Fluid Properties | Definition, Types, Overview

By Mohit Uniyal|Updated : May 30th, 2022

To understand the various fluid properties, we must first understand what is meant by the term fluid. A fluid is a liquid, gas, or other material that deforms continuously under applied shear stress, or external force, according to physics. They have a zero shear modulus, or, to put it another way, they are substances that cannot withstand any shear stress. The fluid properties are defined and classified based on kinematic properties, thermodynamic properties, and physical properties.

Although the term fluid encompasses both the liquid and gas phases, it has different definitions in different fields of research. Solid definitions vary, and some substances can be both fluid and solid depending on the field. When a sudden force is applied to viscoelastic fluids like Silly Putty, they appear to behave like solids. Pitch, for example, has a very high viscosity and appears to act like a solid. Here, we will explore all the fluid properties in brief here.

Table of Content

What Do You Mean by Fluid Properties?

Fluid properties enable us to understand the behavior of fluids under a variety of forces and atmospheric conditions, as well as to select the appropriate fluid for a wide range of applications. These properties can be classified into the following categories:

Kinematic properties: These characteristics help in understanding fluid motion. The kinematic properties of fluids are velocity and acceleration.

Thermodynamic properties: These qualities help to determine the fluid's thermodynamic state. The thermodynamic properties of fluids are temperature, density, pressure, and specific enthalpy.

Physical properties: These characteristics, such as color and odor, help in determining the fluid's physical state.

What are the Types of Fluid Properties?

Even though each fluid is unique in terms of composition and specific attributes, there are several characteristics that all fluids share. Let's take a closer look at fluid properties.

There are several types of properties. The most important properties are given below.


Specific Gravity


Surface Tension


Vapor Pressure



Specific Volume


Specific Weight



The density of a substance is its mass per unit volume (more accurately, its volumetric mass density; also known as specific mass). The symbol for density is ρ. Density is defined mathematically as mass divided by volume:


where the density is ρ, the mass is m, and the volume is V.


A fluid's viscosity is a measurement of its resistance to deformation at a specific rate. It corresponds to the informal sense of "thickness" in liquids: syrup, for example, has a higher viscosity than water. The internal frictional force between neighbouring layers of fluid in relative motion is measured by viscosity. 

When a viscous fluid is driven through a tube, for example, it flows faster towards the tube's axis than near the tube's walls. Experiments have shown that some stress is required to keep the flow going. This is because a force is necessary to overcome the friction between the fluid layers that are in relative motion. The compensatory force is proportional to the viscosity of the fluid in a tube with a constant rate of flow.


Temperature is a measure of the average kinetic energy of the atoms or molecules in a system or a physical quantity that expresses hot and cold When a body comes into contact with another that is colder or hotter. It is the manifestation of thermal energy, which is present in all matter and is the source of the occurrence of heat, a flow of energy.

 Heat should not be confused with temperature. A thermometer is used to determine the temperature. Thermometers are calibrated in a variety of temperature scales that have traditionally defined temperature using a variety of reference points and thermometric substances. The Celsius scale, Fahrenheit scale, and Kelvin scale are the three most popular scales.


The physical force exerted on an item is known as pressure. The force applied per unit area is perpendicular to the surface of the objects. Pressure is calculated using the F/A formula (Force per unit area). Pascals is the unit of pressure (Pa). There are four different kinds of pressure:

  • Atmospheric Pressure
  • Absolute Pressure
  • Differential Pressure
  • Gauge Pressure

Specific Volume

The specific volume of a substance (symbol v) is a property of the substance defined as the ratio of the substance's volume (V) to its mass (M) in thermodynamics (m). It is the reciprocal of density (ρ) and has the following relationship with molar volume and mass:

ν = V/m = 1/ρ

The cubic meter per kilogram (m3/kg) is the standard unit of specific volume.

Specific Weight

A material's specific weight (γ), is calculated by multiplying its density (ρ) by its standard gravity (g).

