What Is Viscosity?

Viscosity is the fundamental characteristic property of all fluids. It is usually defined as the measure of internal friction or resistance of the fluid. It can also be termed as the drag force and can be defined as the measure of the frictional properties of the fluid. The study of the behavior of flowing fluid is known as rheology,which has already been discussed in detail.

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Viscosity is the real factor behind the thickness or concentration of the fluid, i.e. fluid having more viscosity is thicker then the less viscosity fluid hence resists more in the flow. The oil and water are the most common examples of this viscosity and thickness relationship. Oil viscosity is greater then the water viscosity, that’s why oil is thicker then water, and sustain more resistance in flow then water. Viscosity itself is a complete science, having applications in numerous sectors like petroleum, coating, printing, food and beverages, combustion,image processing, power, environment etc.   

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Viscosity can be expressed as two distinct forms:

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• Dynamic viscosity

Dynamic viscosity is also named as Absolute viscosity. It is basically the tangential force per unit area, which is required to drag one layer of fluid to another.

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Mathematically, the above described phenomena can be written as:

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τ  = F / A

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            This equation can also be written in the differential form:

       

   τ  = µ (∂u / ∂y)

            Where:

                                   τ          =          shearing stress

µ          =          dynamic viscosity

∂u/∂y   =          velocity gradient

           

• Kinematic Viscosity

Kinematic viscosity is simply the dynamic viscosity of the fluid divided by the density of the fluid. Mathematically, kinematic viscosity is expressed as:

υ = µ / ρ

            Where:

                                  υ          =           kinematic viscosity

µ         =           dynamic viscosity

ρ         =          density of the fluid

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Viscosity Units:

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The viscosity units are different for different forms of viscosity. i.e.

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• The dynamic viscosity units are often expressed in CGS units. However common units of dynamic viscosity are:

• CGS units      : Poise , g/cm.s , dyne.s/cm²


• British units : lb/ft.s , lbf.s/ft²

• The kinematic viscosity units are expressed as Stokes (St) , Centistokes (cSt), or m2/s.

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Viscosity Of Some Common Substances:

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The viscosity of some most common substances at room temperatures are given below for references.

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• Viscosity of air                =  10^-5     Pa.S


• Viscosity of water          =  10^-3    Pa.S


• Viscosity of olive oil       =  10^-1     Pa.S


• Viscosity liquid honey    =   10¹    Pa.S


• Viscosity glass                 =  10⁴⁰   Pa.S

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What Is Lubricant?

Lubricant is a material or substance applied to reduce the friction between the moving surfaces. There are number of industrial applications of the lubricants. One of the most common applications of lubricants is in the form of motor oil, which is used to protect the combustion chambers of the vehicles, as well as other powered equipments.

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Characteristics Of A Good Lubricant:

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A lubricant is considered as effective, and efficient, if it possess the following properties:

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• Low freezing point


• High boiling point


• Thermal stability


• Prevention from corrosion


• Higher viscosity index


• High resistance to oxidation

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Types Of Lubricants

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• Petroleum Lubricants:

Petroleum products are the most common lubricants among others.These are the first preference for typical processes, because of being in expensive in comparison with other lubricants. These petroleum lubricants are used for a wide range of application sectors, like engines, gears,transmissions etc. these petroleum products are known by their viscosity indexes or ASTM codes. However some of the common petroleum lubricants are gear oils, motor oils, automatic transmission fluids, valve oils etc.

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• Synthetic oils

These are also the petroleum based lubricants, but are more modified, and that is why quite expensive then the simple petroleum products.The typical uses of different types of synthetic oils are in auto engines, jet engines, air craft hydraulics, rubber seals, fire resistance fluids, and number of others.

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• Greases

Grease is basically lubricating oil which is thickened with a gelling agent. Greases are the first considerations for the lubrication of bearings in electric motors, machine tools, house hold appliances, as well as the slow speed moving equipments. Common gelling agents used to make greases are the fatty acids of soap (like oleic, palmitic and other carboxylic acids),clay particles of bentonite and hectorite, carbon black, silica and several others.

