Viscosity Measurement

Chapter 1Measurement of Viscosity

Krunal H. Parmar

Assistant Professor

Department of Instrumentation & Control

Sardar Vallabhbhai Patel Institute of Technology, Vasad

Outlines

1. Introduction- What is Viscosity?- Why should I measure Viscosity?

2. Newtonian & Non Newtonian behavior- Newtonian behavior- Non Newtonian behavior

- Pseudo plastic- Dilatants- Plastic- Thixotropy- Rheopexy

1. Introduction

• Viscosity is a quantitative measure of a fluid’s resistanceto flow.

• It is defined as the internal friction of fluid.

• Lack of slipperiness is called as viscosity

• Rheology: The study of the change in form and the flowof matter, embracing elasticity, viscosity, and plasticity.

• We generally measures two types of viscosity :

1. Absolute/ Dynamic Viscosity (ƞ)

2. Kinematic Viscosity (Ʋn)

Viscosity

• Viscosity is the measure of the internal friction of a fluid.

• This friction becomes apparent when a layer of fluid ismade to move in relation to another layer.

• The greater the friction, the greater the amount of forcerequired to cause this movement, which is called shear.

• Shearing occurs whenever the fluid is physically movedor distributed, as in pouring, spreading, spraying, mixing,etc. Highly viscous fluids, therefore, require more force tomove than less viscous materials.

• Isaac Newton defined viscosity by considering the modelrepresented in the figure below.

• Two parallel planes of fluid of equal area A are separatedby a distance dx and are moving in the same direction atdifferent velocities V1 and V2.

• Newton assumed that the force required to maintain thisdifference in speed was proportional to the difference inspeed through the liquid, or the velocity gradient. Toexpress this, Newton wrote:

• where Ƞ is a constant for a given material and is called itsviscosity.

• The velocity gradient, dv/dx , is a measure of the changein speed at which the intermediate layers move withrespect to each other. It describes the shearing the liquidexperiences and is thus called shear rate.

• The term F/A indicates the force per unit area required toproduce the shearing action. It is referred to as shearstress.

• So, mathematically Viscosity can be defines as

Why should I measure Viscosity?

• From viscosity measurement, we can obtain much usefulbehavioral and predictive information for variousproducts.

• A frequent reason for the measurement of rheologicalproperties can be found in the area of quality control,where raw materials must be consistent from batch tobatch.

• For this purpose, flow behavior is an indirect measure ofproduct consistency and quality.

Types of Viscosity

Newtonian fluids

• This type of flow behavior Newton assumed for all fluidsis called Newtonian.

• It is, however, only one of several types of flow behavioryou may encounter.

• Graph A shows that the relationship between shearstress and shear rate is a straight line.

• Graph B shows that the fluid's viscosity remains constantas the shear rate is varied.

• Typical Newtonian fluids include water and thin motoroils.

Non Newtonian fluids

• A non-Newtonian fluid is broadly defined as one forwhich the relationship is not a constant.

• It means that there is non-linear relationship betweenshear rate & shear stress.

• In other words, when the shear rate is varied, the shearstress doesn't vary in the same proportion (or evennecessarily in the same direction).

• E.g. Soap Solutions & cosmetics, Food such as butter,jam, cheese, soup, yogurt, natural substances such aslava, gums, etc.

Pseudo plastic

• This type of fluid will display a decreasing viscosity withan increasing shear rate.

• Probably the most common of the non-Newtonian fluids,pseudo-plastics include paints, emulsions, anddispersions of many types.

• This type of flow behavior is sometimes called"shear-thinning.

Dilatants

• Increasing viscosity with an increase in shear ratecharacterizes the dilatant fluid.

• Although rarer than pseudo plasticity, dilatancy isfrequently observed in fluids containing high levels ofdeflocculated solids, such as clay slurries, candycompounds and sand/water mixtures.

• Dilatancy is also referred to as shear-thickening flowbehavior.

Plastic (Bingham Plastic)

• This type of fluid will behave as a solid under staticconditions.

• A certain amount of force must be applied to the fluidbefore any flow is induced; this force is called the yieldstress (f').

• Tomato catsup is a good example of this type fluid; itsyield value will often make it refuse to pour from thebottle until the bottle is shaken or struck, allowing thecatsup to gush freely.

• Once the yield value is exceeded and flow begins, plasticfluids may display Newtonian, pseudoplastic, or dilatantflow characteristics.

• Some fluids will display a change in viscosity withtime under conditions of constant shear rate.

• There are two categories to consider:

1) Thixotropy

2) Rheopexy

Thixotropy

Rheopexy

Viscosity coefficients

• Viscosity coefficients can be defined in two ways:

1) Dynamic or Absolute viscosity

2) Kinematic Viscosity

• Viscosity is a tensorial quantity that can be decomposedin different ways into two independent components. Themost usual decomposition yields the following viscositycoefficients:

1) Shear Viscosity

2) Extensional Viscosity

Dynamic Viscosity/ Absolute Viscosity

Proportionality constant between shear stress andvelocity gradient is often called as “Dynamic Viscosity/ Absolute Viscosity”.

