Shear Thinning and Thickening
Name: Ali S.
Please tell me what is the meaning of the shear thinning
and shear thickening?
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.
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.
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Update: June 2012