Table of Contents

Introduction to Viscosity

  • Viscosity is a fundamental property of fluids that describes the internal friction or resistance to flow when subjected to an external force.
  • It can also be understood as the thickness or stickiness of a fluid.

Historical Background

Ancient Civilizations:

  • Early societies observed differences in the flow properties of liquids but lacked a scientific framework to explain them.

Newton’s Law of Viscosity:

  • Sir Isaac Newton formulated a law describing the relationship between shear stress and shear rate in a fluid, laying the foundation for understanding viscosity.

Mathematical Models:

  • Over time, scientists and engineers developed mathematical models and experimental methods to quantify and predict viscosity, enhancing its practical applications.

Understanding Viscosity


  • Viscosity is the measure of a fluid’s resistance to shear or flow. It indicates how thick or sticky a fluid is.



  • The relationship between shear stress () and shear rate (˙) in a fluid is described by the formula:
  • Where is the dynamic viscosity of the fluid.

Units of Viscosity in SI and CGS Systems

SI System:

  • The unit of viscosity in the SI (International System of Units) is the Pascal-second (Pa·s) or Newton-second per square meter (N·s/m^2).

CGS System:

  • In the CGS (Centimeter-Gram-Second) system, viscosity is measured in Poise (P) or dyne-second per square centimeter (dyne·s/cm^2).

Newton’s Law of Viscosity

Newton’s Law:

What is Viscosity ? States Newton's Law of Viscosity & define ...

  • Also known as the Newtonian Law of Viscosity, it states that the shear stress () in a fluid is directly proportional to the shear rate (�˙).
  • The linear relationship can be expressed as: Where is the viscosity coefficient or dynamic viscosity of the fluid.

Newtonian Fluids:

  • Fluids that obey Newton’s Law of Viscosity are termed as Newtonian fluids. Examples include water, air, and most common liquids and gases under normal conditions.

Factors Affecting Viscosity

Temperature and Viscosity

  • General Trend: In most fluids, viscosity decreases with an increase in temperature.
  • Explanation: As temperature rises, the kinetic energy of the fluid molecules increases, leading to weaker intermolecular forces and thus reduced viscosity.

Oil Viscosity Index and Viscosity Temperature Relation – About Tribology


  • Water: Becomes less viscous and flows more easily as it’s heated.
  • Motor Oil: Thins out and becomes less sticky when warmed up, improving engine lubrication.

Nature of Fluid and Viscosity

Different Liquids and Gases:

  • Variability: Viscosity values can vary widely among different fluids due to differences in molecular structure and composition.

The nature of fluids

Newtonian vs. Non-Newtonian Fluids:

  • Newtonian Fluids: Exhibit constant viscosity regardless of the applied shear stress, e.g., water and air.
  • Non-Newtonian Fluids: Viscosity changes with shear rate or stress, e.g., ketchup, honey, and certain types of industrial slurries.

Pressure and Viscosity

Effect of Pressure:

  • General Observation: In gases, viscosity tends to increase with an increase in pressure.
  • Non-Newtonian Fluids: In some non-Newtonian fluids like polymers, pressure can significantly alter viscosity, making it a crucial factor to consider in industrial processes.

Pressure-Viscosity Coefficient and Characteristics of Lubricants – About  Tribology


  • Compressibility: Gases can be compressed, leading to closer molecular interactions and increased viscosity.
  • Shear-Thinning or Shear-Thickening:
    • Shear-Thinning: Viscosity decreases with increasing shear rate, typical for certain non-Newtonian fluids under pressure.
    • Shear-Thickening: Viscosity increases with increasing shear rate, observed in some non-Newtonian fluids under pressure.

Types of Fluids Based on Viscosity

Newtonian Fluids

  • Definition:
    • Newtonian fluids are fluids that obey Newton’s Law of Viscosity, meaning they have a constant viscosity at a given temperature, regardless of the shear rate or stress applied.
  • Examples:
    • Water: At standard conditions, water exhibits consistent viscosity and follows Newton’s Law.
    • Air: For practical purposes and under normal conditions, air behaves as a Newtonian fluid.

