Rheology — Definition Check: In a rheopectic (time-dependent shear-thickening) fluid, how does apparent viscosity change under continuous mixing at roughly constant shear conditions?

Introduction:
Rheopecty is a time-dependent rheological behavior in which a fluid becomes progressively more viscous the longer it is sheared or mixed at a roughly constant shear rate. This question checks whether you can distinguish rheopectic behavior from thixotropy (time-dependent thinning) and from simple shear-rate effects seen in non-time-dependent non-Newtonian fluids.

Given Data / Assumptions:

• Sustained, approximately constant shear rate during mixing.
• Temperature and composition are not changing appreciably during the observation window.
• Focus is on time dependence of viscosity, not just shear-rate dependence.

Concept / Approach:

A Newtonian fluid has viscosity independent of shear rate and time. A dilatant (shear-thickening) fluid shows viscosity increasing with shear rate, but not necessarily with time at a fixed shear rate. Rheopectic fluids, however, exhibit an increase in viscosity with time under constant shear, the mirror image of thixotropic fluids, which show a decrease with time under constant shear. Recognizing the time component is key to identifying rheopecty.

Step-by-Step Solution:

Verification / Alternative check:

Plot apparent viscosity versus time at a fixed shear rate. A rheopectic fluid shows a monotonically increasing trend during the shearing interval and partial structural persistence after shear removal compared to the pre-shear baseline.

Why Other Options Are Wrong:

A: Describes ideal Newtonian behavior, not rheopecty. B: Describes thixotropy and immediate recovery, not rheopecty. D: Time-dependent thinning (thixotropy), the opposite of rheopecty. E: Ignores the defining role of shear history in time-dependent fluids.

Common Pitfalls:

Confusing shear-rate dependence (instantaneous) with time dependence (history effects). Another mistake is assuming rheopecty and dilatancy are the same; dilatancy is rate-dependent thickening, whereas rheopecty is time-dependent thickening at roughly fixed rate.

Final Answer:

It increases with mixing time under sustained shear (time-dependent thickening).