Jul 15, 2008

Posted by in Physics Talk | 1 Comment

Non-Newtonian fluid physicsness, put to music!

So simple to set up; a metal sheet on top of a booming subwoofer and pour some non-newtonian fluid (cornstarch and water) on it and watch a new type of universe emerge right in front of your eyes!

 

What is a non-newtonian fluid?

A non-Newtonian fluid is a fluid whose viscosity is variable based on applied stress. The most commonly known non-Newtonian fluid is cornstarch dissolved in water. Contrast with Newtonian fluids like water, whose behavior can be described exclusively by temperature and pressure, not the forces acting on it from second to second. Non-Newtonian fluids are fascinating substances that can be used to help us understand physics in more detail, in an exciting, hands-on way.

 

If you punch a bucket full of non-Newtonian fluid, the stress introduced by the incoming force causes the atoms in the fluid to rearrange such that it behaves like a solid. Your hand will not go through. However, if you shove your hand into the fluid slowly, it will penetrate successfully. If you pull your hand out abruptly, it will again behave like a solid, and you can literally pull a bucket of the fluid out of its container in this way. However, the effect doesn’t last for long – if stress is not continuously applied, the non-Newtonian fluid turns back into a liquid, and will ooze right off your hand.

Here is another video of large pool of cornstarch+water!

 

One of the best suggestion that I have heard recently is to make the speed bumps out of non-newtonian fluids, so if you are moving slowly, you will go through them but if you are moving too fast, it will act as a real speed bump.

A comment from John Spevacek

His blog
It’s the Rheo Thing has a nice tag line “Everything flows, but only the macromolecules are worth the time.”

Non-Newtonian fluids can be fit into at least 4 categories, of which the cornstarch/water mixture you describe is only 1: rheopexy. This is where the viscosity of the fluid increases over time while a constant shear rate is applied. This is often confused with dilatancy (shear thickening), where the viscosity increases as the shear rate increase. The opposite behaviors are also possible. Shear thinning is very common with molten plastics – the viscosity decreases as the shear rate increases, and thixotopric fluids will show a decrease in viscosity over time at a constant shear rate.

Beyond those four categories, there are also Bingham plastics which do not flow until a critical stress is exceeded and Boger fluids which show elasticity but a Newtonian viscosity.

So you can see that some of your statements above are not exactly correct – they are overly broad.

Lastly, I would submit that a cornstarch/water is NOT the most common non-Newtonian fluid as it takes a fairly large amount of cornstarch to acheive the effect (~ 6:5 cornstarch:water on a weight basis IIRC). Considering that polyethylene is the largest volume plastic and it is shear-thinning in the molten state, I would submit that material for the title of most-common non-Newtonian fluid. But if you are looking for “everyday” materials found around the house, there are other more popular examples than cornstarch in water. Paint, tomato juice, peanut butter, toothpaste, …

Talk Like a Physicist

  1. Non-Newtonian fluids can be fit into at least 4 categories, of which the cornstarch/water mixture you describe is only 1: rheopexy. This is where the viscosity of the fluid increases over time while a constant shear rate is applied. This is often confused with dilatancy (shear thickening), where the viscosity increases as the shear rate increase. The opposite behaviors are also possible. Shear thinning is very common with molten plastics – the viscosity decreases as the shear rate increases, and thixotopric fluids will show a decrease in viscosity over time at a constant shear rate.

    Beyond those four categories, there are also Bingham plastics which do not flow until a critical stress is exceeded and Boger fluids which show elasticity but a Newtonian viscosity.

    So you can see that some of your statements above are not exactly correct – they are overly broad.

    Lastly, I would submit that a cornstarch/water is NOT the most common non-Newtonian fluid as it takes a fairly large amount of cornstarch to acheive the effect (~ 6:5 cornstarch:water on a weight basis IIRC). Considering that polyethylene is the largest volume plastic and it is shear-thinning in the molten state, I would submit that material for the title of most-common non-Newtonian fluid.

    But if you are looking for “everyday” materials found around the house, there are other more popular examples than cornstarch in water. Paint, tomato juice, peanut butter, toothpaste, … all of which are ready to go with no preparation.

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