Newton's Law of Viscosity, Newtonian and Non-Newtonian Fluids | SpringerLink

Non-Newtonian Fluids

  • Explanation:
    • Non-Newtonian fluids are fluids whose viscosity changes with the applied shear rate or stress. They do not follow Newton’s Law of Viscosity.
  • Classification:
    • Shear-Thinning Fluids: Viscosity decreases with increasing shear rate. Example: Ketchup, where shaking or squeezing reduces its viscosity.
    • Shear-Thickening Fluids: Viscosity increases with increasing shear rate. Example: Cornstarch and water mixture (sometimes called oobleck), which becomes thicker and more resistant when stirred or pressed quickly.
    • Bingham Plastic: Exhibits a yield stress before it begins to flow. Example: Toothpaste, which stays in place until a certain pressure is applied.
    • Thixotropic Fluids: Viscosity decreases over time under constant shear stress. Example: Paint, which becomes thinner when stirred but thickens when left undisturbed.

Applications of Non-Newtonian Fluids

  • Daily Life:
    • Food Industry:
      • Non-Newtonian fluids like ketchup, mayonnaise, and yogurt exhibit shear-thinning properties, making them easier to pour or spread.
    • Personal Care Products:
      • Toothpaste and certain lotions demonstrate Bingham plastic behavior, maintaining shape until squeezed or dispensed.
  • Industry:
    • Oil Drilling:
      • Drilling muds often consist of non-Newtonian fluids tailored to shear-thinning or shear-thickening properties to optimize drilling efficiency and safety.
    • Manufacturing:
      • Non-Newtonian fluids are used in various processes like mixing, coating, and 3D printing due to their adjustable viscosity and flow properties.

Importance and Applications of Viscosity

Fluid Mechanics:

  • Viscosity is paramount in understanding the behavior of fluids, influencing flow patterns, turbulence, and pressure drops in pipes and channels.
  • It determines the efficiency of fluid machines like pumps, turbines, and propellers.

Daily Life:

  • Examples:
    • The slow flow of honey compared to water illustrates viscosity.
    • The thicker consistency of motor oil and the pourability of ketchup are also examples.
  • Practical Implications:
    • Understanding viscosity helps in choosing the right fluids for various applications, like lubricants for machinery or paints for surfaces.

Industrial Processes:

  • Mixing and Stirring:
    • Viscosity affects the ease and efficiency of mixing processes in industries such as food and pharmaceuticals.
  • Pumping and Transportation:
    • Proper viscosity is crucial for the smooth and efficient pumping of liquids in chemical, petroleum, and wastewater industries.
  • Lubrication:
    • In automotive and machinery sectors, viscosity determines the effectiveness of lubricants in reducing friction and wear.


Conclusion on Viscosity

Viscosity, as a fundamental property of fluids in physics, serves as a cornerstone in understanding fluid behavior and dynamics. It defines a fluid’s resistance to flow or shear, effectively representing its thickness or stickiness.

  • Newtonian Fluids: These fluids, like water and air, adhere to Newton’s Law of Viscosity, maintaining a constant viscosity regardless of the applied shear stress. They are foundational in many fluid mechanics applications due to their predictable behavior.

  • Non-Newtonian Fluids: In contrast, non-Newtonian fluids, which include categories like shear-thinning, shear-thickening, Bingham plastic, and thixotropic fluids, do not follow this law. Their viscosity varies based on shear rate or stress, leading to a wide range of intriguing and often useful flow behaviors.


Viscosity is a measure of a fluid’s resistance to flow. In simpler terms, it’s how thick or thin a fluid is. Honey, for example, has a high viscosity, while water has a low viscosity.

The SI unit of viscosity is pascal-second (Pa·s). It can also be expressed as newton-second per square meter (N·s/m²).

Viscosity is a property of all fluids, both liquids and gases. So, you’ll encounter it everywhere you find fluids – from honey in your kitchen to the oil in your car engine.


