what is viscosity

What is viscosity in Physics? How does it work? Examples

When we pour liquids from one bottle to another bottle, it takes different times for different fluids. In general concept, water will take less time compared to syrup and honey. Because, water is thin, honey and syrup are thick. In physics, to explain this fact a special term is used –“Viscosity”. Now the question arises, what is viscosity?

Viscosity is the measure of the resistance of a fluid to attempt to move through it. In another word, viscosity is the measure resistance to deformation at any state.

What is Viscosity or definition of Viscosity

In general, the more the viscosity of a fluid, the more force requires to flow it. We can easily come to the conclusion that honey is thick so its viscosity is more.

Viscosity of a fluid is the measure of its resistance to deformation at a given rate.

When we want to move a solid block on the ground, we have to put force on the block. And the amount of force depends on measure friction between two layers of solids. Similarly, viscosity is the results of interaction and friction among fluid molecules and layers.

This concept is similar to Newton’s second law of motion, it says that when a force is applied on an object it tends to accelerate (accelerate when the applied force is greater than resistant). Here, mass acts as a major factor, the more the mass of the object the more force is to be applied to accelerate it.

Read also: What is impulse in physics and how it Works?

Viscosity formula

  1. Viscosity is represented by the symbol η “eta”. Mathematically viscosity is the ratio of shearing stress to the velocity gradient.

η = (F/A)/(dvx/dz)

Shearing stress= (F/A)

Velocity gradient= (∆vx/∆z or dvx/dz)

2. Another way of measuring viscosity of a fluid is using Newton’s equation for fluid.

F / A = n (dv / dr)

 F represents force and A represents the area. So, F/A, or force divided by area= viscosity. Dv divided dr represents the “sheer rate,” or the speed the liquid is moving. And, is a constant unit equal to 0.00089 Pa s (Pascal-second), which is a dynamic viscosity measurement unit.

 3. If a sphere is dropped into a fluid, the viscosity can be determined using the following formula:

η= 2g*square of a(Δρ)/9v


  • ρ is the density difference between fluid and sphere tested
  • a is the radius of the sphere
  • g is the acceleration due to gravity
  • v is the velocity of the sphere

Types of Viscosity

There are two types of viscosity; 1) Dynamic Viscosity and 2) Kinematic viscosity

Dynamic Viscosity: Dynamic viscosity is the measure of the ratio of shear stress to the shear rate for a fluid

Kinematic viscosity: Kinematic viscosity is the measure of the ratio of the viscous force to the inertial force on the fluid

In general, the two types of viscosity seems to be same, but they are different in broad concept. Use of kinematic viscosity is more than dynamic viscosity.

Unit of Viscosity

The unit of kinematic viscosity is square meter per second (m2/s), in CGS unit is “Stokes“, named after the British physicist, Sir George Gabriel Stokes. A stoke is defined as one centimetre squared per second.

The SI unit of dynamic viscosity is the newton-second per square meter (N·s/m2) or pascal-second (Pa·s) or kilogram per meter per second (kg·m−1·s−1).

The CGS unit is the poise (P, or g·cm−1·s−1 = 0.1 Pa·s)

Why Should You Measure Viscosity?

We often see different types of containers are made to keep different fluids. Different fluids have different thickness and viscosity. Manufacturing companies measure the viscosity of the fluid and then decides which type of container is to be made. And what process is to be applied to get out fluids from the container. For example, if we keep honey in a scent bottle and try to spry it. It will not spray like scent or perfume because the viscosity of honey is more than liquid in scent bottle.

There are also many fields in which the measure of viscosity becomes important. For example, inkjet printing, protein formulations and injections etc.

Newtonian Fluid and non-Newtonian Fluid

Most of the fluids are Newtonian fluids. The viscosity of Newtonian fluids does not depend on shearing forces acting on it. The viscosity of Newtonian fluids only depends on temperature and pressure. And the resistance to flow is directly proportional to the motion of a fluid. So we can conclude that the viscosity of Newtonian fluids remains the same no matter how much force applied to it.

In contrast, viscosity of Non-Newtonian fluids changes with acting forces. Alternatively,  viscosity of Non-Newtonian fluids is a function of either shear stress or shear rate.

There are different types of Non-Newtonian fluids;

  1. The viscosity of Non-Newtonian fluids vary with the duration of shear stress applied to it (Rheopectic and Thixotropic)
  2. The viscosity of Non-Newtonian fluids varies with shear stress (Dilatants and Shear Thinning or Pseudoplastic)
  3. The fluids act as a solid up until a certain level of stress and then begin to acts as a fluid (Bingham plastic)

and many other types. The example of one type of Non-Newtonian fluid is “Oobleck” also known as “Slime” which acts as solid when a large amount of force applied to it.

“Magnetorheological fluids” are other types of Now-Newtonian fluids that become solid with the change of the magnetic field, and viscosity also changes with state matter.

Importance of viscosity in daily life

We can not measure viscosity in an easy way and most importantly we will not find daily life of use of viscosity. But in industrial sector it has wide use to make the product, transportation and use of product convenient to people.

Food Industry

In food industry, fruit juices, ghee flow through pipes. As different fluid have different viscosity, the inclination of pipes are made according to the viscosity to maintain smooth flow of fluids.

Automobile industry

In automobile industry, mobiles and many other oils are used as lubricant. When machines run for long time, heat generates and it decreases the viscosity of a fluids. It may create problem if oils become thinner.

viscosity in car industry

In contrast, if the place is cold, where machines kept, viscosity of oil may increase due to low temperature. It also hamper machines functions. So, maintain of viscosity is really important in automobile industry.


Maintaining blood viscosity is important, otherwise, it may cause serious damage to our life. Some medicines may increase the viscosity of blood, that leads to blood clotting. On the other hand, if the viscosity of blood decreases, that means if the blood becomes thinner, this can lead to heavy blood loss.

In Adhesives

Adhesives help to join cracks, broken things. Sometimes we need to join a small part of a thing, and sometimes need to join to large thing over a large surface. In those two cases, the use of adhesives with different viscosity is recommended.

For the first case, the high viscous adhesive needs so that it does not spread over the whole surface and for the second case low viscous adhesive needs so that it spreads over the whole surface. We might not consider the adhesives from the point of viscosity rather use the thin and thick word.


Viscosity is a scientific term. Physicist, chemist, biologist and normal people use the word “Viscosity” differently. In simple word viscosity is the measure of resistance of fluids to deform or to move a particle through it or it flow it.

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