Newtonian Fluids — Which statement best describes viscosity behavior during mixing at constant temperature?

Introduction:
A Newtonian fluid has a constant viscosity at a given temperature and pressure, independent of shear rate and mixing time. This is a foundational concept used in pump sizing, pipe flow calculations, and reactor design for simple liquids like water and many light oils.

Given Data / Assumptions:

• Temperature and composition are constant.
• Shear rates fall within the continuum regime where Newtonian behavior applies.
• No time-dependent structural changes occur in the liquid.

Concept / Approach:

Newtonian behavior is defined by a linear relationship between shear stress tau and shear rate gamma_dot: tau = mu * gamma_dot, with constant mu. As such, varying the impeller speed (changing shear rate) changes shear stress proportionally, but does not change mu. Time under shear does not alter mu either.

Step-by-Step Solution:

Verification / Alternative check:

Viscometry with concentric cylinders shows straight-line tau versus gamma_dot passing through the origin for Newtonian liquids; slope equals mu and is independent of the test duration at constant temperature.

Why Other Options Are Wrong:

B, C, D: Describe non-Newtonian behaviors (shear-thickening, shear-thinning, time dependence). E: Blade-passage pressure fluctuations do not change the intrinsic viscosity.

Common Pitfalls:

Confusing operational power requirements (which do change with speed) with the material property viscosity (which does not for Newtonian fluids).

Final Answer:

Viscosity remains essentially constant with time and power input (independent of shear history).