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• Solid film lubricants

These solid film lubricants provide thin films of solids,between the moving surfaces, to reduce friction and wear. Solid film lubricants are classified as inorganic (molybdenum disulfide, graphite etc) and organics(ethylene-propylene copolymer etc).

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Lubricants Additives

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The common additives used in the lubricants are

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• Foam inhibitors


• Oxygen inhibitors


• Viscosity index improver


• Rust inhibitors


• Anti-wears


• Pour point depressants


• Extreme pressure agents


• Friction modifiers


• Detergents and dispersant

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What Is Lubrication?

Lubrication is the process or method used for the reduction of friction and wear of the moving surfaces by applying an additional substance between the surfaces. This substance is called as Lubricant. While the study of this lubrication science,friction, and wear is called as Tribology.

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Lubrication is necessary for the correct operations of the mechanical operations works on the contacting- rubbing phenomena, like pumps, pistons, turbines, bearings etc.so that the pressure generated by the contact between the surfaces can be minimized,and so is the risk of wear- tear as well as friction losses.

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Regimes Of Lubrication

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There are several distinct regimes which are commonly used to describe the fundamental principles of lubrication. When the load increases on the surfaces in contact, usually following distinct regimes are observed, with respect to the lubrication modes. These regimes are called as regimes of lubrication.

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• Fluid film lubrication

In fluid film lubrication regime, the moving surfaces have a liquid or gaseous lubricant film between them, so that the contact between the surfaces can be avoided. So both the frictional power loss, as well as the pressure is the functions of lubricant viscosity, as well as the shear rate and geometry of the contacting surfaces. This regime is common in electric motors,and generators.

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• Boundary lubrication

Boundary lubrication regime is used for the worse conditions,where the fluid film lubrication is not enough to cover the applied load severity. I.e. when the bodies come in contact, heat generated by the by the load causes stick-slip conditions, which can lead to breakage. At high pressure and temperature conditions, the reactive particles of the lubricant reacts with the contacting surfaces forming a layer on the moving surfaces, this layer is highly resistive and can support the load, so can avoid the breakage. This lubrication layer is often considered as the boundary film lubrication. The most common example of boundary film lubrication is Hypoid gears in automobiles.

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• Elastohydrodynamic lubrication (EHL)

This regime of lubrication is the lubrication between the non-conformal surfaces contact, i.e. the contact between those surfaces, which do not fit each other, such as bearings (ball and roller), cams, gear teeth, or other friction drivers. By understanding EHL, it can be predicted that how thick lubricant films should be formed on the basis of the contacting pressure magnitudes and the surface stresses.

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It is not enough to know about the lubrication regimes, as there are several different lubricants, which are used in any of the lubrication regime. So a detailed study of tribology is needed to understand any friction-lubrication case.

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What Is Tribology?

what is tribology

The term“Tribology” is derived from a Greek work “Tribos” which originally means“rubbing”. So the simplest way to explain What Is Tribology is that “Tibology”is the analysis of the rubbing surfaces, i.e. the friction, wear and lubrication analysis. It is a complete science it self, which deals with the detailed study and analysis of four major factors involved for the prevention from wear and tear of the sliding surfaces:

• Corrosion: destruction or degradation of material by electrochemical or chemical reactions between the surface and environment.


• Abrasion : It is a mechanical process, in which the sliding surfaces start scratching or scuffing away.


• Adhesion: It is the attraction between dissimilar particles or the surfaces


• Erosion: Layer by layer degradation or destruction of material surface.

It not only helps in minimizing the wear and tear of the surfaces, but also helps in extending the working life of the equipments. Although tribology is more about mechanical engineering, then chemical engineering; but the processes under tribology tree are more concerned with the chemical engineering. Out of them, Lubrication is the most important.

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Lubrication is probably the vital science, used in the field of Tribology. Lubrication works by making the surfaces in contact more slippery and greasy to eliminate several factors like overheating, friction, or tearing. There are several different types of Lubricants, which are used for different surfaces; most important of them are Petroleum lubricants, Synthetic Oils, Greases, Solid film lubricants and Metal films.

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The application sectors of the tribology are unlimited, from a little lip gloss to the very large machinery or equipment, every single substance, which has to slide on any other substance, is designed by keeping the rules of tribology in mind. With the help of tribology, the friction losses, and other heat losses are minimized, and so is the operational cost of the process.