Reciprocal of Dynamic Viscosity is “Fluidity”.

Ratio of Shear stress to the velocity gradient of thefluid is known as Absolute Viscosity.

Units of Absolute Viscosity

Kinematic Viscosity

Shear Viscosity

• The most important one, often referred to as simplyviscosity, describing the reaction to applied shearstress; simply put, it is the ratio between thepressure exerted on the surface of a fluid, in thelateral or horizontal direction, to the change invelocity of the fluid as you move down in the fluid(this is what is referred to as a velocity gradient).

Volume Viscosity or Bulk Viscosity

• Bulk viscosity becomes important only for sucheffects where fluid compressibility is essential.Examples would include shock waves and soundpropagation.

• It appears in the Stokes' law (sound attenuation) thatdescribes propagation of sound in Newtonian liquid.

Extensional Viscosity

• A linear combination of shear and bulk viscosity,describes the reaction to elongation, widely used forcharacterizing polymers.

• For example, at room temperature, water has adynamic shear viscosity of about 1.0 * 10−3 Pa·s andmotor oil of about 250 * 10−3 Pa·s.

Instruments for Measuring Viscosity

Measurement of Shear viscosity

Depends upon dynamics of shear force acting upon fluideither Newtonian / non- Newtonian.

Instruments which measures the viscosity are called asViscometers. Viscometers only measures under one flowcondition.

Most popular viscometers to measure shear viscosity are

1. Capillary flow viscometer

2. Circular couette flow viscometer

3. Cone & plate flow viscometer

4. Parallel plate flow viscometer

Principle of Viscosity Measurement

Capillary Flow Viscometer

• Capillary flow viscometers are based on pressuredrop

• With this as a starting point we can derive theHagen-Poiseuille Equation:

Q (P)R4

8L

(P)R4

8LQ

Couette Viscometer

Cone & Plate Viscometer

Parallel Plate Viscometer

Shop Floor Viscometers

• Mainly used in Industrial applications to measureviscosity of mostly Newtonian types of fluid.

• These viscometers consisting simple & convenientmethod for viscosity measurement.

• Most popular shop-floor viscometers :

1. Rotational Viscometer

2. Flow through restriction type Viscometer

3. Flow around type Viscometer

Rotational Viscometer (Brook field Viscometer)

• Determine viscosity by measuring the

resistance on a shaft rotating in the

fluid .

• They are designed to make a direct

measurement of the absolute

Viscosity.

• The theory of operation of a rotational

viscometer is based on the Couette

flow model for fully developed,

steady and laminar flow between two

surfaces, one of which is moving.

• Pointer displacement is directly

proportional to fluid viscosity.

Flow through restriction type

Flow around obstruction type

Viscometer standard table

Most popular flow around type Viscometer

Measurement of Extensional Viscosity

Operating Principle

Effect of Temperature

• The viscosity of liquids decreases with increase thetemperature.

• The viscosity of gases increases with the increase thetemperature.

• The lubricant oil viscosity at a specific temperaturecan be either calculated from the viscosity -temperature equation or obtained from theviscosity-temperature chart.

Viscosity Temperature equations

Viscosity Index

• An entirely empirical parameter which would accuratelydescribe the viscosity- temperature characteristics of theoils.

• The viscosity index is calculated by the followingformula:

VI = (L - U)/ (L - H) * 10

where ,

VI is viscosity index

U is the kinematic viscosity

of oil of interest

L and H are the kinematic

viscosity of the reference oils

Effect of Pressure

• Lubricants viscosity increases with pressure.

• For most lubricants this effect is considerably largest

than the other effects when the pressure is

significantly above atmospheric.

• The Barus equation :

Online Viscosity Measurement

Applications

• Selection of lubricants for various purpose.- we can choose an optimum range of viscosity forengine oil.

- for high load and also for speed operation highviscous lubricants is required.

• In pumping operation

- for high viscous fluid high power will require.

- for low viscous fluid low power will require.

• In making of blend fuel

- less viscous fuels easy to mix.

• In the operation of coating and printing.

Important Questions

1. Explain Coutte Viscometer in detail.

2. Define Viscosity. What are the different methods of Viscositymeasurement? Explain any one method with basic Principle, working &suitable application.

3. Explain the measurement of viscosity under extremes of temperature and pressure.

4. Explain in detail construction and working of cone and plate viscometerand parallel plate viscometer.

5. Define the viscosity with necessary equation and discuss Newtonianand non-Newtonian behavior of various fluids

6. Explain the measurement of shear viscosity using capillary viscometerwith necessary equation

7. Give explanation about following terms: Viscosity, Bingham Plastic,Pseudoplastic, Shear Thinning, Shear Thickening, Thixotropy &Rheopexy