Question

Which of the following describes a Rheopectic fluid?

• A. Viscosity decreases with time under constant shear
• B. Viscosity increases with time under constant shear
• C. Viscosity remains constant but the fluid has a yield stress
• D. Viscosity increases as the shear rate increases

Hint:

Thixotropic and Rheopectic behaviors are time-dependent:
- Thixotropic: Viscosity decreases with time under constant shear (e.g., ketchup).
- Rheopectic: Viscosity increases with time under constant shear (e.g., gypsum paste).

Answer 1

The Correct Option is B

Step 1: Understanding the Problem:
The question asks for the correct rheological definition of a "rheopectic" fluid, which exhibits time-dependent non-Newtonian flow behavior.

Step 2: Detailed Explanation:

• Time-Dependent Fluid Rheology: Some fluids exhibit viscosity changes that depend not only on the shear rate but also on the duration of the applied shear stress. These are classified into:

• Thixotropic Fluids: Viscosity decreases over time when a constant shear stress is applied. Common in ketchup, mayonnaise, and paint. This matches option (A).

• Rheopectic (Anti-thixotropic) Fluids: Viscosity increases over time when a constant shear stress is applied. The fluid solidifies or thickens upon prolonged agitation but returns to a lower-viscosity state when allowed to rest. Examples include print inks, gypsum suspensions, and certain starch pastes. This matches option (B).

• Why Other Options are Incorrect:

• Viscosity remains constant with a yield stress: Describes a Bingham Plastic or viscoplastic fluid (e.g., chocolate paste), corresponding to option (C).

• Viscosity increases as shear rate increases: Describes a time-independent Dilatant (shear-thickening) fluid (e.g., concentrated corn starch suspension), corresponding to option (D).

Step 3: Final Answer:
A rheopectic fluid is defined by an increase in viscosity over time under a constant shear rate, corresponding to option (B).