A fluid is a substance that does not have a definite shape of its own but conforms to the shape of the vessel. A fluid is a substance that when subjected to a shearing force, however small, undergoes deformation continuously as long as the force is applied.
Classifications of Fluids
Fluids are of two types on the basis of Theoretical and Partially. And based upon the behaviour of fluids under the action of applied pressure, and temperature, the fluids are classified.
(i) Ideal fluids.
(ii) Real fluids.
(i). Ideal Fluids
Ideal liquids are those which show no resistance to the displacement of any of their particles. Ideal fluids are incompressible and frictionless, ideal fluids do not have the properties of surface tension, and viscosity, but in reality, no fluid is an ideal fluid. But practically, the liquids for which the viscosity value is very less, are considered ideal fluids.
(ii). Real fluids
Real liquids are those which show some resistance to the displacement of any of their particles. They can be pressed and have the properties of friction and surface tension.
Fluids are classified on the basis of density
The density of all liquids depends on their temperature. And along with the change in their value, small and large changes occur in the value of density. There are two types of fluids on the basis of density.
Incompressible Fluids
If the effect of pressure and temperature on fluids is negligible, then liquids are called incompressible liquids, these liquids cannot be easily compressed and the temperature has little effect on them. There is a non-linear change in the shape of liquids upon applying a little external pressure, but liquids are considered to be incompressible liquids.
Compressible Fluids
If the effect of temperature and pressure on a liquid is more, then the liquid is called compressible. These liquids can be easily pressurized, they are most or most affected by temperature changes. Gases are considered compressible liquids.
Fluids are Classified on the Basis of the Action of Shear Stress, and on the basis of Newton's Law
According to Newton's third law, there is the following relationship between shear stress (τ) and rate of angular deformation (dv/dy).
τ ∝ dv/dy
τ = μdv/dy
On the basis of this relationship, liquids can be classified as follows.
On the basis of shear stress there are two types of fluids (i) Newtonian Fluids and (ii) Non-Newtonian Fluids,
Newton's Fluids
Fluids that obey Newton's law of viscosity, i.e. fluids for which the ratio of shear stress to the rate of shear is a constant, are called Newtonian fluids. Newton's law obeys τ = μ dv/dy.
Examples of Newton's Fluids
All gases, air, and liquids, such as kerosene, alcohol, glycerine, hexane ether, benzene etc., solutions of salts and of sugar in water.
Non-Newton Fluids
Fluids that do not obey Newton's law of viscosity, i.e. fluids for which the ratio of shear stress (shear stress) to the rate of shear is not a constant, are called non-Newtonian fluids. does not obey Newton's law τ = μ dv/dy.
Examples of Non-Newton Fluids
Toothpaste, paints, gels, jellies, slurries, polymers solutions etc.
(a) If τ = μ dv/dy + constant then it is Bingham plastic. Ex. Sewage sludge, toothpaste, jellies, paints and some slurries.
(b) When τ = μ (dv/dy)^n+constant, n ≠ 1 that is Thyrolotropic fluids
(c) When τ = μ (dv/dy)^n, n<1 that is Pseudoplastic Fluids, Ex. Rubber, leather, mud, blood, and solutions of high molecular weight polymers.
(d) When τ = μ (dv/dy)^n, n≽1 that is Dilatant fluids, Ex. Suspension of starch in water, pulp in water and sand-filled emulsions.
Properties of Fluids
The following are the properties of fluids:
1. Mass density or simply density (specific density)
2. Specific weight (Weight density).
3. Specific gravity.
4. Vapour pressure.
5. Viscosity.
6. Specific volume.
7. Thermal conductivity.
8. Surface tension and capillarity.
9. Compression and elasticity.