Comments on: Non-Newtonian fluid physicsness, put to music! http://talklikeaphysicist.com/2008/non-newtonian-fluid-physicsness-put-to-music/ Talk Like A Physicist Day : March 14, 2009 Wed, 03 Dec 2008 00:04:39 +0000 http://wordpress.org/?v=2.6.3 By: John Spevacek http://talklikeaphysicist.com/2008/non-newtonian-fluid-physicsness-put-to-music/#comment-4232 John Spevacek Thu, 24 Jul 2008 13:36:54 +0000 http://talklikeaphysicist.com/2008/non-newtonian-fluid-physicness-put-to-music/#comment-4232 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. 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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