GATE BT 2025 • Q25
MSQ • 1 Mark
Section 4: Bioprocess Engineering and Process Biotechnology
Mass transfer in bioprocesses (kLa)
Subtopic: Fundamentals of Mass transfer in bioprocesses (kLa)
Question Statement:
Which of the following fluids exhibit(s) non-Newtonian behaviour at 25 C?
Options:
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(A) Toothpaste
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(B) Mercury
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(C) Brine
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(D) Blood plasma
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2025 Q.25)**
**1. Governing Concept**:
- **Topic**: Fluid Mechanics & Rheology of Fluids
- Fluids are classified by their response to applied shear stress:
- **Newtonian Fluids**: Exhibit constant dynamic viscosity independent of shear rate or duration of shear (e.g. pure water, ethanol, benzene, dilute sucrose solutions).
- **Non-Newtonian Fluids**: Viscosity varies with shear rate or time:
- Pseudoplastic (shear-thinning, e.g. blood, polymer solutions like xanthan gum, toothpaste).
- Dilatant (shear-thickening, e.g. cornstarch in water).
- Bingham plastic / Herschel-Bulkley (yield stress fluids).
**2. Option Analysis**:
- **Option (A) [Human Blood] is CORRECT**: Exhibits non-Newtonian, shear-thinning (pseudoplastic) and thixotropic behavior due to erythrocyte rouleaux formation at low shear and deformability/alignment at high shear.
- **Option (B) [Pure Water] is INCORRECT**: Canonical Newtonian fluid with constant viscosity $\mu \approx 1\text{ cP}$ at room temperature.
- **Option (C) [Ethanol] is INCORRECT**: Newtonian fluid with linear shear stress vs. shear rate response.
- **Option (D) [Xanthan gum aqueous solution] is CORRECT**: Highly pseudoplastic (shear-thinning) non-Newtonian polymer fluid due to alignment of rigid polysaccharide chains under shear.
**3. Final Answer**:
- **Correct Options**: **A, D**
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