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Name: Ali S.
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 1999-2001


Question:
Please tell me what is the meaning of the shear thinning and shear thickening?


Replies:
These terms refer to the response of a fluid's viscosity to a shearing stress, that is, a force tending to make part of the fluid slide past another part. Most familiar fluids are approximately "Newtonian", that is, their viscosities are not affected by shear. Shear-thickening and shear-thinning fluids are non-Newtonian, as their viscosities increase or decrease, respectively, as the applied shearing stress increases. Silly Putty is a shear-thickening fluid; ketchup is shear-thinning.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois


The time-independent behavior of fluids falls into 3 general classes: Shear stress (resistance to flow) is proportional to shear rate (speed of motion of the fluid with respect to nearby fluid elements (in lay terms it means: How fast am I stirring the stuff) -- this is Newtonian flow. Another way of saying the same thing is the fluid viscosity is a constant with change in the shear rate. Shear stress decreases with shear rate. Another way of saying this is: The viscosity of the fluid decreases with increasing shear rate. This type of behavior is also referred to as pseudoplastic. Shear stress increases with shear rate. Another way of saying this is viscosity increases with increasing shear rate. These types of fluids are referred to as "shear thickening" and also "dilatant". Low molecular weight liquids, like water etc. are usually Newtonian. Water borne latex paints are shear thinning, and wet concrete is an example of a dilatant fluid.

In contrast, but often confused with the above are fluids that become less viscous as a function of TIME. These types of fluids are called thixotropic, although this term is often confused with shear thinning. Fluids that become more viscous as a function of time are called "rheopectic" although again this term is often applied when the term dilatant is meant.

The source of the confusion is that in real uncontrolled situations it is often difficult to separate the time-dependent and time-independent behavior of a fluid.

Vince Calder



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