**FLUIDS**

Fluid any liquid or gas or generally any material that cannot sustain a tangential, or shearing, force when at rest and that undergoes a continuous change in shape when subjected to such a stress. This continuous and irrecoverable change of position of one part of the material relative to another part when under shear stress constitutes flow, a characteristic property of fluids. In contrast, the shearing forces within an elastic solid, held in a twisted or flexed position, are maintained; the solid undergoes no flow and can spring back to its original shape. (See deformation and flow.) Compressed fluids can spring back to their original shape, too, but while compression is maintained, the forces within the fluid and between the fluid and the container are not shear forces. The fluid exerts an outward pressure, called hydrostatic pressure, that is everywhere perpendicular to the surfaces of the container.

Various simplifications, or models, of fluids have been devised since the last quarter of the 18th century to analyze fluid flow. The simplest model, called a perfect, or ideal, fluid, is one that is unable to conduct heat or to offer drag on the walls of a tube or internal resistance to one portion flowing over another. Thus, a perfect fluid, even while flowing, cannot sustain a tangential force; that is, it lacks viscosity and is also referred to as an inviscid fluid. Some real fluids of low viscosity and heat conductivity approach this behaviour.

Fluids of which the viscosity, or internal friction, must be taken into account are called viscous fluids and are further distinguished as Newtonian fluids if the viscosity is constant for different rates of shear and does not change with time. The viscosity of non-Newtonian fluids either varies with the rate of shear or varies with time, even though the rate of shear is constant. Fluids in a class in this last category that become thinner and less viscous as they continue to be stirred are called thixotropic fluids.

Although the term “fluid” includes both the liquid and gas phases, in common usage, “fluid” is often used as a synonym for “liquid”, with no implication that gas could also be present. For example, “brake fluid” is hydraulic oil and will not perform its required incompressible function if there is gas in it. This colloquial usage of the term is also common in medicine and in nutrition (“take plenty of fluids”).

Liquids form a free surface (that is, a surface not created by the container) while gases do not. The distinction between solids and fluid is not entirely obvious. The distinction is made by evaluating the viscosity of the substance. Silly Putty can be considered to behave like a solid or a fluid, depending on the time period over which it is observed. It is best described as a viscoelastic fluid. There are many examples of substances proving difficult to classify. A particularly interesting one is pitch, as demonstrated in the pitch drop experiment currently running at the University of Queensland.

**TYPES OF FLUIDS**

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**WHAT ARE TYPES OF FLUIDS?**

Of what types/classifications are fluids in science and engineering study? What do you think is the definition of fluid? Fluids can be defined as substances that flow or deform under the application of shear stress, and these include liquids and gases. They are part of engineering study in many tertiary institutions of the world. Basically, in the study of science, fluids are divided into two broad groups. These divisions in this write-up are known as types of fluids which are Newtonian and non- Newtonian fluids. Newtonian fluids are those fluids that obey Newton Law of viscosity. Non-Newtonian fluids are the opposite of Newtonian fluid in the sense that they do not obey Newton Law of viscosity. Non Newtonian fluids in this text are sub-divided into time-independent, time-dependent and elasticoviscous or viscoelastic fluids.

**NEWTONIAN FLUIDS**

**WHAT ARE NEWTONIAN FLUIDS **

Newtonian fluids as written in the introductory part of this text are those fluids that concur (agree) with the Newton Law of viscosity. Viscosity is the opposition to the flow of fluids and it is measured in force per unit area of the fluid. The generally accepted unit of viscosity is Newton per meter square (NM-2). This is known as the SI unit of viscosity which is the same with that of stress. Mathematically, viscosity is expressed as Force per unit area or simply F̸̸A. Newton law of viscosity states that the shear stress on a fluid element layer is directly proportional to the rate of shear strain. In Newtonian fluids, coefficient of viscosity does not change with the rate of deformation of the fluid. Examples of Newtonian fluids are: water, kerosene and air.It is shown mathematically as:

**τ = ηγ; where τ = shear stress, η and γ are coefficient of viscosity and share strain respectively.**

**NON- NEWTONIAN FLUIDS**

**WHAT ARE NON-NEWTONIAN FLUIDS AND THEIR CLASSIFICATIONS? **

Non-Newtonian fluids are those fluids that do not obey Newton’s Law. They are the opposite of Newtonian fluids. Examples of non-Newtonian fluids are colloids, emulsions, pastes, sols, gels, thick slurry, latex-based paints, and Biological fluids. Note that non-Newtonian fluids are many but these are few examples given. Non-Newtonian fluids do not exhibit the property of Newtonian fluids where shear stress is directly proportional to shear rate.

