Which description best defines a thixotropic fluid in mixing operations, considering both time dependence and recovery after shear?

Introduction:
Thixotropy is a time-dependent rheological behavior commonly observed in structured fluids such as gels, cell broths, and polymer solutions. In bioprocessing, recognizing thixotropy is important for predicting torque trends during start-up and for interpreting off-line rheology tests that show hysteresis between ascending and descending shear ramps.

Given Data / Assumptions:

• Mixing is performed at approximately constant shear rate over time.
• Fluid microstructure can break down under shear and rebuild at rest.
• Process conditions allow observation of recovery when agitation is reduced or stopped.

Concept / Approach:
Under constant shear, thixotropic fluids exhibit a gradual decline in apparent viscosity as internal structures break down. Once shear is removed, partial or full recovery of the original structure occurs, causing viscosity to rise toward its initial value. Thus, thixotropy encompasses both the time-dependent decrease during mixing and the recovery afterward, which is why the definition emphasizing change and return is most complete.

Step-by-Step Solution:

Verification / Alternative check:
Time sweep tests on a rheometer at fixed shear show viscosity decay, while subsequent low-shear recovery tests demonstrate rebuild toward the initial value, validating the definition.

Why Other Options Are Wrong:

• Constant viscosity: Describes Newtonian, not thixotropic, behavior.
• Progressive increase with time: That is rheopectic behavior.
• Decrease with time only: Captures part of thixotropy but ignores recovery, making it incomplete.

Common Pitfalls:
Confusing thixotropy with pseudoplasticity (which lacks time dependence) or with rheopecty (the opposite time trend). Recovery may be partial and depend on rest duration.

Final Answer:
Viscosity changes during mixing but returns to its original state after mixing stops