Rheological properties in bioprocessing MCQs With Answer

Introduction: Rheological properties in bioprocessing are central to understanding how biological fluids—such as fermentation broths, cell suspensions, and polymer solutions—behave under flow and deformation. For M.Pharm students, mastering rheology helps optimize mixing, mass and oxygen transfer, pumping, and scale-up of bioprocesses, and improves downstream operations like filtration and centrifugation. This collection of MCQs focuses on conceptual and applied aspects: Newtonian vs non-Newtonian behavior, rheological models (power-law, Bingham, Herschel-Bulkley), viscoelasticity, thixotropy, measurement techniques, and how rheology influences equipment selection and process performance in pharmaceutical bioprocess engineering.

Q1. Which statement best distinguishes a Newtonian fluid from a non-Newtonian fluid?

• Newtonian fluid has viscosity independent of shear rate; non-Newtonian fluid has viscosity dependent on shear rate
• Newtonian fluid is compressible; non-Newtonian fluid is incompressible
• Newtonian fluids are always dilute solutions; non-Newtonian fluids are always suspensions
• Newtonian fluids show viscoelastic behavior while non-Newtonian fluids do not

Correct Answer: Newtonian fluid has viscosity independent of shear rate; non-Newtonian fluid has viscosity dependent on shear rate

Q2. In the power-law (Ostwald–de Waele) model τ = K·(γ̇)^n, what does n < 1 indicate about the fluid?

• Shear-thinning (pseudoplastic) behavior
• Shear-thickening (dilatant) behavior
• Newtonian behavior
• Bingham plastic with yield stress

Correct Answer: Shear-thinning (pseudoplastic) behavior

Q3. Which rheological parameter in the power-law model represents the consistency (apparent viscosity magnitude)?

• n (flow behavior index)
• K (consistency index)
• τ0 (yield stress)
• γ̇ (shear rate)

Correct Answer: K (consistency index)

Q4. A fermentation broth exhibits a finite stress below which it does not flow. Which model commonly describes this behavior?

• Newtonian model
• Power-law model without yield stress
• Bingham plastic model
• Maxwell viscoelastic model

Correct Answer: Bingham plastic model

Q5. Thixotropy in bioprocess fluids is best defined as:

• Permanent increase in viscosity with time under shear
• Time-dependent decrease in viscosity under constant shear and recovery when shear is removed
• Instantaneous elastic response to shear followed by viscous flow
• Viscosity independent of both shear and time

Correct Answer: Time-dependent decrease in viscosity under constant shear and recovery when shear is removed

Q6. Which rheological measurement device is most appropriate for characterizing viscoelastic behavior (storage and loss moduli) of fermentation broths?

• Capillary viscometer
• Ubbelohde viscometer
• Rotational rheometer with oscillatory mode
• Anemometer

Correct Answer: Rotational rheometer with oscillatory mode

Q7. How does increased broth viscosity generally affect oxygen transfer rate (OTR) in aerobic bioreactors?

• Increases OTR by improving bubble dispersion
• Reduces OTR due to lowered gas-liquid mass transfer coefficient (kLa)
• Has no effect on OTR
• Increases OTR by raising oxygen solubility

Correct Answer: Reduces OTR due to lowered gas-liquid mass transfer coefficient (kLa)

Q8. For scale-up of stirred bioreactors processing non-Newtonian broths, which criterion is often modified to account for non-Newtonian flow?

• Maintain constant impeller tip speed only
• Use Reynolds number based only on Newtonian viscosity
• Use generalized Reynolds number or Metzner–Otto correlation incorporating apparent viscosity
• Ignore viscosity and scale by geometric similarity alone

Correct Answer: Use generalized Reynolds number or Metzner–Otto correlation incorporating apparent viscosity

Q9. Apparent viscosity is most properly defined as:

• The true viscosity measured at infinite shear
• Viscosity of a fluid at a specific shear rate for non-Newtonian fluids
• Viscosity independent of shear history
• Viscosity measured only in capillary flow

Correct Answer: Viscosity of a fluid at a specific shear rate for non-Newtonian fluids

Q10. Which phenomenon describes materials that exhibit both solid-like elastic and liquid-like viscous responses?

