# Properties of Fluids and Its Classification

Matter is generally found in three states or phases i.e. solid, liquid and gases (or vapour). Solids are definite in shape and volume where molecules are tightly packed together. While on the other side, both the liquid and gases do not have definite shape, or volume and particles are compressible and adjust according to the container. Liquid and Gases (or vapour) are collectively termed as **Fluids. **Fluids can include the matter which has the** tendency to flow**.

In medicine or biology, fluids are the body fluids (liquid constituent of the body).

In physics, Fluids are those matter (liquid, gas or other material ) that has the tendency to deform (flow) when external force (or shear stress). The shear constant for fluids remains **zero** means they are not able to resist the shear stress.

The branches which study about the fluids and its properties in rest state are called Fluid** Statics **and the branches which study about the fluids in motion are called **Fluid Dynamics.**

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## Classification of Fluids

There are various types of fluids known to humankind. They are diversified on different bases;

- Perfect fluids.
- Depending upon the
**shear stress**and**the rate of stress**and its derivatives i.e.**viscous**or**newtonian fluid**. - On the basis of the compressibility i.e.
**compressible**or**Incompressible fluids**. - On the basis of
**Conductivity**. - On the basis of density.

### Perfect Fluids

Those virtual fluids that completely ignore the effect of Viscosity and compressibility are called **Perfect Fluids.**

In a more detailed manner, a perfect fluid is that which can be completely characterized by its rest frame mass density ⍴m and the isotropic pressure **P. ** The Real fluids are” stick” and contain (or might conduct ) heat.

Perfect Fluids have no **shear stresses, viscosity **or **heat conduction.** Perfect Fluids admit to the **Lagrangian formulations **but the heat conduction and anisotropic stresses cannot be treated as generalized formulations.

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### Newtonian Fluids or Viscous Fluids

On the basis of the shear stress and the rate of stress or its derivatives the fluids are generally divided into ;

#### Newtonian Fluids

Those fluids that follow the Newton law of viscosity and have stress directly proportional to ratio of strain.

Newtonian fluids do not have any elastic properties. They are incompressible, isotropic and unreal.

**Viscosity **in these fluids is **inversely proportional **to the increase in its temperature. For a **constant **temperature, viscosity tends to remain **constant**.

#### Non – Newtonian Fluids

Those fluids that will not follow Newton’s law of viscosity and the stress are not proportional to the ratio of the strain or its higher derivatives.

Non – Newtonian Fluids can be further divided into subcategories;

#### Time Dependant Fluid:

Fluids are the ones for whose stress or viscosity decreases with time due to isothermal condition and steady shear are known as **Thixotropic.**

Those that increase with time under the same circumstances are known as **Rheopectic **or **Anti- thixotropic.**

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#### Time Independent Fluid:

Fluids are the ones for which the rate of shear at a given point depends on the instantaneous shear stress at the point. These are also known as **Non Newtonian Viscous **or **Purely Viscous Fluids.**

They can still be classified as further into types;

- Those which do yield stress.
- Fluids that do not yield stress. Further classified as;
- Pseudoplastic fluids
- Dilatant Fluids.

These fluids follow the Newton law of Viscosity.They can be also called as **Viscous Fluids.**

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### Compressibility and Incompressibility

On the basis of their compressibility, fluids are categorised as

### Compressible Fluids

A fluid in which volume reduces or density changes when external pressure is applied to the fluid or when it becomes **Supersonic.**

### Incompressible Fluids

A fluid whose volume does not vary with variations in pressure or flow velocity (i.e. ρ= constant ) such as water or oil.

### Ideal & Real Fluids

- Ideal Fluids:
- The fluids which have a constant density and zero viscosity are called as
**Ideal Fluids** - No Fluid can be termed as Ideal Fluid that is why they are called
**Imaginary Fluids.**

- The fluids which have a constant density and zero viscosity are called as
- Real Fluids:
- The Fluids which have a variable density and non – zero viscosity are called
**Real Fluids.** - For example: Kerosene, petrol, Castor Oil, etc.

