Viscosity and Rheology One Shot Notes and MCQs

Viscosity and Rheology One Shot Notes and MCQs

Viscosity and Rheology One Shot Notes and MCQs

Viscosity and Rheology One Shot Notes


Viscosity and rheology are essential concepts in pharmaceutical science, especially for formulating and characterizing liquids, suspensions, and semi-solids. Understanding the flow behavior of various systems ensures pharmaceutical products’ stability, uniformity, and usability.


1. Viscosity and Law of Flow

  • Viscosity:
    It is the internal resistance offered by a fluid to flow. It measures how easily a fluid flows under an applied force.
    Formula:
    η=FA⋅dxdv\eta = \frac{F}{A} \cdot \frac{dx}{dv}
    Where:

    • η\eta = viscosity
    • FF = applied force
    • AA = area
    • dxdv\frac{dx}{dv} = velocity gradient
  • Kinematic Viscosity:
    It is the ratio of dynamic viscosity (η\eta) to the fluid’s density (ρ\rho).
    Formula:
    ν=ηρ\nu = \frac{\eta}{\rho}
    Units: Stokes (St) or centistokes (cSt).
  • Effect of Temperature on Viscosity:
    • Liquids: Viscosity decreases with an increase in temperature.
    • Gases: Viscosity increases with temperature due to increased molecular collisions.

2. Newtonian and Non-Newtonian Systems

Newtonian Systems

  • Fluids that obey Newton’s Law of Flow:
    F=η⋅dvdxF = \eta \cdot \frac{dv}{dx}
    (Shear stress is directly proportional to shear rate).
    Examples: Water, ethanol, glycerin, simple oils.

Characteristics:

  • Constant viscosity, independent of applied shear rate.
  • Straight-line graph of shear stress vs. shear rate.

Non-Newtonian Systems

  • Fluids whose viscosity changes with the applied shear rate or shear stress.

Types of Non-Newtonian Systems:

  1. Pseudoplastic Systems:
    • Viscosity decreases with an increase in shear rate (shear-thinning).
    • Example: Polymer solutions, ketchup, blood.
    • Graph: Curved line with a decreasing slope.
  2. Dilatant Systems:
    • Viscosity increases with an increase in shear rate (shear-thickening).
    • Example: Suspensions with a high concentration of solids (e.g., corn starch in water).
    • Graph: Curved line with an increasing slope.
  3. Plastic Systems:
    • A minimum force (yield stress) must be applied before the fluid flows.
    • Example: Toothpaste, butter.
    • Graph: Shows a straight line after overcoming the yield point.

3. Thixotropy and Negative Thixotropy

  • Thixotropy:
    • A time-dependent decrease in viscosity when shear is applied, followed by recovery when shear is removed.
    • Example: Gel-based formulations such as lotions and creams.
    • Important for pharmaceutical formulations like suspensions, where a high viscosity at rest prevents sedimentation but low viscosity under shear ensures easy flow.
  • Negative Thixotropy (Anti-thixotropy):
    • A time-dependent increase in viscosity with shear application.
    • Occurs in some colloidal suspensions.

4. Determination of Viscosity

Several methods and instruments are available for measuring the viscosity of fluids and semi-solid materials.

4.1 Capillary Viscometer

  • Principle: Measures the time taken for a liquid to pass through a capillary tube under gravity.
  • Example: Ostwald viscometer.
  • Application: Suitable for low-viscosity fluids such as solvents and water.

4.2 Falling Ball Viscometer

  • Principle: A ball falls through a fluid, and the time taken for the ball to pass between two points is recorded.
  • Stokes’ Law:
    η=2g(ρs−ρf)r29v\eta = \frac{2 g (\rho_s – \rho_f) r^2}{9 v}
    Where:

    • gg = acceleration due to gravity
    • ρs\rho_s = density of the sphere
    • ρf\rho_f = density of the fluid
    • rr = radius of the sphere
    • vv = velocity of the sphere
  • Application: Used for moderately viscous liquids.

4.3 Rotational Viscometer

  • Principle: Measures the torque required to rotate a spindle in the fluid at a constant speed.
  • Example: Brookfield viscometer.
  • Application: Used for measuring the viscosity of creams, gels, and semi-solid formulations.

5. Derived Properties of Powders

Porosity:

  • The fraction of void space within a bulk of powder.
  • Formula:
    Porosity(%)=VvoidsVbulk×100\text{Porosity} (\%) = \frac{V_{\text{voids}}}{V_{\text{bulk}}} \times 100

Packing Arrangement:

  • Refers to how particles are arranged in a bulk (close packing or loose packing).

Density:

  • Bulk density: Density of a powder when loosely packed.
  • Tapped density: Density after tapping to reduce voids.

Bulkiness:

  • The reciprocal of bulk density, indicating the volume occupied by a unit mass of powder.

