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What is Viscosity?

Last Updated : 23 Jul, 2025
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Viscosity is a fundamental property of liquids that describes their internal resistance to flow. Imagine three bowls—one filled with water and the other with oil and honey. If you were to tip the three bowls and observe the flow, you’d quickly notice that water pours out much faster than oil and honey. This simple observation highlights the difference in the viscosity of liquids. Honey and Oil, being thicker and more resistant to flow, have a higher viscosity than water. Here, we'll explore viscosity, its definition, formula, and measurement.

Viscosity
Viscosity of different fluids

What is Viscosity?

Viscosity is the measure of a fluid's resistance to flow. It occurs due to the internal friction between the fluid's layers as they move past each other. Essentially, viscosity reflects how "thick" or "sticky" a fluid is. A fluid with high viscosity, like honey, has stronger molecular forces that create significant resistance to flow.

A fluid with low viscosity, like water, flows more easily because its molecular interactions are weaker, leading to less internal friction. While gases also exhibit viscosity, it's less noticeable in everyday situations compared to liquids. This resistance to flow caused by internal friction between the fluid layers is called viscous force, which plays an important role in how fluids behave and interact with objects moving through them.

Viscosity Definition

Viscosity of any liquid is defined as,

"The measure of the resistance of the flow of the liquid."

Viscosity is the property of any liquid which tells us how a liquid flow under the action of gravity. It tells that any liquid with a lower viscosity flows easily whereas the liquid with high viscosity flows with great difficulties.

Unit of Viscosity

SI unit for measuring the viscosity of liquid is Poiseiulle (PI). Other units used for measuring viscosity are Newton-Second per Square Metre (Nsm-2) or Pascal-Second (Pas).

Viscosity Dimensional Formula

The dimensional formula of the viscosity is [ML-1T-1]

Viscosity Formula

The viscosity of a fluid is defined as the ratio of the shearing stress to the rate of change in velocity (velocity gradient) within the fluid. In simpler terms, it measures how much force is required to move one layer of the fluid over another.

For example, when an object, such as a sphere, is dropped into a fluid, the fluid's viscosity can be determined by observing how the fluid resists the motion of the object as it moves through it. This relationship can be expressed mathematically using the formula:

η = 2ga2(∆ρ) / 9v

where,

  • g is Acceleration due to Gravity
  • a is Radius of Sphere
  • ∆ρ is Density difference between fluid and sphere tested
  • v is Velocity of Sphere

Types of Viscosity

We can measure Viscosity by two methods and the viscosity measured by these two methods is called the types of viscosity. The two types of viscosity are:

  1. Dynamic Viscosity (Absolute Viscosity)
  2. Kinematic Viscosity

One way is to measure the fluid’s resistance to flow when an external force is applied. This is known as Dynamic Viscosity. And the other way is to measure the resistive flow of a fluid under the weight of gravity. We call this measure of fluid viscosity kinematic viscosity.

The Formula for Dynamic Viscosity is given as,

Dynamic Viscosity = Shearing Stress/Shearing Rate Change

The Kinematic Viscosity Formula is given as,

Kinematic Viscosity = Absolute Viscosity/Density of the Liquid

Read More, Dynamic Viscosity and Kinematic Viscosity

Co-efficient of Viscosity

According to Newton’s law of viscosity, the viscous drag, between these layers is,

  • Directly proportional to area (A) of the layer F ∝ A
  • Directly proportional to velocity gradient (dv/dx) between the layers F ∝ (dv/dx)

Therefore, it can be written as:

F ∝ A (dv/dx)

Lets remove the proportionality sign by introducing a proportionality constant η.

F = η A (dv/dx)

Here, η is called the coefficient of viscosity. 

If A = 1 m2 and dv/dx = 1 s-1 then the above expression becomes: 

F = η

Thus, the coefficient of viscosity of a liquid is defined as the viscous drag or force acting per unit area of the layer having a unit velocity gradient perpendicular to the direction of the flow of the liquid. 

Viscosity Co-efficient Units

Co-efficient of Viscosity is measured in various units as,

  • In the CGS system, the unit of coefficient of viscosity is dynes s cm-2 or Poise
  • In the SI system the unit of coefficient of viscosity N s m-2 or deca-poise
  • Dimensional formula for the coefficient of viscosity is [ML-1 T-1]

Variation of Viscosity

The coefficient of viscosity depends on the following mentioned factors.

  • Effect of Temperature on Viscosity: The viscosity of liquids decreases with an increase in temperature. The viscosity of gases increases with an increase in temperatures as η ∝ √T.  
  • Effect of Pressure on Viscosity: The coefficient of viscosity of liquids rises as pressure increases, although there is no relationship to explain the phenomenon thus far.

The table given below lists some fluids and their coefficient of viscosity at different temperatures:

Fluid

Temperature (in °C)

η (deca-poise)

Air

20

0.018 × 10-3

Water

0

1.8 × 10-3

20

1.0 × 10-3

Blood

100

0.3 × 10-3

37

2.7 × 10-3

Engine Oil

30

250 × 10-3

Glycerine

0

10

20

1.5

Newtonian and Non-Newtonian Fluids

The viscosity of any liquid is directly influenced by the change in pressure and temperature in the liquid. So on this basis we have two types of liquid that are,

  1. Newtonian Fluids
  2. Non-Newtonian Fluids

1. Newtonian Fluids

Any fluid whose viscosity remains constant when the amount of shear is applied at a constant temperature is called Newtonian fluid. There is a linear relationship between viscosity and shear stress in the case of Newtonian Fluid.

Examples: Water, Alcohol, Petroleum, and others.

