# What is Fluid? Difference Between Solids and Fluids. Different Properties of fluids

While studying Hydraulics in Civil Engineering, Fluid is an important term. Fluid Mechanics in civil engineering mainly deal with Liquids because civil engineers have to design Water tanks, Water supply systems, Irrigation systems,Drainage systems, and most important structures such as Dams. So it is important to have detailed knowledge of fluids and its properties. So directly we will discuss fluid and its properties.

## What is fluid?

The fluid is a Substance that having ability to flow or Deform-Continuously under the action of Shear Force ( the amount of shear force doesn’t matter, even it can be very small).

Thus fluids are the substances that are able to flow and take the shape of the vessel in which it is stored. The ability to Deform-Continuously is only present in Fluids it is not present in solids.

Suppose if the fluid is at rest then there is no sharing force acting so all forces are perpendicular to the planes on which they act. Thus For a static Fluid Shear force is zero.

Fluids are considered as Continuum. Let us understand What is Continuum.

## Fluid as a Continuum

As the fluid is made up of different particles and that particles are spaced at a certain distance from each other. But in a fluid system the inter-molecular spacing between the fluid particles considered negligible. The entire fluid mass system is assumed as a continuous distribution of mass. Thus it is Continuous distribution of mass with no voids or empty spaces. This concept is called the continuum.

## Conditions for applying Continuum:-

• Molecules must be uniformly and continuously distributed.
• This assumption is valid only if no molecules large thus Number of molecules must be large such that there is no void.
• Mean free path should be negligible to length, i.e. Knudsen number should be less than 0.01.

Knudsen Number (Kn) = Mean free path / System Dimensions (i.e. Length)

knudsen number should be negligible i.e. less than 0.01.

### Some important Terms for understanding fluids

Free Surface:-The surface over which only atmospheric pressure is acting is called free surface.

No-Slip Condition / Maxwellian Condition:- The fluid contains an infinite number of molecules. From those molecules, some molecules are located near the boundary ( Some fixed object or the surface on which fluid flow). Thus the molecules that come in contact with Boundary have the ability to take the velocity of that boundary. That means if a boundary is at rest then fluid particles near it also comes to rest or if Boundary is moving with a certain velocity then the fluid particles take up that velocity. This property is called as No-slip Condition.

Cohesion:- Cohesion is a property of a fluid that there is an intermolecular attraction between the particles of the same substance. Thus the particles attract each other within the same substance are termed as cohesive. The forces by which particles attract each other are called cohesive forces.

Adhesive:- Adhesion is a property of a fluid that there is an inter-molecular attraction between the particles of Different substances. so adhesive particles are the particles that attract the particles of different substances. The forces between these particles are called adhesive forces.

Cohesion and adhesion properties in fluids depend on the nature of surfaces in contact. For understanding this let us see examples,

Example:- If suppose water and glass are in contact then there is more attraction between particles of different substances so adhesion in more. Water+ Glass then Adhesion is more.

Now Mercury (Hg) and glass are in contact then there are not many adhesive forces are present. Intermolecular forces of attraction between particles of the same substance are more thus cohesive forces dominate in this case.Mercury+ Glass, Cohesion is more.

## Differences between solids and Fluids

The key differences between solids and fluids are mentioned below

In solids the deformation is constant with respect to time.

In solids deformation is important.

On removal of load solids may regain original shape.

In liquids the deformation is continuous with respect to time .

In fluids the rate of deformation is more important than deformation.

on removal of load fluids never regain there original shape.

## Properties of fluids

Properties are certain characteristics that can be measured and also quantified. There are mainly two types of properties First Intensive Properties and Second Extensive properties.

Intensive properties:- These properties are independent of the mass of the system. Examples -Temperature, Pressure, Density, etc.

Extensive properties:- The extensive properties depend on the size or extent of the system. Examples- total mass, total volume, total momentum, etc

## List of the Important Properties of Fluids

• Density (Mass Density)
• Specific weight ( Weight Density)
• Specific gravity
• Viscosity (Dynamic Viscosity and kinematic Viscosity )
• Compress-ability
• Vapour Pressure
• Cavitation
• Surface Tension
• Capillarity
• Elasticity

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