Brookfield DV3T Rheometer - Manual (Page 83)

Brookfield Engineering Labs., Inc. Page 83 Manual No. M13-2100
VII. MATH MODELS
VII.1 The Power Law (Ostwald) Model
(τ = 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 VII-1
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.