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Rheology

Rheology is the study of deformation and flow behavior of materials. Rheology is generally associated with the behavior of the fluids, but the semi-solids materials (under specific conditions) some times also possess the rheological properties.

Rheology normally accounts for the flow of unusual materials, generally non-Newtonian fluids, as rheological properties are the parameters in any quantitative functional relation between the stress and strain.And as we have already discussed that the Newtonian fluids have the linear relationship between the stress and strain.

All non Newtonian fluids do not possess same properties. For example paints, blood, custard, ketchup etc do not fall in the same category of the non Newtonian fluids. So these fluids are divided according to their rheological properties. i.e.

1) Non Newtonian fluids with Time independent viscosity:

1.1)          Shear thinning (Pseudo plastic) fluids

In pseudo plastic fluids, the viscosity decreases with the increase in stress or disturbance. i.e. these fluids are also named as shear thinning fluids. The most common examples of these pseudo plastic fluids are paint, blood, syrups, molasses etc.

           

1.2)          Shear thickening (dilatant) fluids

Like the name, the dilatant fluids are those, whose viscosity increases with the increase in the stress. The most common example is the corn syrup, or the sand in water.

1.3)          Bingham plastic fluids

The bingham plastic fluids are also named as the generalized Newtonian fluids. These fluids have the constant viscosity, but unlike non Newtonian fluids, the stress is dependent on the strain rate and the pressure applied on the fluid. Blood plasma and custard are the common examples of these fluids.

2) Non Newtonian fluids with Time dependent viscosity:

2.1)      Rheopectic

The viscosity of the fluid increases with the increase in the duration of stress,

2.2)          Thixotropic

The viscosity of the fluid decreases with the increase in the duration of stress,

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Rheology

Rheology is the study of deformation and flow behavior of materials. Rheology is generally associated with the behavior of the fluids, but the semi-solids materials (under specific conditions) some times also possess the rheological properties.

Rheology normally accounts for the flow of unusual materials, generally non-Newtonian fluids, as rheological properties are the parameters in any quantitative functional relation between the stress and strain.And as we have already discussed that the Newtonian fluids have the linear relationship between the stress and strain.

All non Newtonian fluids do not possess same properties. For example paints, blood, custard, ketchup etc do not fall in the same category of the non Newtonian fluids. So these fluids are divided according to their rheological properties. i.e.

1) Non Newtonian fluids with Time independent viscosity:

1.1)          Shear thinning (Pseudo plastic) fluids

In pseudo plastic fluids, the viscosity decreases with the increase in stress or disturbance. i.e. these fluids are also named as shear thinning fluids. The most common examples of these pseudo plastic fluids are paint, blood, syrups, molasses etc.

           

1.2)          Shear thickening (dilatant) fluids

Like the name, the dilatant fluids are those, whose viscosity increases with the increase in the stress. The most common example is the corn syrup, or the sand in water.

1.3)          Bingham plastic fluids

The bingham plastic fluids are also named as the generalized Newtonian fluids. These fluids have the constant viscosity, but unlike non Newtonian fluids, the stress is dependent on the strain rate and the pressure applied on the fluid. Blood plasma and custard are the common examples of these fluids.

2) Non Newtonian fluids with Time dependent viscosity:

2.1)      Rheopectic

The viscosity of the fluid increases with the increase in the duration of stress,

2.2)          Thixotropic

The viscosity of the fluid decreases with the increase in the duration of stress,

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types of fluid

Fluid mechanics is one of the major sciences involved in chemical engineering, so a chemical engineer is mostly interested in many aspects of the problems involved in the fluid flow. To understand the fluid mechanics, a chemical engineer must have to understand that what is fluid, and the types of the fluid.

The simplest definition of the fluid is that “Any substance which can flow under pressure is fluid”. Fluid can also be defined as “any substance that has no fixed structure, shape or size, and yields easily to the external pressure”.

There are generally two ways to classify the fluids, i.e.

> Compressible and Incompressible Fluids

> According to viscosity change

Compressible And Incompressible Fluids

The nature of the fluid is said to be compressible or incompressible according to its behavior under applied pressure, i.e.

