Brookfield DV-III Ultra Rheometer - Manual (Page 57)

Brookfield Engineering Laboratories, Inc. Page 57 Manual No. M98-211-E0912
IV.7 Math Models
IV.7.1 The Power Law (Ostwald) Model
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t = shear stress, k = consistency index, = shear rate, and n = flow index)
What does it tell you?
The Power Law model provides a consistency index, k, which is a product’s viscosity at one
reciprocal second. (Reciprocal seconds are the units of measurement for shear rate.) It also
provides a flow index, n, which indicates the degree with which a material exhibits non-Newtonian
flow behavior. Since Newtonian materials have linear shear stress vs. shear rate behavior and n
describes the degree of non-Newtonian flow, the flow index essentially indicates how “non-linear”
a material is.
Figure IV-48
When n < 1 the product is shear-thinning or Pseudoplastic. This means the apparent viscosity
decreases as shear rate increases. The closer n is to 0, the more shear thinning the material is.
When n > 1 the product is shear-thickening or Dilatant. Their apparent viscosity increases as
shear rate increases.
When should you use it?
This model should be used with non-Newtonian, time-independent fluids that do not have a yield
stress. These fluids will begin to flow under any amount of shear stress. Graphs of such material
generally intersect the y-axis at 0.