Non-Newtonian behavior: A fluid whose apparent viscosity decreases as stirrer speed increases and also decreases progressively with mixing time is best classified as which combination?

Introduction:
Industrial broths and formulated media often deviate from Newtonian behavior. Correctly identifying shear-rate and time-dependent effects is essential for scale-up, mixing, and mass-transfer predictions. This item checks recognition of a fluid that thins with shear rate and also thins with time under constant shear.

Given Data / Assumptions:

• Apparent viscosity decreases as stirrer speed (shear rate) increases.
• At a fixed speed, viscosity further drops over time and then recovers when resting.
• No plastic yield stress is mentioned.

Concept / Approach:
Shear-thinning behavior indicates pseudoplasticity: viscosity falls as shear rate rises. Time-dependent viscosity decrease at constant shear indicates thixotropy, a reversible structural breakdown under shear with time that recovers at rest. The combined description “pseudoplastic, thixotropic” accurately captures both rate and time dependencies.

Step-by-Step Solution:
Map rate dependence: viscosity ↓ with shear rate ⇒ pseudoplastic.Map time dependence: viscosity ↓ with time at constant shear ⇒ thixotropic.Combine classifications to obtain “pseudoplastic, thixotropic.”Exclude rheopectic and dilatant behaviors (opposites).

Verification / Alternative check:
Flow curves (apparent viscosity vs. shear rate) and hysteresis loops from up/down shear ramps show classic thixotropic loops for such fluids. Many cell broths and polymer solutions exhibit this profile.

Why Other Options Are Wrong:
Newtonian: viscosity is constant, independent of shear rate and time.

Dilatant, rheopectic: both imply thickening (viscosity increases) with rate and time, opposite to the description.

Dilatant, pseudoplastic: self-contradictory pairing (dilatant = shear-thickening; pseudoplastic = shear-thinning).

Common Pitfalls:

• Confusing thixotropy (time-dependent thinning) with mere shear-thinning (rate-dependent only).
• Assuming all non-Newtonian fluids are thixotropic; many are not time-dependent.

Final Answer:
Pseudoplastic, thixotropic fluid