Fluid rheology — If the apparent viscosity of a fluid decreases with the length of time that it is being mixed (at roughly constant shear), what is this fluid behavior called?

Introduction:
Engineers frequently encounter non-Newtonian fluids whose apparent viscosity depends on not only shear rate but also the duration of shearing. The question tests recognition of time-dependent thinning versus thickening behaviors and distinguishes these from purely shear-rate–dependent or elastic responses.

Given Data / Assumptions:

• Mixing occurs at roughly constant shear conditions.
• The measured apparent viscosity decreases as mixing continues over time.
• No large temperature drift or composition change during the observation.

Concept / Approach:
Thixotropy is the phenomenon where viscosity decreases with time under constant shear, typically due to reversible breakdown of a microstructure (e.g., flocculated networks). Rheopexy is the opposite: viscosity increases with time under shear. Pseudoplastic or dilatant labels refer primarily to dependence on shear rate at a given moment, not on elapsed time at constant shear. Viscoelasticity refers to storage and loss moduli and elastic recoil, not inherently to time-thinning under constant conditions.

Step-by-Step Solution:
Identify time dependence: viscosity change is tied to mixing time at constant shear, indicating a structural breakdown process.Map to terminology: time-dependent thinning corresponds to thixotropy.Exclude alternatives: rheopexy would thicken; Newtonian viscosity would be constant; viscoelasticity is about elastic response; dilatancy is shear-thickening with rate, not time.

Verification / Alternative check:
Perform a three-interval thixotropy test on a rotational rheometer (low–high–low shear). A hysteresis loop in the flow curve (downward shift after high-shear interval) confirms thixotropy. Recovery with a rest period further corroborates a reversible structure breakdown–rebuild cycle.

Why Other Options Are Wrong:

• Newtonian: viscosity independent of shear rate and time.
• Rheopectic: time-dependent thickening, opposite of the observation.
• Viscoelastic: describes elastic effects, not necessarily time-thinning.
• Dilatant: shear-thickening with increasing rate, not a time effect at constant shear.

Common Pitfalls:
Confusing pseudoplasticity (rate effect) with thixotropy (time effect); misattributing temperature drift to rheology; ignoring rest-time recovery behavior.

Final Answer:
Thixotropic (time-dependent thinning)