There are three broad classifications of non-Newtonian fluids. These three classifications are: time-independent, time-dependent and viscoelastic fluids. The viscoelastic fluids can also be called elasticoviscous fluids. One should not be confused because some textbooks relating to fluids may only one of these two names. Notwithstanding the three broad classifications of non-Newtonian fluids, there are also some other divisions of the three.

**TIME-INDEPENDENT FLUIDS**

As the name sounds, time-independent fluids are those non-Newtonian fluids that do not depend on time. They are those fluids in which the shear rate at a given point is a function of stress at that point only. Examples of time-independent fluids are Casson, Bingham, Dilatent and Pseudoplastic fluid.

**THE BINGHAM FLUID AS AN**

exampleof time-independent fluid does not flow at all until the shear stress exceeds certain critical value called yield stress. In this fluid, the flow behaviors appear like that of Newtonian once the system begins to flow. There is an internal structure in this type of fluid which breaks down before flow of the fluid can start. Notable examples of Bingham fluids are tomato puree, wood pulp suspensions, butter, drilling mud and toothpaste. When equation is used to represent Bingham fluid, it is represented as:

**τ = τy + ƞγ, where τy is yield stress.**

Casson fluids also require a critical shear stress to overcome before flow can occur in the system. The type of flow in this type of time-independent fluid is non-Newtonian, non-linear and parabolic in shape. Casson and Bingham fluids are called plastic fluids.

Dilatent and Pseudoplastic fluids exhibit different characters on their own. Dilatent fluid is also called shear thickening fluid. Dilatant fluid becomes more viscous as the shear stress increases. The shear stress increases much more rapidly than the shear rate in this kind of fluid. Examples of dilatent fluids are slurry and highly concentrated suspensions, like, Poly Vinyl Chloride. Pseudoplastic fluid is opposite to Dilatent fluid because the share rate increases much more rapidly than the shear stress. It is known as shear thinning fluid. As the shear stress increases, pseudoplastic fluid becomes less viscous.

**TIME-DEPENDENT FLUIDS**

Time-dependent fluids are fluids whose shear rate is a function of shear stress and time. In this type of non-Newtonian fluid, the property of the fluid flow such as apparent viscosity changes with time. It is further classified into thixotropic and rheopectic fluid. In relation of thixotropic with rheopectic fluids, if the shear stress and shear strain relationship are observed with increasing shear rate, both sets of data do not coincide. This results to formation of hysteresis loop. In thixotropic and rheopectic fluids, at a given shear rate; there are two apparent viscosities depending on when the readings were taken. The difference between the two is that thixotropic fluid becomes less viscous on application of stress while rheopectic fluid becomes more viscous on application of stress.

**ELASTICOVISCOUS FLUIDS**

Fluids that are predominantly viscous but show partial elastic recovery after deformation are termed elasticoviscous fluids. Examples of such fluids are multi-grade oils, polymer melts and liquid detergents. The term viscoelastic fluid is also used in place of elasticoviscous fluids as the former denotes solids with viscous properties while the later (elasticoviscous) denotes fluids that possess elastic property.

**CONCLUSION**

This article is purely written for academic purpose. In summary, this write-up has dealt seriously on types of fluids based on science and engineering study. Fluids cannot be done without in our everyday life and this is one of the reasons that makes scientists to show more interesting in categorizing them and for more in-depth study of their flow. One of the basic types of food which people neglect is fluid. Do you know what that important fluid is? It is no other thing but the water we drink on our daily basis and I do not think you can do without it. Gasoline is another basic fluid used in automobiles and this is of great help to man. We cannot be able to power or motors on without this energy supplier. So, respect is to be given to fluids as they contribute to both technological and human development. Fluids were categorized broadly as Newtonian and Newtonian fluids. The non-Newtonian fluids were further divided into other classes and explained in sub-headings.