Time-dependent rheology: A fluid whose viscosity changes with stirrer speed or mixing time but then returns to the original value once mixing stops is best described as what?

Introduction:
Beyond rate dependence, many process fluids exhibit time dependence. Distinguishing thixotropy from rheopexy and from viscoelasticity is key for predicting start-up, shutdown, and intermittent mixing behavior in reactors and pipelines.

Given Data / Assumptions:

• Viscosity changes with speed or time during mixing.
• Viscosity returns to its original value after mixing ceases and the fluid is allowed to rest.
• No permanent structural change or chemical reaction occurs.

Concept / Approach:
Thixotropy describes time-dependent structural breakdown under shear and recovery at rest. Apparent viscosity decreases during sustained shear and rebuilds when shear is removed. This is distinct from rheopexy (viscosity increases with time at constant shear), viscoelasticity (time-dependent stress–strain response but not necessarily viscosity recovery behavior), and Newtonian behavior (no time or rate dependence).

Step-by-Step Solution:
Observe time-dependent viscosity decrease under shear ⇒ thixotropic signature.Cease mixing and note recovery to initial value ⇒ reversible microstructure rebuilds.Exclude rheopexy (time-dependent thickening) and purely elastic phenomena.Classify the fluid as thixotropic.

Verification / Alternative check:
Hysteresis loops on up/down shear ramps show area between curves for thixotropic fluids, and creep–recovery tests demonstrate structural rebuilding over time at rest.

Why Other Options Are Wrong:
Newtonian: viscosity is constant; no time or rate effects.

Rheopectic: opposite time dependence (thickening under sustained shear).

Viscoelastic: describes stress relaxation and elastic recoil but not necessarily the reversible viscosity drop/recovery described.

Common Pitfalls:

• Confusing viscoelasticity with thixotropy; many thixotropic fluids are also viscoelastic, but the defining feature here is time-dependent thinning with recovery.
• Attributing irreversible changes to thixotropy; by definition it is reversible.

Final Answer:
Thixotropic (time-dependent thinning with recovery at rest)