- The Fluids which have a variable density and non – zero viscosity are called

- Ideal Plastic Fluids:
- The fluids for which the shear stress is more than the yield value and shear stress is proportional to the rate of shear strain or velocity gradient, is called an
**Ideal plastic fluid.**

- The fluids for which the shear stress is more than the yield value and shear stress is proportional to the rate of shear strain or velocity gradient, is called an

On the basis of the **DENSITY**, Fluids are generally classified into two categories;

- Gas
- Liquid

Also, check out the notes on Oxidation Number, here.

## Fluid Properties

Fluids possess different properties of different basis which can be used to characterise the fluids. The main properties of the fluids are Viscosity, Density, Specific Weight, Specific Gravity, Bulk Modulus, kinematic Viscosity. We will discuss all of them in a detailed way in this section of the article.

Fluids are generally classified under three types of properties;

### Kinematic Properties

These properties help in understanding fluid motion. Velocity and acceleration are the kinematic properties of the fluids.

### Thermodynamics Properties

These properties help in understanding the thermodynamic state of the fluid. Temperature, density, pressure, and specific enthalpy are the thermodynamic properties of the fluids

### Physical Properties

These properties help in understanding the physical state of the fluid such as colour and odour.

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### Density

Density is the intensive property of matter. Density is the mass per unit volume for a substance or matter.

For fluids, the density is the mass per unit volume. It is denoted by the **rho (ρ).**

** Volume**

**\(\rho={\text{mass}\over{\text{Volume}}}\)**

Values of densities for

Substance | Density |

Air \((ρair) \rho_{air}\) | \(1.2 kg\over{m^3}\) |

Water \((ρwater) \rho_{water}\) | \(1000 kg\over{m^3}\) |

Mercury\((ρHg) \rho_{Hg}\) | \(13600 kg\over{m^3}\) |

### Specific Weight & Gravity

Specific Weight is the weight per unit volume. It is denoted by **W. **

**Volume**

\(W={\text{weight}\over{\text{Volume}}}\)

Since, weight **w**=**mg**

Density **ρ=m/v**

So, W= mg/v

**W = ρg**

The dimensional formula of Specific weight \([ M1 L-2 T-2 ] [M^{1}L^{-2}T^{-2}]\)

Now, for Specific Gravity, denoted by **S** is defined as the ratio of the density of the fluid to the density of the standard fluid or the ratio of the weight density of fluids to the weight density of standard fluids.

\(S={\text{Density of Fluid}\over{\text{Density of the standard Fluids}}}\)

It is the unitless and dimensionless quantity.

**Specific Volume (V)** is defined as the volume per unit mass of liquid. It is denoted by V.

\(V={V\over{m}}\text{ or }{1\over{\rho}}\)

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### Bulk Modulus

Bulk Modulus, is defined as the measurement of the ability of the fluid to resist changes in the volume when it is exposed under compression on all sides. Simply could be referred to as Incompressibility. It is usually expressed to determine the elastic properties of fluids.It is a numerical constant denoted by **K **

\(K={-VdP\over{dV}}\)

or

\(K=\rho{dP\over{d\rho}}\)

The SI Unit is **N/m****2 **or **Pa**

The dimensional Formula is \([ M1 L-1 T-2 ] [M^{1}L^{-1}T^{-2}]\)

### Compressibility

**Compressibility** can be easily explained as the ability of the substance to decrease in size and volume on applying external pressure**.**

For Fluids, **Compressibility **is a measure of the relative volume change of a solid or **fluid **in a response to applied external pressure change. For a given mass of fluid, an increase in pressure, Δp > 0, will cause a decrease in volume, ΔV < 0.

It is denoted by **β. **It’s SI unit is **Pa****-1** and the dimensional unit is \([ M-1 L1 T2 ] [M^{-1}L^{1}T^{2}]\)

\(\beta={1\over{K}}\)

OR

\(\beta={-1\over{V}}.{dV\over{dP}}\)

Also, check out the notes on Adsorption and its Types, here.

### Viscosity

**Viscosity** is a measure of the tendency of the fluid to resist its flow over a surface.

It can be described as the internal friction of a moving fluid. A fluid with large viscosity will tend to resist the motion because its molecular structure will give it a lot of internal friction that increases the fluid’s resistance.

