Discussion
Which one of the following figures shows the CORRECT dependence of apparent viscosity (\(\eta\)) on rate of shear strain (\(du/dy\)) for pseudoplastic fluids?
- A.
- B.
- C.
- D.
Answer: B
The correct plot is (B), apparent viscosity falling as the shear rate rises.
A pseudoplastic fluid is shear-thinning: in the power-law model \(\tau = K(du/dy)^{n}\) its index is \(n \lt 1\), so the apparent viscosity \(\eta = K(du/dy)^{n-1}\) decreases with increasing shear rate. Polymer solutions, paints and blood behave this way.
Plot (A), a constant viscosity, describes a Newtonian fluid. Plots (C) and (D), viscosity rising with shear rate, describe a dilatant (shear-thickening) fluid with \(n \gt 1\), such as a dense starch suspension.
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