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Fluid and Liquid are two related but different terms. Fluid is a matter which moderates all the processes that occur in living beings and it is everywhere around us. It flows from one place to another flexibly. The term fluid also constitutes water and gas. Fluids should not be considered the same as liquids. The two terms, fluid and liquid, are distinct from each other in many ways.
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| Table of Content |
Key Takeaways: Fluids, Liquids, Viscosity, Conductivity, Density, Compressibility, Surface Tension, Ideal Fluid, Real Fluid
What is Fluid?
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Fluid is a substance which can flow. It shows continuous relative motion between different particles when there is a difference of pressure between two points. A fluid has no definite shape of its own and therefore it deforms when a sheer force or stress is applied onto it. The deformation is viewed as the motion of the fluid or the flow of the fluid. Fluids constitute liquids, gases, plasma, and some types of plastic solids. The fluids are classified based on: viscosity, conductivity, density, and compressibility.
Fluid
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Classification of Fluids
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Fluids can be classified into various types on different basis:
Based on Viscosity
- Ideal Fluids - A fluid, which is incompressible which means that the density of fluid does not change with the change in external force or pressure and has no viscosity, is known as an ideal fluid. Ideal fluid is imaginary as all the fluids, which exist, have some amount of viscosity.
- Real Fluids - Fluids, which possess viscosity and are compressible, are known as real fluid. In reality, All the fluids are real fluids. For example - water, air etc.
- Newtonian Fluids - A real fluid, which obeys Newton’s law of Viscosity in which shear stress is directly proportional to the rate of shear strain or velocity gradient for a given temperature and pressure, is known as a Newtonian fluid.
- Non-Newtonian Fluids - A real fluid, which disobeys Newton’s law of viscosity due to which the shear stress is not directly proportional to the rate of shear strain or velocity gradient, is known as a Non Newtonian fluid. This includes plaster, slurries, pastes etc.
- Ideal Plastic Fluid - An ideal plastic fluid is a fluid in which shear stress is more than the yield value and shear stress is proportional to the rate of shear strain or velocity.
Classification of Fluids
Based on Compressibility
- Compressible Fluids - When we apply an external pressure to the fluid and its volume or density changes, then such fluids are called compressible fluids.
- Incompressible Fluids - When we apply an external pressure to the fluid and its volume or density does not change, then they are called incompressible fluids.
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Based on Conductivity
The conductivity of a fluid influences its charging capacity. The conductivity is expressed as siemens per meter (S/m) or most commonly picosiemens per meter (pS/m). As per NFPA-77, the US consensus practice on static electricity, fluids can be partitioned into three classes:
- Conductive (>10,000 pS/m)
- Semi-conductive (50-10,000 pS/m)
- Non-conductive (<50 pS/m)
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Density vs Viscosity Between Fluids
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The difference in the values of density and viscosity of some fluid types are given below:
| Types of Fluids | Density | Viscosity |
|---|---|---|
| Ideal Fluids | Constant | Zero |
| Real Fluids | Variable | Non-zero |
| Newtonian Fluids | Constant/Variable | T = u (du/dy) |
| Non-Newtonian Fluids | Constant/Variable | T ≠ u (du/dy) |
| Incompressible Fluids | Constant | Non zero / zero |
| Compressible Fluids | Variable | Non zero / zero |
Graph showing Shear stress and Velocity gradient of different fluids
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What is Gas?
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Gas is a type of fluid in which the particles (atoms or molecules) are separated by several times the particle diameter, so that intermolecular forces are negligible. Gasses constitute of individual atoms (e.g. noble gasses like argon, neon), elemental molecules made up of one type of atom (e.g. oxygen, nitrogen), and compound molecules made from a variety of atoms (e.g. carbon dioxide, carbon monoxide etc.). Air is basically a gas which is present in the atmosphere surrounding the Earth. The molecules in gasses are really spread out, are full of energy, and constantly move randomly and so is the reason that they are highly compressible.
Gas Molecules are spread out
What is Liquid?
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Liquid is a state of matter composed of molecules that flows freely but is of constant volume with no fixed shape. It possesses the following characteristics:
- Liquids can flow and can change its shape.
- It maintains a relatively fixed volume because the molecules are pretty close to each other.
- Liquids constitute very small particles (atoms, molecules, and/or ions).
- The particles that make up a liquid are close together with no regular arrangement and vibrate, move about, and slide past each other.
The liquid state of matter is an intermediate phase between solid and gas and has no defined structure and a fixed amount of matter. Like the particles of a solid, particles in a liquid are subject to intermolecular attraction, however, it is so weak that the molecules travel relative to each other. The liquid particles have more spaces in between them, so they are not fixed in position. The liquid always has a small surface area owing to surface tension. The attraction between the particles in a liquid keeps the volume of the liquid constant.