For fluid, it is due to **cohesion force **while in Gases, viscosity is observed due to **molecular momentum transfer. **

\(F=\mu{A}{u\over{y}}\)

If velocity does not varies with the y, then it is expressed as;

\(\tau=\mu{\partial{u}\over{\partial{y}}}\)

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#### Kinematic Viscosity

When we talk about the Fluid DynamIcs, the viscosity is treated as **kinematic viscosity** for more convenience. It can sometimes be termed as **momentum diffusivity. **

It is defined as the ratio of the dynamic viscosity **μ **to the density of the fluid **ρ** .

\(\nu={\mu\over{\rho}}\)

It is denoted ν. Its SI unit is \(m2/s. m^{2}\over{s} \) It’s dimensional formula is \([ M0 L2 T-1 ] [M^0L^2T^{-1}]\)

#### Dynamic Viscosity

Dynamic Viscosity describes the Absolute viscosity of the fluid which is obtained mathematically by dividing the Shear stress by the rate of shear strain.

The units of dynamic viscosity are: Force / area x time

The Pascal unit (Pa) is used to describe pressure or stress = force per area.This unit can be combined with time (sec) to define dynamic viscosity.

It is denoted by **μ**

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## Effect of Temperature and Pressure on Viscosity

### For Liquids

The viscosity of the fluid (liquid) is inversely proportional to the temperature. For an increase in the temperature, both the viscosity i.e. Kinematic (ν) and Dynamic Viscosity (μ) decreases. But both the viscosities are independent of pressure, for increase in pressure, both ν and μ remain the same.

### For Gases

The viscosity of the fluid (gas) is directly proportional to the temperature. For an increase in the temperature, both the viscosity i.e. Kinematic (ν) and Dynamic Viscosity (μ) increases. But this time the dynamic viscosity is not independent of pressure, for increase in pressure, μ tends to increase but the kinematic viscosity (ν) still remains the same.

**Testbook Tricks:**

- Dynamic Viscosity \(\nu\propto{T^{3/2}} ν ∝ T3/2\)
- Kinematic Viscosity \(μ ∝ √T \mu\propto\sqrt{T}\)
- Specific Gravity is a function of temperature.
- Specific gravity of water is
**1**at**4****0****C.** - For isothermal process, K=P & for adiabatic process, K=𝛄P
- 1 Stoke= \(10^{-4}{m\over{s^2}} 10-4 m/s^2\)
- 1 Poise= 0.1 Pa-s
- Specific gravity measuring device is
**Hydrometer** - Viscosity is also defined as the slope of the line or curve drawn in the shear stress v/s rate of shear stress or velocity gradient.

Also, check out the notes on Amines, here

## Surface Tension

Surface tension is the property exhibited by the fluids. Surface tension is basically the tendency of the fluid (liquids or gases) at rest to shrink into the minimum surface area possible. This property will allow objects to float on the surface without submerging in the liquid.

It has liquid-liquid interfaces and liquid-air interfaces, surface tension results from the great attraction of liquid molecules to each other due cohesion then to the molecules in the air due to adhesion.

Due to the cohesive forces (the force of attraction acting between the molecules of same types), a molecule experiences equal pull in every direction by surrounding liquid molecules, which results in net force **zero. **The molecules at the surface do not have the same molecules on the sides of them and therefore are pulled inwards. This will create some internal pressure and force liquid surfaces to contract to the minimum area.

Mathematically, Surface tension is the force applied per unit of length. Also, it can also refer to energy per unit of area.

𝝈 = **F/L**

Its SI unit of Surface Tension is **Newton/meter.**

The dimensional formula of surface tension is\([M^{1}L^{1}T^{-2}]\)

\(\sigma={1\over{2}}.{F\over{L}}\)

Here, **F**= Force applied to surface

**L**= Length

𝛄 or 𝝈 = the surface tension at room temperature.

The reason for ½ is that film has two sides (two surfaces) each of which contributes equally to the force, so the force contributed by a single side will 𝝈 **L=F/2.**

Surface Tension 𝝈 in the form of work **W** is defined as the ratio of the change in Energy of the liquid to the change in the surface area of the liquid.