The incompressible fluid is the one, whose volume is independent of its temperature and pressure, i.e. its volume will not be affected by the change of its temperature and pressure. There is no real fluid, which is completely incompressible, however liquids are assumed to be the incompressible fluids, as they sustain the change of temperature and pressure, more then gases.

The compressible fluids change their volume according to the change in their temperature or pressure. The gases are real example of such type of fluid. However, if the percent change is small, then for practical purposes, a gas may be treated as the compressible fluid.

Classification Of Fluids According To Viscosity Change

The fluids can also be classified according to the effects produced on the fluid by the action of the shear stress. This classification is important, as it determines the way in which the fluid will flow. This classification is based on a most important physical property “viscosity”.

Then main two types under this classification are:

> Newtonian fluid

> Non Newtonian fluid

A Newtonian fluid is the fluid, whose viscosity remains constant regardless of any applied stress. That’s why these fluids are also named as “linear viscous fluids”. The most common example of Newtonian fluid is water. The flow of water remains same, whether it flows alone, or in vigorously agitation condition. Its simplest meaning is that, the fluid will continue to flow, regardless of the forces acting on it. The Newtonian fluids behave according to following equation:

                                                            τ = µ (du/dy)

τ is the shear stress exerted by fluid

µ is fluid viscosity, constant for Newtonian fluids

(du/dy) is the velocity gradient, or the strain.

So for Newtonian fluid, according to the equation, the ratio of stress to strain is constant, there fore, the viscosity is constant.

The viscosity of the non Newtonian fluid is variable, and is dependent upon the applied stress on the fluid. These type of fluids also exhibits the rheological properties .The common examples of non Newtonian fluids are solution of corn starch and water, paints, ink, tooth paste, etc.

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types of fluid

Fluid mechanics is one of the major sciences involved in chemical engineering, so a chemical engineer is mostly interested in many aspects of the problems involved in the fluid flow. To understand the fluid mechanics, a chemical engineer must have to understand that what is fluid, and the types of the fluid.

The simplest definition of the fluid is that “Any substance which can flow under pressure is fluid”. Fluid can also be defined as “any substance that has no fixed structure, shape or size, and yields easily to the external pressure”.

There are generally two ways to classify the fluids, i.e.

> Compressible and Incompressible Fluids

> According to viscosity change

Compressible And Incompressible Fluids

The nature of the fluid is said to be compressible or incompressible according to its behavior under applied pressure, i.e.

The incompressible fluid is the one, whose volume is independent of its temperature and pressure, i.e. its volume will not be affected by the change of its temperature and pressure. There is no real fluid, which is completely incompressible, however liquids are assumed to be the incompressible fluids, as they sustain the change of temperature and pressure, more then gases.

The compressible fluids change their volume according to the change in their temperature or pressure. The gases are real example of such type of fluid. However, if the percent change is small, then for practical purposes, a gas may be treated as the compressible fluid.

Classification Of Fluids According To Viscosity Change

The fluids can also be classified according to the effects produced on the fluid by the action of the shear stress. This classification is important, as it determines the way in which the fluid will flow. This classification is based on a most important physical property “viscosity”.

Then main two types under this classification are:

> Newtonian fluid

> Non Newtonian fluid

A Newtonian fluid is the fluid, whose viscosity remains constant regardless of any applied stress. That’s why these fluids are also named as “linear viscous fluids”. The most common example of Newtonian fluid is water. The flow of water remains same, whether it flows alone, or in vigorously agitation condition. Its simplest meaning is that, the fluid will continue to flow, regardless of the forces acting on it. The Newtonian fluids behave according to following equation:

                                                            τ = µ (du/dy)

τ is the shear stress exerted by fluid

µ is fluid viscosity, constant for Newtonian fluids

(du/dy) is the velocity gradient, or the strain.

So for Newtonian fluid, according to the equation, the ratio of stress to strain is constant, there fore, the viscosity is constant.

The viscosity of the non Newtonian fluid is variable, and is dependent upon the applied stress on the fluid. These type of fluids also exhibits the rheological properties .The common examples of non Newtonian fluids are solution of corn starch and water, paints, ink, tooth paste, etc.

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