Non-Newtonian Fluids — What changes the viscous behavior most: power input or time? Identify the correct statement about non-Newtonian fluids in mixing operations.

Introduction:
Unlike Newtonian liquids, many process fluids (polymer solutions, slurries, cell broths) are non-Newtonian. Their apparent viscosity depends on shear rate, and in time-dependent cases, on how long shear is applied. This question distinguishes non-Newtonian responses from Newtonian behavior and other irrelevant analogies.

Given Data / Assumptions:

• Mixing imparts shear that depends on impeller type, speed, and geometry.
• Shear history (duration) can alter microstructure (e.g., floc breakup, polymer alignment).
• Temperature is assumed constant for the time frame considered.

Concept / Approach:

Non-Newtonian fluids show shear-rate dependence (pseudoplastic or dilatant) and may also be time-dependent (thixotropic or rheopectic). Power input affects local shear rates; sustained mixing introduces time-dependent restructuring. Therefore, viscosity can change with both intensity (power) and duration (time) of mixing, unlike Newtonian liquids where viscosity at given temperature/pressure is constant.

Step-by-Step Solution:

Verification / Alternative check:

Rheometer tests at different shear rates and hold times show viscosity curves that shift with both rate and time for many industrial fluids (e.g., paints, food pastes, biopolymer solutions).

Why Other Options Are Wrong:

A: Describes Newtonian behavior. C and D: Irrelevant analogies; gases and water do not represent non-Newtonian complexity. E: Temperature matters, but history effects are also critical in non-Newtonian systems.

Common Pitfalls:

Assuming a single viscosity value applies across all operating conditions. In design, always specify shear rate and, if needed, shear history.

Final Answer:

Viscosity can change with the level of power input and/or with the duration of mixing (time under shear).