• Newtonian behavior
• Thixotropy
• Viscoelasticity
• Yield stresslessness

Correct Answer: Viscoelasticity

Q11. In microbial suspensions, an increase in cell concentration typically results in:

• Lower viscosity due to lubrication between cells
• Higher viscosity and increased non-Newtonian behavior
• No change in rheological properties
• Transformation to ideal gas behavior

Correct Answer: Higher viscosity and increased non-Newtonian behavior

Q12. Which correlation or concept helps relate shear rate in a stirred tank to impeller speed for non-Newtonian fluids?

• Einstein viscosity correlation
• Metzner–Otto concept relating apparent shear rate to impeller speed through a constant
• Arrhenius temperature dependence
• Henry’s law for gas solubility

Correct Answer: Metzner–Otto concept relating apparent shear rate to impeller speed through a constant

Q13. Which downstream operation is most directly hindered by high broth viscosity and shear-thinning behavior?

• Lyophilization
• Filtration and membrane fouling
• Sterile filtration at micron scale is improved
• Autoclaving time is reduced

Correct Answer: Filtration and membrane fouling

Q14. The flow behavior index (n) of 0.2 in a broth indicates what practical consequence for mixing?

• Very close to Newtonian; standard mixing rules apply
• Strong shear-thinning—high apparent viscosity at low shear, making mixing difficult in dead zones
• Shear-thickening; impeller will become more effective at higher speeds
• Negative viscosity indicates unstable flow

Correct Answer: Strong shear-thinning—high apparent viscosity at low shear, making mixing difficult in dead zones

Q15. Which rheological test is most useful to determine yield stress of a concentrated biomass slurry?

• Steady shear flow curve with stress ramp including low-stress region
• Simple capillary viscometry at high shear only
• Measuring density by pycnometer
• Gas chromatography

Correct Answer: Steady shear flow curve with stress ramp including low-stress region

Q16. Effect of temperature on viscosity of polymeric fermentation broths is commonly described by:

• Henry’s law
• Arrhenius-type or Williams-Landel-Ferry (WLF) type temperature dependence
• Ostwald–de Waele with n>1 only
• Reynolds number independence

Correct Answer: Arrhenius-type or Williams-Landel-Ferry (WLF) type temperature dependence

Q17. In laminar flow of a power-law fluid in a pipe, how is pressure drop affected compared to a Newtonian fluid with same apparent viscosity at a reference shear rate?

• Identical pressure drop at same mean velocity always
• Depends on flow index n; pressure drop relation differs and must use analytical solution for power-law
• Pressure drop is zero for power-law fluids
• Pressure drop is inversely proportional to pipe diameter but independent of n

Correct Answer: Depends on flow index n; pressure drop relation differs and must use analytical solution for power-law

Q18. Which of the following describes wall slip in rheometry and bioprocessing contexts?

• Enhanced adhesion at the fluid-solid interface
• Relative motion between bulk fluid and bounding surface causing underestimation of bulk viscosity
• Complete solidification at the wall
• An increase of measured viscosity at all shear rates

Correct Answer: Relative motion between bulk fluid and bounding surface causing underestimation of bulk viscosity

Q19. For oxygen mass transfer in non-Newtonian broths, which factor is most directly modified by rheology?

• Gas composition of supplied air
• Interfacial area and kL (liquid-side mass transfer coefficient) due to bubble size and coalescence behavior
• Enzyme kinetics inside cells
• Boiling point of the medium

Correct Answer: Interfacial area and kL (liquid-side mass transfer coefficient) due to bubble size and coalescence behavior

Q20. Which impeller selection principle is true for high-viscosity, shear-thinning fermentation broths?

• Use high-speed Rushton turbines to maximize shear
• Use pitched-blade or helical ribbon impellers to generate axial flow and good bulk mixing at low shear rates
• Impeller type is irrelevant for non-Newtonian fluids
• Always use gas-lift reactors instead of mechanical agitation

Correct Answer: Use pitched-blade or helical ribbon impellers to generate axial flow and good bulk mixing at low shear rates

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