# What do you mean by Fluid? Types of Fluid

## What is Fluid?

The substance that tends to flow is called fluid. It is defined as a substance that is capable of spreading and changing its shape according to surroundings, without offering internal resistance. It is a substance that is continuously deformed under constant shear stress. Fluid refers to a substance that does not have a definite shape and surrender easily to external pressure. It can be liquid or gas.

There are 3 phases of matter around us.

- Solid
- Liquid
- Gas

From the above 3 phases, liquid and gas are known as fluids.

## Types of Fluid (Based on Viscosity)

- Ideal Fluid
- Real Fluid
- Ideal Plastic Fluid
- Newtonian Fluid
- Non-Newtonian Fluid

### Ideal Fluid

The fluid does not have any viscosity and surface tension known as an ideal fluid. Ideal fluid has no friction and is incompressible. It is only an imaginary fluid that does not exist in reality because all fluids have some viscosity.

- Zero viscosity
- Incompressible i.e. the density is constant.
- Irrotational i.e. the flow is smooth hence turbulence.
- Inviscid i.e. fluid has no internal friction ( η=0 ).
- No surface tension.
- Infinite bulk modulus.
- Imaginary and do not exist in nature.

### Real Fluid

The fluid that is compressible and has viscosity and surface tension is known as real fluid. In a real fluid, viscosity produces resistance to motion by causing shear or friction forces between fluid particles and boundary walls.

- Real fluids are compressible in nature.
- Have some viscosity.
- Real fluid implies friction effects.
- Finite bulk modulus.
- Surface tension exists.
- Exists in nature.
- Examples are petrol, kerosene, castor oil, etc.

### Ideal Plastic Fluid

A fluid in which shear stress is more than the yield value and shear stress is proportional to the rate of velocity gradient (shear strain) is known as ideal plastic fluid.

- Examples are sewage sludge, drilling mud, toothpaste, etc.

### Newtonian Fluid

A real fluid in which shear stress is directly proportional to the rate of velocity gradient (shear strain) is known as a Newtonian fluid. It obeys newton’s law of viscosity.

- Viscosity is independent of the shear rate.
- Obeys newton’s law of viscosity.
- A linear relationship between shear stress and viscosity.
- Comprised of small isotropic molecules that are not oriented by the flow.
- Examples are water, alcohol, gasoline, mineral oil, honey, organic solvents, etc.

### Non-Newtonian Fluid

A real fluid in which shear stress is not proportional to the rate of velocity gradient (shear strain) is known as a Non-Newtonian fluid. It does not obey newton’s law of viscosity.

- Viscosity is dependent on the shear rate.
- Does not obey newton’s law of viscosity.
- No linear relationship between shear stress and shear rate.
- Examples are blood, paints, ketchup, soap, cosmetics, butter, jam, cheese, yogurt, lava, gums, etc.

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