Fluid | Definition, Types, Properties, Examples [Brief Guide]

What is a fluid?

Before discussing fluids let’s first discuss phases of matter. Matter around us exists in three phases, excluding plasma. Plasma is superheated matter – so hot that the electrons are ripped away from the atoms forming an ionized gas. There three phases of matter are

1. Solid
2. Liquid
3. Gas

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

The main difference between fluids and solids lies in their ability to resist shear stresses.

Let’s understand fluids with their definition.

When a constant shear force is applied, a solid eventually stops deforming, whereas a fluid never stops deforming and approaches a constant rate of strain

(ref. ‘Fluid Mechanics’ by Cengel & Cimbala)

Types of fluids

On the basis of shear stress and velocity gradient relationship, fluids can broadly be classified into following five types.

Image Source: NPTEL

1. Ideal fluid

Ideal fluid is incompressible and has no viscosity. It is an imaginary fluid and does not exists in reality.

2. Real fluid

Real fluid possesses viscosity. In practice all fluids are real.

3. Newtonian fluid

A real fluid which obeys Newton’s law of viscosity is known as Newtonian fluid.

Note: According to Newton’s law of viscosity shear stress is directly proportional to strain rate.

4. Non-Newtonian fluids

A real fluid which does not obeys Newton’s law of viscosity is known as Non-Newtonian fluid.

5. Ideal plastic fluid

A fluid, in which shear stress is more than the yield value and shear stress is directly proportional to the rate of shear strain, is known as ideal plastic fluid.

Properties of fluids

There are some basic properties associated with fluids which help us to understand them in a better way.

• Viscosity

It is the ratio of applied shear stress to the obtained shear strain rate in fluids. Viscosity offers resistance to the movement of fluid layers.

• Pressure

It is the term used in fluids which is analogues to the term stress used in solids. Both are the ratio of applied force and area of surface (on which the force is applied).

• Specific gravity

Specific gravity of fluids is defined as the ratio of density of a particular fluid to the density of standard fluid (generally water).

• Mass Density

Mass per unit volume is known as mass density of fluid.

• Specific weight

Weight per unit volume is known as mass density of fluid.

• Specific volume

Volume per unit mass is known as specific volume.

• Vapor pressure

Pressure exerted by the vapor when it is in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system is known as vapor pressure.

• Compressibility

It is a measure of the change in volume of a fluid in response to a change in pressure.

• Surface tension

It is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible.

Examples of fluids

• Water
• Air
• Blood
• Mercury
• Honey
• Gasoline
• Any other gas or liquid

Also read:

What are cutting fluids?

What are Lubricating Fluids?

What is Liquefied Petroleum Gas?