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Difference Between Fluids & Liquids
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The key differences between fluids and liquids are as follows:
| Fluids | Liquids |
|---|---|
| Fluid has a higher viscosity than liquid. | Liquids have lower viscosity than fluid. |
| All fluids may or may not be liquids. | All liquids are fluids. |
| It basically describes anything that can flow as a result of a difference in pressure between two points. | It is a subset of fluid. |
| Fluids consist of two mutually exclusive groups of compressible and incompressible fluids. | Liquids are incompressible fluids and hence form a subset of the more general term fluid. |
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Things to Remember
- Newtonian Fluids do not possess any elastic properties and are incompressible,isotropic and are not real.
- The viscosity of Newtonian Fluid is inversely proportional to the increase in temperature.
- The ratio of shear stress to the velocity gradient in a fluid is called viscosity.
- When you are defining a fluid substance, you must also mention its viscosity. Liquids, on the other hand, flow and are defined as having volume without a fixed shape.
- The SI unit of viscosity is poiseuille (PI). Its other units are newton-second per square meter (Ns m-2) or pascal-second (Pas).
Sample Questions
Ques. What is the movement of particles in a liquid? (2 Marks)
Ans. In liquids, particles are quite close together and move with random motion throughout the container. Particles move rapidly in all directions but collide with each other more frequently than in gasses due to shorter distances between particles.
Ques. Define Compressibility of Fluids. (2 Marks)
Ans. The measure of the relative volume change of a solid or fluid in response to the external pressure change is called compressibility. It is represented by \(\beta\).
\(\beta\)= 1/K or \(\beta\)= -1/V.dV/dP
The SI Unit of Compressibility is Pa-1. Its Dimensional unit is [M-1L1T2]
Ques. Explain Density of Fluids. (2 Marks)
Ans. Density is defined as mass per unit volume of a fluid. It is denoted by \(?\).
\(?\)= mass/volume
The Density of air is 1.2 kg/m3 and that of water is 1000 kg/m3.
Ques. Define Bulk Modulus. (2 Marks)
Ans. The measurement of the ability of the fluid to resist changes in the volume when it is being subjected to compression from every side is called Bulk Modulus and is usually expressed to determine the elastic property of a fluid. It is denoted by K.
K = -vdP/dV or K = . dP/dp
Its SI unit is Newton per meter square (N/m2) or Pascal (Pa). The Dimensional Formula is [M1L-1T-2].
Ques. What are conductive, semi-conductive and low-conductive fluids? (2 Marks)
Ans. For conductive fluids any static which is generated within the fluid can be conducted through the pipe/vessel and be dissipated safely via the grounding. For semi-conductive fluids, when the rate of charge generation is rapid, there may not be time for the charge to be dissipated. Low conductivity fluids are unable to dissipate the static charge.
Ques. How do pressure changes force some fluids to change phase? (3 Marks)
Ans. By compressing a gas, we reduce its volume. We are forcing gas particles to get closer and closer to each other. Eventually, they will liquefy. Liquids and solids, on the other hand, are composed of molecules that are more strongly attached to one another. Applying pressure to a liquid won't do much but perhaps re-arrange a few of its molecules. Applying pressure to a solid will perhaps result in breaking it, but will not cause a phase change.
Ques. How do temperature changes force a fluid to undergo a phase change? (2 Marks)
Ans. Gas particles behave erratically. Heating up a container of gas will transfer heat to the gas particles and excite them. As a result, their velocities will increase. No phase change will occur with added heat, but if cooled, a gas may liquefy.
The rigid bonds of a liquid and solid can be broken with enough heat, which gives the energy needed to ultimately destroy these tight bonds and change phase. Cooling a solid won't change any phases, but cooling a liquid will freeze it if the temperature is below the freezing point.
Ques. Why is it impossible for a fluid to accelerate towards a smaller area and simultaneously increase its pressure? (2 Marks)
Ans. If a fluid is accelerating, then it is under the influence of a force since F = ma. We've seen that this force actually comes from the fluid itself. Pressure is given by force divided by area, or P = F/A. Since area A decreases, this means that pressure P increases.
Ques. What is Thixotropic and how does it apply to my needs? (3 Marks)
Ans. The fluids whose shear stress or viscosity decreases with time due to isothermal conditions and steady shear are called Thixotropic. Thixotropic materials reduce viscosity when stirred or put under pressure. An example of Thixotropic is ketchup. When static, it is generally thick and reluctant to pour out of the bottle. However, if stirred, it becomes much more fluid and will pour easily. Polymeric engineering fluids exhibit this same characteristic to varying degrees, and systems designed to manage them must be engineered on a case-by-case basis for optimum results.
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