickening are honey and quicksand. These materials, along with our friend oobleck, belong in a class dubbed rheological fluids.

What is Rheology?

Rheology is the study of the flow of matter, and it explores many fascinating properties including rheopexy/dilatancy (mentioned above), pseudoplasticity, thixotropy, viscoelasticity, and various electric, magnetic, and thermal properties. If these terms scare you, don’t worry! I’ll make them easy.

  • Rheopexy/dilatancy- Stirring a material, or imposing a shear rate, causes the material to thicken. For example, quicksand.
  • Pseudoplasticity- An increase in shear rate decreases viscosity. Viscosity is time dependent. For example, ink and paint.
  • Thixotropy- Material will return to original viscosity some time after the shear rate is imposed. Becomes less viscous, or thins, when a stress is applied. Viscosity is time independent. For example, toothpaste.
  • Thermorheological (TR)- Viscosity responds with temperature. Note: this does not make something like a milkshake TR. Milkshakes become thinner because they melt, whereas TR materials have an intrinsic property relating temperature and viscosity.
  • Electro-/Magnetorheological (ER, MR)- Viscosity increases drastically when an electric or magnetic field is imposed, even to the point of becoming a viscoelastic solid.
  • Viscoelasticity- Material does not initially respond to an imposed stress. When it does, it will eventually return to its initial state, usually incompletely. For example, skin and wood.

So what do you think about rheological fluids? Did I miss anything worth mentioning? Leave a comment and let me know!

If you would like to read more, here’s an intro to rheology, another one, and a little more advanced look at rheology.


Images courtesy of:

Oobleck: https://www.wired.com/2014/06/wuwt-oobleck-non-newtonian/

Shear diagram: http://earthquake.usgs.gov/learn/glossary/?term=shear%20stress

Liquid magnet: http://kaustav-sengupta.blogspot.com/2012/09/the-robots-blood.html

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