\(\sigma={F\over{2L}}={F\Delta{x}\over{2L\Delta{x}}}={W\over{\Delta{A}}}\)

Here, **W**= work done

F= Force applied on the surface.

ΔA= change in surface area

𝛄 or 𝝈 = the surface tension at room temperature

**Testbook tricks:**

- Surface tension of mercury is more than water at room temperature.
- The surface tension of water increases with decrease of temperature.
- If cohesive force is more than the liquid level dips while if adhesive force is more then liquid level rises.

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## Wetting and Non Wetting Phenomenon

Contact Angle is an angle which is formed between the solid surface or capillary walls of a porous material when they come in contact. This angle can be used to determine the properties of the substance. Those interactions are described by cohesion and adhesion forces which are intermolecular forces

Contact angle is the most common way to measure the wettability of a surface or material.

**Wetting** refers to the study of how a liquid deposited on a solid (or liquid) substrate spreads out or the ability of liquids to form boundary surfaces with solid states. The wetting, as mentioned before, is determined by measuring the contact angle, which the liquid forms in contact with the solids or liquids.

The wetting tendency is larger, the smaller the contact angle or the surface tension is.

On the basis phenomenon is of this phenomenon liquids are of two types:

- Wetting liquids
- Non Wetting liquids.

A **wetting liquid** is a liquid that forms a contact angle with the solid which is **smaller than 90º **.In this case, the adhesive force is more effective than the cohesive force .

A **non-wetting liquid** creates a contact angle **between 90º and 180º** with the solid. Here the cohesive force will be more prominent than the adhesive force.

The wetting phenomena can be measured by various methods:

- The Sessile Drop Method.
- Wilhelmy Plate Method.
- Capillary Rise

Learn more about Surface Chemistry, here.

## Capillary Action

Capillary action is another property that is exhibited by the fluids. Intermolecular forces cause this phenomenon. The tendency of a polar liquid to rise against gravity into a small-diameter tube (a capillary tube) is called **capillary action**.

This capillary action is actually the net resultant of the two opposing forces i.e. Cohesive force, the intermolecular forces that will hold the liquid together and Adhesive Forces, which are the attractive forces between a liquid and the substance that composes the **capillary actions**.

This phenomenon is explained as when a glass capillary is in contact with liquid, say water, water tends to rise up into the capillary tube. This rising of the water is called **capillary action**. Capillary action depends on the diameter of the tube and the temperature of the liquid (here water).

Mathematically, the capillary action is used to calculate the height upto which liquid rises

\(h={2\gamma{cos\theta}\over{\rho{gr}}}\)

Here, h= height of the capillary tube

r= the radius of the capillary tube

𝜭= the contact angle between the surfaces

𝝆= the density of liquid

𝛄 or 𝝈 = the surface tension at room temperature

**g**= the acceleration due to gravity.

For a water-filled glass tube in air at standard laboratory conditions, *γ* = 0.0728 N/m at 20 °C, *ρ* = 1000 kg/m3, and *g* = 9.81 m/s2. For these values, the height of the water column is

\(h\approx{1.48\times10^{-5}\over{r}}\)

Also, check out other topics of Chemistry, here.

The above notes on Fluid Properties are intended to guide candidates to understand the important concept with detailed explanations and solved examples. Do practice it now on the Testbook App through the free mock tests.

## Fluids Properties FAQs

**Q.1 Who is the father of Fluid Mechanics?**

**Ans.1**Ludwig Prandtl is known as the father of modern aerodynamics.

**Q.2 Why is Fluid Mechanics important ?**

**Ans.2**It will help us to understand the nature of various fluids under different circumstances of the atmosphere like forces, temperature.

**Q.3 What is the relation between Specific Volume and specific density?**

**Ans.3**Specific Volume of fluids is inversely proportional to Specific Density of fluids.

**Q.4 What are the basic properties of fluid?**

**Ans.4**The basic properties of fluids are density, viscosity, surface tension, capillarity, specific volume and specific weight.

**Q.5 Is buoyancy a property of fluid?**

**Ans.5**Yes, buoyancy is a property of fluid