A fluid shows an increase in apparent viscosity the longer it is mixed at an approximately constant shear rate. Which time-dependent rheological behavior does this indicate?

Introduction:
Some complex fluids change viscosity over time even when shear rate is held roughly constant. Distinguishing between thixotropy and rheopecty is important for predicting how torque and power draw evolve during prolonged mixing, and for preventing equipment overloading or insufficient mixing performance.

Given Data / Assumptions:

• Shear rate is maintained approximately constant.
• Apparent viscosity increases with mixing time.
• No indication of elastic phenomena dominating the response.

Concept / Approach:
Rheopecty, sometimes called anti-thixotropy, describes the time-dependent increase in viscosity under sustained shear, implying structure builds up or orders in a way that resists flow. This is the opposite of thixotropy, where viscosity decreases with time due to structural breakdown. Newtonian fluids exhibit no time dependence, and viscoelasticity refers to recoverable elastic response, not necessarily a monotonic time trend in viscosity.

Step-by-Step Solution:

Verification / Alternative check:
Rheometer time sweeps demonstrate a monotonic increase in viscosity at constant shear, and hysteresis loops may invert compared to thixotropic fluids, confirming rheopecty.

Why Other Options Are Wrong:

• Newtonian: No time dependence.
• Viscoelastic: Describes elastic storage and loss behavior, not necessarily time-dependent thickening.
• Thixotropic: Viscosity would decrease with time, opposite of the observation.

Common Pitfalls:
Misidentifying rheopecty as temperature rise effects; always control temperature during tests to isolate time-dependent structure changes.

Final Answer:
rheopectic