Question

Assertion A: Bingham plastic fluid shows solid characteristics when stress applied is low.
Reason R: Before beginning of flow Bingham plastic fluid have finite yield stress.
In the light of the above statements, choose the most appropriate answer:

• A. Both A and R are correct and R is the correct explanation of A
• B. Both A and R are correct but R is NOT the correct explanation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

Think of a Bingham plastic as a "Lazy Fluid." It refuses to move (stays solid) until you push it hard enough (exceed the yield stress). Once it starts moving, it often behaves linearly like a Newtonian fluid.

Answer 1

The Correct Option is A

Concept: Bingham plastics are a type of non-Newtonian fluid that require a certain amount of stress to be applied before they begin to flow. Common examples include toothpaste and mayonnaise.

Step 1: Analyze the behavior of Bingham plastics (Assertion A).
A Bingham plastic behaves as a rigid body (solid) at low shear stresses. It will not deform or flow until the applied stress exceeds a critical value. Thus, Assertion A is correct.

Step 2: Analyze the Yield Stress (Reason R).
The "finite yield stress" ($\tau_y$) is the threshold stress that must be overcome to initiate flow. The rheological equation is $\tau = \tau_y + \eta(du/dy)$. Below this $\tau_y$, the shear rate is zero. Thus, Reason R is correct.

Step 3: Connect A and R.
The reason the fluid behaves like a solid at low stress is specifically $because$ of the existence of this yield stress. Therefore, R is the correct explanation of A.