6. Rheological Properties and Their Importance in Formulations

  • Rheology is essential for understanding the flow behavior of pharmaceutical products like suspensions, emulsions, and ointments.
  • Importance in Formulation:
    • Ensures proper flow during manufacturing (e.g., mixing, pumping, filling).
    • Affects drug delivery (e.g., easy flow through syringes or inhalers).
    • Helps maintain product stability (e.g., suspensions should not settle quickly).

MCQs on Viscosity and Rheology

  1. The resistance of a liquid to flow is known as:
    • a) Viscosity
    • b) Surface tension
    • c) Compressibility
    • d) Density
      Answer: a
  2. Which law describes the flow behavior of Newtonian fluids?
    • a) Stokes’ law
    • b) Arrhenius law
    • c) Law of flow
    • d) Boyle’s law
      Answer: c
  3. In Newtonian systems, viscosity is:
    • a) Dependent on shear rate
    • b) Independent of shear rate
    • c) Affected by applied pressure
    • d) Variable with concentration
      Answer: b
  4. A Newtonian fluid follows which of the following relationships?
    • a) Shear stress is directly proportional to shear rate
    • b) Shear stress is inversely proportional to shear rate
    • c) Viscosity increases with shear rate
    • d) Viscosity decreases with temperature
      Answer: a
  5. Which of the following fluids is an example of a Newtonian system?
    • a) Blood
    • b) Glycerin
    • c) Toothpaste
    • d) Paint
      Answer: b
  6. Kinematic viscosity is defined as:
    • a) Absolute viscosity divided by density
    • b) Density divided by absolute viscosity
    • c) Absolute viscosity times density
    • d) The shear stress per unit area
      Answer: a
  7. What is the effect of increasing temperature on the viscosity of a Newtonian liquid?
    • a) Increases viscosity
    • b) Decreases viscosity
    • c) No effect on viscosity
    • d) First increases, then decreases viscosity
      Answer: b
  8. Which of the following is a unit of kinematic viscosity?
    • a) Pascal
    • b) Poise
    • c) Centistoke
    • d) Joule
      Answer: c
  9. What type of flow behavior does a non-Newtonian fluid exhibit?
    • a) Constant viscosity
    • b) Viscosity changes with shear rate
    • c) Viscosity remains independent of temperature
    • d) Viscosity depends only on pressure
      Answer: b
  10. Which of the following describes a pseudoplastic fluid?
    • a) Viscosity increases with increasing shear rate
    • b) Viscosity decreases with increasing shear rate
    • c) Viscosity remains constant regardless of shear rate
    • d) Viscosity decreases with temperature
      Answer: b
  11. A dilatant system is also known as:
    • a) Thixotropic fluid
    • b) Shear-thinning fluid
    • c) Shear-thickening fluid
    • d) Bingham plastic fluid
      Answer: c
  12. Which of the following is a characteristic of plastic flow?
    • a) Flows only after a certain stress threshold
    • b) Viscosity remains constant with shear rate
    • c) Viscosity increases indefinitely with shear rate
    • d) Flows immediately with zero stress
      Answer: a
  13. The minimum stress required to initiate flow in a plastic system is known as:
    • a) Flow index
    • b) Shear rate
    • c) Yield value
    • d) Viscous modulus
      Answer: c
  14. Thixotropy is defined as:
    • a) Decrease in viscosity with constant shear over time
    • b) Increase in viscosity with constant shear over time
    • c) Viscosity independent of time
    • d) None of the above
      Answer: a
  15. Thixotropy is most commonly observed in:
    • a) Newtonian systems
    • b) Non-Newtonian systems
    • c) Ideal solutions
    • d) Solid systems
      Answer: b
  16. Which of the following is an example of a thixotropic material?
    • a) Water
    • b) Toothpaste
    • c) Paint
    • d) Sugar solution
      Answer: c
  17. Negative thixotropy is also known as:
    • a) Anti-thixotropy
    • b) Dilatancy
    • c) Shear-thickening
    • d) Pseudoplasticity
      Answer: a
  18. Which method is commonly used to measure viscosity in Newtonian fluids?
    • a) Capillary viscometer
    • b) Falling ball viscometer
    • c) Rotational viscometer
    • d) All of the above
      Answer: d
  19. In a capillary viscometer, the viscosity is determined by measuring:
    • a) The time for a fluid to flow through a capillary tube
    • b) The force required to push a fluid through a tube
    • c) The angle of flow
    • d) The rate of cooling
      Answer: a
  20. A rotational viscometer is particularly useful for:
    • a) Determining viscosity in Newtonian fluids only
    • b) Determining viscosity in non-Newtonian fluids
    • c) Determining viscosity at zero shear rate
    • d) Measuring viscosity of solid materials
      Answer: b
  21. The unit of viscosity in the SI system is:
    • a) Centipoise
    • b) Pascal-second
    • c) Newton-meter
    • d) Joule-second
      Answer: b
  22. The phenomenon where viscosity decreases with increasing shear rate is known as:
    • a) Pseudoplasticity
    • b) Dilatancy
    • c) Plastic flow
    • d) Bingham flow
      Answer: a
  23. For a falling ball viscometer, which parameter is used to determine viscosity?
    • a) Time taken by a ball to fall through the liquid
    • b) Weight of the ball
    • c) Volume of the liquid
    • d) Temperature of the room
      Answer: a
  24. In a dilatant fluid, viscosity:
    • a) Increases with increasing shear rate
    • b) Decreases with increasing shear rate
    • c) Remains constant regardless of shear rate
    • d) Is inversely related to temperature
      Answer: a
  25. An example of a dilatant material is:
    • a) Cornstarch suspension
    • b) Water
    • c) Olive oil
    • d) Alcohol
      Answer: a
  26. A fluid with a high yield value would most likely exhibit:
    • a) Plastic flow
    • b) Newtonian flow
    • c) Pseudoplastic flow
    • d) Dilatant flow
      Answer: a
  27. The term “shear-thinning” refers to:
    • a) An increase in viscosity with increased shear rate
    • b) A decrease in viscosity with increased shear rate
    • c) Viscosity independent of shear rate
    • d) None of the above
      Answer: b
  28. The study of deformation and flow behavior of materials is known as:
    • a) Rheology
    • b) Hydrology
    • c) Fluid dynamics
    • d) Capillarity
      Answer: a
  29. Which instrument is commonly used to measure viscosity in non-Newtonian fluids?
    • a) Rotational viscometer
    • b) Capillary viscometer
    • c) Falling ball viscometer
    • d) Ostwald viscometer
      Answer: a
  30. Which of the following does NOT affect the viscosity of a liquid?
    • a) Temperature
    • b) Shear rate
    • c) Density
    • d) Pressure
      Answer: c
  1. What type of flow does toothpaste exhibit when squeezed from a tube?
    • a) Newtonian
    • b) Plastic
    • c) Pseudoplastic
    • d) Dilatant
      Answer: b
  2. Which of the following exhibits Newtonian behavior?
    • a) Creams
    • b) Paint
    • c) Water
    • d) Toothpaste
      Answer: c
  3. Thixotropic behavior is desirable in which type of pharmaceutical formulations?
    • a) Intravenous solutions
    • b) Oral suspensions
    • c) Topical gels
    • d) Tablets
      Answer: c
  4. A substance that shows increased viscosity over time when subjected to constant shear stress demonstrates:
    • a) Positive thixotropy
    • b) Negative thixotropy
    • c) Newtonian flow
    • d) Bingham flow
      Answer: b
  5. What does a Brookfield viscometer measure?
    • a) Yield value
    • b) Shear stress
    • c) Viscosity
    • d) Shear rate
      Answer: c
  6. In which type of fluid does viscosity increase when subjected to a continuous force?
    • a) Newtonian fluid
    • b) Dilatant fluid
    • c) Pseudoplastic fluid
    • d) Plastic fluid
      Answer: b
  7. The viscosity of which fluid increases with an increase in temperature?
    • a) Water
    • b) Olive oil
    • c) Glycerin
    • d) None of the above
      Answer: d
  8. The time-dependent decrease in viscosity is a characteristic of:
    • a) Dilatant fluids
    • b) Newtonian fluids
    • c) Thixotropic fluids
    • d) Pseudoplastic fluids
      Answer: c
  9. Which term describes the phenomenon where a material returns to a higher viscosity after shear is removed?
    • a) Rheopexy
    • b) Thixotropy
    • c) Dilatancy
    • d) Shear-thickening
      Answer: a
  10. The flow characteristic of ketchup is an example of:
    • a) Newtonian behavior
    • b) Plastic flow
    • c) Thixotropy
    • d) Dilatancy
      Answer: c
  11. For a capillary viscometer, the flow time is directly proportional to:
    • a) Viscosity
    • b) Density
    • c) Shear rate
    • d) Temperature
      Answer: a
  12. As temperature increases, the viscosity of a gas:
    • a) Increases
    • b) Decreases
    • c) Remains the same
    • d) Becomes zero
      Answer: a
  13. What is the effect of a shear rate increase on a pseudoplastic fluid?
    • a) Viscosity increases
    • b) Viscosity decreases
    • c) Viscosity remains constant
    • d) Density increases
      Answer: b
  14. Which device measures viscosity by observing the time for a ball to fall through a liquid?
    • a) Capillary viscometer
    • b) Falling ball viscometer
    • c) Rotational viscometer
    • d) Brookfield viscometer
      Answer: b
  15. A Bingham plastic behaves like:
    • a) A solid until the yield stress is exceeded
    • b) A liquid under any amount of stress
    • c) A gas at high temperatures
    • d) A non-flowing solid
      Answer: a

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