2. Non-Newtonian Fluids

Non-Newtonian fluids are the opposite of Newtonian fluids i.e. on applying shear, the viscosity of non-Newtonian fluids changes, depending on the fluid. 

Examples: Ketchup, Quicksand, Silly Putty, etc.

Measurement of Viscosity

The viscosity of a liquid is often determined by observing the fall of a metal ball through the substance and recording the time it takes for the ball to reach the bottom. A slower fall indicates a higher viscosity. This method doesn't provide an accurate idea of the viscosity, but a more precise measurement is given by the viscometer.

Viscometer
Viscometer

U-Tube Viscometer

  • A U-tube viscometer, also known as a Glass Capillary or Ostwald Viscometer, measures the viscosity of liquids.
  • It consists of a U-shaped tube with two reservoir bulbs and a narrow capillary tube in one arm.
  • The capillary tube has a fine, uniform bore to control the liquid's flow.
  • The upper bulb is located above the capillary, and the lower bulb is positioned at the opposite end of the U-tube.
  • The liquid is drawn into the capillary tube via suction from the upper bulb.
  • The liquid flows through the capillary and into the lower bulb.
  • The device has two reference points, one above and one below the upper bulb, indicating a known liquid volume.
  • The time it takes for the liquid to travel between these two points is measured.
  • This time is proportional to the liquid's kinematic viscosity.
  • To determine viscosity, the flow time is recorded and multiplied by a conversion factor specific to the viscometer.
  • The method provides an accurate measurement by accounting for the liquid's resistance to flow, offering more precision than simpler viscosity measurement methods.

Bernoulli's Theorem

Bernoulli's Theorem states that for an incompressible, non-viscous fluid (ideal fluid) flowing through a streamline, the total mechanical energy along the streamlines remains constant. This total energy is the sum of the pressure energy, kinetic energy, and potential energy of the fluid.

For Unit Volume: 

P+1/2 ​ρv2+ρgh=Constant

  • P is the pressure at a point in the fluid
  • ρ is the density of the fluid
  • v is the velocity of the fluid
  • g is the gravitational acceleration
  • h is the height of the fluid above a reference point

For Unit Mass:

P+1/2 ​ρv2+gh=Constant

where,

  • P/ρrepresents the pressure head, the potential energy per unit mass of the fluid
  • v2represents the kinetic energy per unit mass
  • gh represents the potential energy per unit mass due to gravity

Read More, Bernoulli’s Principle

Solved Examples on Viscosity

Example 1: There is a 3 mm thick layer of glycerin between a flat plate and a large plate. If the viscosity coefficient of glycerin is 2 N s/m2 and the area of the plane plate is 48 cm. How much force is required to move the plate at a speed of 6 cm/s? 

Solution:

Given,
Thickness of the layer, dx = 3 mm = 3 × 10-3 m.
Coefficient of viscosity, η = 2 N s/m2 
Change in speed, dv = 6 cm/s = 6 × 10-2 m/s.
Area of the plate, A = 48 cm2 = 48 × 10-4 m2
Formula to calculate the force required to move the plate is, 
F = ηA × (dv/dx)
Substitute the given values in the above expression as:
F = 2 N s/m2 × 48 × 10-4 m2 × (6 × 10-2 m/s / 3 × 10-3 m)
= 192 × 10-3 N
= 0.192 N

Example 2: The diameter of a pipe is 2 cm. what will be the maximum average trick of water for level flow? The viscosity coefficient for water is 0.001 N-s/m2.

Solution: 

Given,
Diameter of the pipe, D = 2 cm = 0.02 m
Viscosity coefficient, η = 0.001 N-s/m2
Density of water, ρ = 1000 kg/m3
Since, the maximum value of K for level flow is 2000
Therefore, the formula to calculate the maximum speed of water is,    
v = Kη / ρD
Substitute the given values in the above expression to calculate v as,
v = 2000 × 0.001 N-s/m2 / 1000 kg/m3 × 0.02 m
 = 0.1 m/s 

Example 3: The shear stress at a point in a liquid is found to be 0.03 N/m2. The velocity gradient at the point is 0.21 s-1. What will be its viscosity?

Solution:

Given,
Shear stress, F/A is 0.03 N/m2
Velocity gradient, dv/dx is 0.21 s-1
Formula to calculate the viscous force is,
F = -ηA (dv/dx)
η = -(F/A) / (dv/dx)
Substitute the given values in the above expression to calculate η
η = - 0.03 N/m2 / 0.21 s-1
 = - 0.14 N s/m2

Example 4: Water is flowing slowly on a horizontal plane, the viscosity coefficient of water is 0.01 poise, and its surface area is 100 cm2. What is the external force required to maintain the velocity gradient of the flow 1 s-1?

Solution: 

Given,
Viscosity coefficient of water, η = 0.01 poise = 0.001 kg/ms
Surface area, A = 100 cm2 = 10-2 m2
Velocity gradient of the flow, dv/dx = 1 s-1
Formula to calculate the viscous force, 
F = -ηA (dv/dx)
Substitute the given values in the above expression to calculate F
F = 0.001 kg/ms × 10-2 m2 × 1 s-1
= 10-5

Conclusion

Viscosity refers to a fluid's resistance to flow, indicating how easily it deforms or changes shape when exposed to stress. A high viscosity means the fluid flows slowly, such as honey, whereas a low viscosity allows the fluid to flow more freely, like water. Viscosity is influenced by factors such as temperature and the molecular makeup of the fluid. As temperature increases, the viscosity of most fluids decreases, allowing them to flow more easily.

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