Shear-rate dependence — If the apparent viscosity of a fluid decreases as stirrer speed (shear rate) increases, what is the fluid type?

Introduction:
Non-Newtonian fluids often show apparent viscosity changes with shear rate. Recognizing shear-thinning (pseudoplastic) behavior is critical for sizing pumps, predicting pressure drops, and selecting impellers for bioprocess media and polymer solutions.

Given Data / Assumptions:

• Stirrer speed is a proxy for shear rate over a range of operation.
• Temperature and composition are constant.
• Observation concerns instantaneous rate dependence, not elapsed time at constant shear.

Concept / Approach:
Pseudoplastic fluids follow an apparent flow curve where viscosity decreases with increasing shear rate (e.g., power-law with n < 1). This arises from alignment of macromolecules or disruption of weak structures under shear. By contrast, dilatant fluids (n > 1) thicken as shear increases. Thixotropy and rheopexy describe time-dependent effects at roughly fixed shear, not the instantaneous rate dependence emphasized here.

Step-by-Step Solution:
Note the key: viscosity decreases as speed increases — hallmark of shear-thinning.Map to terminology: pseudoplastic = shear-thinning; dilatant = shear-thickening.Exclude time-dependent labels (thixotropy/rheopexy) and Newtonian constancy.

Verification / Alternative check:
Plot apparent viscosity vs shear rate on a rheometer; a downward slope confirms shear-thinning and fits a power-law model eta = K * gamma^(n-1) with n < 1.

Why Other Options Are Wrong:

• Newtonian: viscosity independent of shear rate.
• Dilatant: predicts increasing viscosity with speed, opposite of observation.
• Thixotropic/Rheopectic: concern time effects at constant shear, not rate dependence.

Common Pitfalls:
Confusing time- and rate-dependent behaviors; interpreting aeration-induced temperature rise as shear-thinning; neglecting measurement dwell time in rheometry.

Final Answer:
Pseudoplastic (shear-thinning)