Subtopic: Time-dependent rheology: Thixotropy vs Rheopexy
Question Statement:
Which of the following statements regarding rheological behavior of food products is/are TRUE?
Options:
(A) Thixotropic food materials show a reversible decrease in apparent viscosity over time under constant shear rate (e.g. yogurt, condensed milk)
(B) Rheopectic materials show an increase in viscosity over time under constant shear
(C) Pseudoplastic fluids ($n < 1$) display shear thinning where apparent viscosity decreases as shear rate increases
(D) In a Newtonian fluid, viscosity is dependent on the applied shear rate
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
1. Thixotropy: time-dependent breakdown of structural networks under shear (A is true).
2. Rheopexy: time-dependent shear thickening (B is true).
3. Pseudoplastic: shear thinning with $n < 1$ (C is true).
4. In Newtonian fluids, dynamic viscosity is strictly constant and independent of shear rate (D is false).