γ =ρg

Specific weight, unlike density, is not a material's fixed fluid property. It is determined by gravitational acceleration, which changes depending on location. Pressure can alter results depending on the bulk modulus of the material, but it has a less significant effect than the other components at modest pressures.

Specific Gravity

The ratio of a substance's density (mass per unit volume) to the density of specified reference material is known as relative density or specific gravity. For liquids, the reference is almost always water at its densest temperature (4 °C); for gases, the reference is air at room temperature (20 °C).

In scientific circles, the term "relative density" is frequently used. If the relative density of a substance is less than one, it is less dense than the reference; if it is larger than one, it is denser. The densities are equivalent if the relative density is exactly 1; that is, equal volumes of the two substances have the same mass. A substance having a relative density (or specific gravity) less than 1 will float in water if the reference material is water. An ice cube, for example, with a relative density of 0.91 will float. A substance sinks if its relative density is larger than one.

Surface Tension

Surface tension is the propensity of liquid surfaces to shrink to the smallest possible surface area while they are at rest. Surface tension is what allows items with a higher density than water, such as razor blades and insects, to float on a water surface without submerging.

Vapour Pressure

Equilibrium or vapour pressure At a given temperature in a closed system, vapour pressure is defined as the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phases (solid or liquid). The evaporation rate of a liquid is determined by the equilibrium vapour pressure. It relates to the tendency of particles to escape from the liquid. Volatile refers to a substance that has a high vapour pressure at room temperature. Vapour pressure is the pressure exerted by vapour existing above a liquid surface.


Capillary action (also known as capillarity, capillary motion, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the aid of, or even against, external forces such as gravity. Liquids can be drawn up between the hairs of a paintbrush, in a narrow tube, in porous materials like paper and plaster, in non-porous materials like sand and liquefied carbon fibre, or in a biological cell. Intermolecular forces between the liquid and the surrounding solid surfaces create this.


Cavitation occurs when the static pressure of a liquid falls below the vapour pressure of the liquid, resulting in the creation of small vapour-filled cavities in the liquid. These cavities, known as "bubbles" or "voids," collapse under increasing pressure and can generate shock waves that might harm machinery. These shock waves are powerful while they are close to the imploded bubble, but they diminish fast as they go away from it.

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FAQs on Fluid Properties

  • The properties and features of any hydraulic oil are critical to your hydraulic system's capacity to function under the operating circumstances you require. This is particularly true for hydraulic fluids used in industry or commerce. To be useful, it should possess the following hydraulic fluid properties:

    • Non-compressible
    • Within a range of operating temperatures, thermally stable
    • Resistance to fire
    • Non-corrosive to its system
    • Anti-wearing to its system
    • Low tendency to cavitate
    • Tolerance to water (resistance to water contamination)
    • Regardless of temperature, The viscosity remains constant.
    • Long-lasting
    • cost-effective
  • The following are the features of liquids: There is no clear shape (takes the shape of its container). Has a distinct volume. Although particles are free to move over one another, they are still attracted to one another.

  • The following are ideal fluid properties.

    it is incompressible, In other words, Its density is constant.

    It has an irrotational flow, i.e. the flow is smooth and free of turbulence.

    It has a low viscosity, The fluid has no viscosity because there is no internal friction in the flow.

    It has a steady flow, i.e. its velocity is constant in time at each point.

  • Liquids expand for the same reason as solids, but they expand more than solids because the connections between molecules are usually less tight. Liquid-in-glass thermometers work on this concept. When the temperature of the liquid rises, it expands, which causes it to rise in the glass.

  • The following are cutting fluid properties.

    • The viscosity of cutting fluid should be low so that it may easily flow over the workpiece.
    • It should have a high flash point and be able to withstand high temperatures.
    • It should have a low tendency for foaming.
    • It should have a high heat absorption rate so that it can easily absorb the heat created during cutting operations.
    • It should have a good lubricating property to reduce friction between the tool and the workpiece, allowing chips to easily escape.
    • Chemical reactions between the coolant and the environment should be avoided.
    • It should be odourless to avoid any unpleasant odours, even at high temperatures.


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