Brookfield DV-III Ultra Rheometer Manual de usuario descargar pdf (Pagina 57)

Brookeld 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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= shear stress, k = consistency index, = shear rate, and n = ow 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 ow index, n, which indicates the degree with which a material exhibits non-Newtonian

ow behavior. Since Newtonian materials have linear shear stress vs. shear rate behavior and n

describes the degree of non-Newtonian ow, the ow 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 uids that do not have a yield

stress. These uids will begin to ow under any amount of shear stress. Graphs of such material

generally intersect the y-axis at 0.

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Brookfield DV-III Ultra Rheometer Manual de usuario Pagina 57