A real fluid, in which the shear stress is NOT proportional to the rate of shear strain is called:
Try it first: pick an option, then read the explanation.
Correct answer: D. Non-Newtonian fluid
Asked in SSC JE 06/11/2024 (09am-11am)
Detailed Explanation
Fluids are classified rheologically by their shear stress response to deformation rate. When viscosity changes with shear rate, the linear relationship breaks down and the fluid is termed non-Newtonian.
Why “Non-Newtonian fluid” Is Correct
By definition, non-Newtonian fluids deviate from Newton's law of viscosity: shear stress is not linearly proportional to the rate of shear strain (apparent viscosity varies with shear rate).
| Fluid type | Viscosity changes with rate of shear? | Examples |
|---|---|---|
| Ideal fluid | No (μ = 0) | Theoretical only |
| Newtonian fluid | No (μ = const) | Water, air, oil, mercury |
| Non-Newtonian | Yes | Blood, toothpaste, ketchup, paint |
| Ideal plastic (Bingham) | No, but needs yield stress τ₀ first | Fresh concrete, drilling mud |
Why the Other Options Are Wrong
- A. Newtonian fluid: In a Newtonian fluid, shear stress is strictly proportional to the rate of shear strain (constant μ).
- B. Ideal plastic fluid: An ideal plastic (Bingham plastic) has a specific yield stress followed by a linear relation, which is a particular sub-class of non-Newtonian behavior.
- C. Ideal fluid: An ideal fluid is frictionless (zero viscosity) and non-existent in reality.
Key Point
Non-Newtonian fluids do not obey Newton's law: τ is not directly proportional to du/dy.
Key Concepts for Students
- Non-Newtonian fluids follow the power-law model: τ = k(du/dy)ⁿ.
- Pseudoplastic (shear-thinning, n < 1): viscosity decreases with shear rate (blood, paint).
- Dilatant (shear-thickening, n > 1): viscosity increases with shear rate (cornstarch in water).
Practice more: this is question 16 in Properties of Fluid MCQs – Part 2. Try the whole set and check every answer instantly.