Explain various properties of Fluids.

Properties of Fluids:

• Density

• Specific gravity 

• Viscosity

• Vapour Pressure

• Cohesion

• Adhesion

• Surface Tension

• Capillarity 

• Compressibility 

Density:

The density (also known as mass density or specific mass) of a liquid may be defined as the mass per unit volume at a standard temperature and pressure. It is usually denoted by `rho`.

`rho=M/V`

Its units are kg/`m^3`

Specific gravity:

Specific gravity is the ratio of the specific weight of the liquid to the specific weight of a standard fluid(air for gases and....Show More

Properties of Fluids:

• Density

• Specific gravity 

• Viscosity

• Vapour Pressure

• Cohesion

• Adhesion

• Surface Tension

• Capillarity 

• Compressibility 

Density:

The density (also known as mass density or specific mass) of a liquid may be defined as the mass per unit volume at a standard temperature and pressure. It is usually denoted by `rho`.

`rho=M/V`

Its units are kg/`m^3`

Specific gravity:

Specific gravity is the ratio of the specific weight of the liquid to the specific weight of a standard fluid(air for gases and water for liquid). It is dimensionless and has no units. It is represented by S.

Viscosity:

Viscosity is defined as the property of the fluid by virtue of which it offers resistance to the movement of one layer of fluid over an adjacent layer. It is primarily due to cohesion and molecular momentum exchange between the fluid layers, and as flow occurs these effects appear as shearing stresses between the moving layers of fluid.

From Newton's law of viscosity,

`tau=mu*(du)/(dy)`

Where, `mu` is the dynamic viscosity.

Its SI unit is Ns/`m^2` and cgs unit is poise.

1 poise=` 1/10` Ns/`m^2`

Kinematic viscosity, `nu`,is defined as the ratio of dynamic viscosity and density of fluid.i.e,

`nu=mu/rho`

Its SI unit is `m^2/(sec)` and CGS unit is stoke.

1 stokes`= 10^(-4) m^2/(sec)`

Variation of viscosity with Temperature:

The viscosity of liquids decreases but that of gases increases with increase in temperature. This is due to the reason that in liquids the shear stress is due to the inter- molecular cohesion which decreases with increase of temperature. In gases the inter- molecular cohesion is negligible and the shear stress is due to exchange of momentum of the molecules, normal to the direction of motion. The molecular activity increases with rise in temperature and so the viscosity of gas also increases with temperature.

Cohesion:

Cohesion means intermolecular attraction between molecules of the same liquid. It enables a liquid to resist small amount of tensile stresses. Cohesion is a tendency of the liquid to remain as one assemblage of particles. �??Surface tension�?� is due to cohesion between particles at the free surface.

Adhesion:

Adhesion means attraction between the molecules of a liquid and the molecules of a solid boundary surface in contact with the liquid. This property enables a liquid to stick to another body.

Capillary action is due to both cohesion and adhesion.

Surface Tension:

Surface tension is defined as the tensile force acting on the surface of a liquid in contact with a gas or on the surface between two immiscible liquids such that the contact surface behaves like a membrane under tension.

For liquid droplets (say water bubble),

`P=(4*sigma)/d`

For hollow bubble (say soap bubble),

`P=(8*sigma)/d`

For liquid jet,

`P=(2*sigma)/d`

Where, `sigma`=surface tension 

P= pressure intensity inside the droplet

d= diameter of the droplet

Capillarity:

Capillarity is a phenomenon by which a liquid (depending upon its specific gravity) rises into a thin glass tube above or below its general level. This phenomenon is due to the combined effect of cohesion and adhesion of liquid particles.

 Expression for capillary rise:

Let us  consider a  glass tube of small diameter d opened at both ends and is inserted in a liquid, say water. The liquid will rise in the tube above the level of the liquid.

Let, 

h= height of liquid in the tube

`sigma=`surface tension

d= diameter of the capillary tube

`theta=`angle of contact between the liquid and the tube

W= weight density

Now, the weight of the liquid inside the tube must be balanced by the vertical components of the surface tension force for equilibrium.

The weight of liquid of height h in the tube is given by,

`W=(pi*d^2)/4*h*rho*g`

Where, `rho` is the density of the liquid.

Also, the vertical component of the surface tension force is, 

`=sigma*pi*d*cos theta`

At equilibrium, these forces must be equal and opposite. i.e,

`sigma*pi*d cos theta=(pi*d^2)/4*rho*g*h`

or, `h=(4 sigma cos theta)/(d rho g)`

or, `h=(r sigma cos theta)/(gamma*d)`

This is the expression for the capillary rise of liquid.

For water, `theta=0^0` so,

`h=(4*sigma)/(gamma*d)`

Similarly, we can obtain results for capillary fall (in case of mercury).

Vapour pressure:

The pressure at which the liquid and its vapour can exist in equilibrium at a certain temperature is termed as the Vapour Pressure of that liquid at that temperature.

Vapour Pressure of liquid increases with rise of temperature due to the increased molecular activity.

Cavitation:

The rapid vaporisation and recondensation of liquid as it flows through a region of low pressure is called cavitation.This phenomenon is not possible in gas because the gas doesnt changes it state at low pressure.

If at any point in flow, the local velocity is so high that the pressure in a liquid is reduced to its vapour pressure, the liquid will then vaporise at that point and bubbles of vapour will form. As a result of flow into a region of high pressure, the bubbles of vapour will suddenly condense i.e, collapse producing high dynamic pressure upon the adjacent solid walls.

Disadvantages of cavitation:

• Turbine runners, pump impellers,  and ship screw propellers are quickly damaged by cavitation phenomenon.

• It may cause a drop in efficiency of a machine or propeller or other devices.

• It increases the maintenance cost.

• It produces undesirable noise and vibration.