1. What does viscosity measure?

  • a) Density
  • b) Speed
  • c) Resistance to flow
  • d) Temperature

Answer: c) Resistance to flow

2. Which of the following fluids is a Newtonian fluid?

  • a) Honey
  • b) Water
  • c) Ketchup
  • d) Cornstarch and water mixture

Answer: b) Water

3. In which system is the unit of viscosity given as Pascal-second (Pa·s)?

  • a) CGS
  • b) FPS
  • c) MKS
  • d) SI

Answer: d) SI

4. What happens to the viscosity of most fluids as temperature increases?

  • a) Increases
  • b) Remains constant
  • c) Decreases
  • d) Fluctuates

Answer: c) Decreases

5. Which type of non-Newtonian fluid becomes thinner when shaken or squeezed?

  • a) Shear-thickening
  • b) Bingham plastic
  • c) Shear-thinning
  • d) Thixotropic

Answer: c) Shear-thinning

6. What does a Bingham plastic fluid exhibit before flowing?

  • a) Constant viscosity
  • b) Increasing viscosity
  • c) Yield stress
  • d) Decreasing viscosity

Answer: c) Yield stress

7. What does the Newtonian Law of Viscosity state?

  • a) Viscosity varies with temperature.
  • b) Viscosity is constant.
  • c) Viscosity increases with pressure.
  • d) Viscosity is independent of shear rate.

Answer: b) Viscosity is constant.

8. What happens to the viscosity of gases with increasing pressure?

  • a) Increases
  • b) Decreases
  • c) Remains constant
  • d) Becomes zero

Answer: a) Increases

9. Which fluid exhibits shear-thickening behavior when stirred or pressed quickly?

  • a) Water
  • b) Air
  • c) Ketchup
  • d) Honey

Answer: c) Ketchup

10. What is the unit of viscosity in the CGS system?

  • a) Poise
  • b) Pascal-second
  • c) Newton-second
  • d) Joule-second

Answer: a) Poise

11. Which of the following is not a factor affecting viscosity?

  • a) Temperature
  • b) Pressure
  • c) Volume
  • d) Nature of fluid

Answer: c) Volume

12. What type of fluid has a viscosity that decreases over time under constant shear stress?

  • a) Newtonian fluid
  • b) Shear-thickening fluid
  • c) Shear-thinning fluid
  • d) Thixotropic fluid

Answer: d) Thixotropic fluid

13. Which fluid is often used in oil drilling due to its non-Newtonian behavior?

  • a) Water
  • b) Air
  • c) Bingham plastic
  • d) Newtonian fluid

Answer: c) Bingham plastic

14. What does the viscosity coefficient represent in the formula ?

  • a) Shear stress
  • b) Shear rate
  • c) Viscosity
  • d) Temperature

Answer: c) Viscosity

15. Which type of fluid exhibits a linear relationship between shear stress and shear rate?

  • a) Newtonian fluid
  • b) Shear-thickening fluid
  • c) Bingham plastic
  • d) Thixotropic fluid

Answer: a) Newtonian fluid

16. What does the term ‘thixotropic’ mean?

  • a) Increasing viscosity with time
  • b) Decreasing viscosity with time
  • c) Constant viscosity
  • d) Variable temperature

Answer: b) Decreasing viscosity with time

17. What is the main factor responsible for the viscosity of fluids?

  • a) Mass
  • b) Pressure
  • c) Molecular structure
  • d) Color

Answer: c) Molecular structure

18. Which fluid is used as a lubricant due to its low viscosity?

  • a) Honey
  • b) Motor oil
  • c) Ketchup
  • d) Toothpaste

Answer: b) Motor oil

19. What is the unit of viscosity in the FPS system?

  • a) Pascal-second
  • b) Poise
  • c) Slug-foot per second
  • d) Newton-second

Answer: c) Slug-foot per second

20. Which of the following fluids has a Bingham plastic behavior?

  • a) Water
  • b) Honey
  • c) Toothpaste
  • d) Air

Answer: c) Toothpaste

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