Rheology is concerned with relating the response of a material to the forces that act upon it. As you have seen from the virtual field trips, the ways in which rocks can respond to forces are highly variable. Not only do different rock types respond in different ways but also the same rock type can show various responses depending on the conditions under which the force is applied. As a general principal however, take it that rocks deform in the easiest way open to them. By that we mean that if it takes less force to break a rock than it does to distort it, the rock will break.
Scientists are still confusedthat how to define rheology Rheology=study of deformation and flow
What is rheology to scientists?Rheology is Yield stresses Viscoelastic effects Memory effects Shear thickening and shear thinning
Rheological measurementsDeformation is the relative displacement of points of abody. It can be divided into two types: flow and elasticity.Flow is irreversible deformation; when the stress isremoved, the material does not revert to its original form.This means that work is converted to heat.Elasticity is reversible deformation; the deformed bodyrecovers its original shape, and the applied work is largelyrecoverable. Viscoelastic materials show both flow andelasticity.
Flow classificationFlows are of following main types Steady simple shear flow Unsteady simple shear flow Extensional flow
Steady simple shear flowShear flow is a flow which occurs when the fluid is placedbetween the two plates and the two plates move at differentvelocitiesThe viscosity function η,the primary and secondary normalstress coefficients ψ 1 and ψ 2 respectively are the three viscometric functionswhich completely determine the state of stress in anyrheologically steady shear flow.
Unsteady simple shear flowUnsteady simple shear flow occurs when the stresses aretime dependentSmall amplitude oscillatory flow, stress growth, stressrelaxation, creep and constrained flow are some examplesof such flows
Extensional FlowThere is no shear flow in this type.The volume of the fluid remains constant in this type offlowIt occurs when the material is longitudinally stretched asfor example in fiber spinning
Flow behaviourThe viscoelastic nature of the polymers whether filled orunfilled bring them under the category called the nonnewtonian fluidsNewtonian fluids as we know are the fluids which obey thenewton’s law of viscosityThe simplest example of newtonian fluid is water
Models for the Shear viscosityAt low shear rate range the unfilled polymer behave asNewtonian fluidsAs shear rate increases the viscosity begins to decreaseshowing the pseudo plastic behavior.Under normalconditions the high shear rate regions are neglected and thecurve of unfilled polymers become similar to the filledpolymers
Polymer applicationsPolymer ProcessingPolymers are used in the construction and a large numberof other applications so these are now indispensableIn most of the cases the melt processing is been carried outbut there are many examples in which processing ofsolution is also taking place such as in the formation offilms and fiber of heat sensitive polymers
Rheological MeasurementsViscometers are used to measure rheological propertiesViscometer is defined as instruments used to measure viscosityThey differ on the basis of geometry and shear ratesThere are three main typesCapillary ViscometerRotational ViscometerMoving Body Viscometer
Choice of a ViscometerThere are number of criteria to be kept in mind;Nature of material to be tested Material’s viscosityMaterials elasticityThe dependence of viscosity on temperatureThe degree of accuracy required
Capillary ViscometerIt is the most oldest and popular wayfollows Hagen_poiseulle equation i.e. η= πr^4pt/8VLIf we are assuming laminar flow and pressure is constant then equation becomesν = η/ρThey are useful for measuring precise viscosities of dilute polymer solutionsThey can’t measure absolute viscositiesAlways measure viscosity relative to a reference liquid
Design of Capillary viscometerThree main design of capillary viscometerOstwald glass capillary viscometer,Cannon–Fenske viscometer,Ubbelohde viscometer Ostwald glass capillary viscometer,It is a u shaped tube with to bulb reservoirsThe time of flow of liquid between to etched marks is taken as function of viscosity
Cannon–Fenske viscometerIt is excellent for general useIt consist of long capillary tubeBoth reservoirs are present on the same vertical axis Ubbelohde viscometer.It is particularly useful for measurement at different concentrationIt is the modified form of Ostwald viscometer
Orifice viscometersIt is also known as cup viscometerIt is typically a cup with a hole in the bottomThe time required for the liquid to flow out is measured to determine viscosityUsesIt is used to measure control flow properties in the manufacturing, processing andapplications of inks, dyes, paints and lubricating oils
LimitationsIt should not be used for setting product specificationsIt is only designed for Newtonian fluidsIt should not be used for thixotropic materials
CONTENTS•What is rotational viscometer•Construction•Working principle•Determination of viscosity•Types and their brief introduction and working•Moving body viscometer•Types and their brief introduction
Rotational viscometerWhat is rotational viscometer?rotational viscometer is an instrument that is“used to find out the viscosity of a fluid by usingaction of rotation ”
construction Rotational viscometers consist of two basic partsseparated by the fluid being tested These parts maybe
DETERMINATION OF VISCOSITYViscosity can be calculated by this formula η=K(stress term/shear rate term)K =is constantStress term=may torque load and deflectionShear rate term=rpm (revolution per min)ASTM followed =D2196
Concentric cylinder viscometerIt consists of two cylinders, one within the other (cupand bob), keeping the specimen between them Inner cylinder(bob) Outer cylinder (cup)
Concentric cylinderThe relationship between viscosity, angularvelocity, and torque for a Newtonian fluid in aconcentric cylinder viscometer is given by theMargules equation M= torque Ω= relative angular velocity H= length of inner cylinder Ri= radius of inner cylinder Ro= radius of outer cylinder Error in calculations can be reduced by reducing the ratio of inner to outer radius that ratio should be equal to 1
Error correctionIn case of Newtonian fluids:Reduce Ro/RiIn case of non Newtonian fulids:The correction appears as an addition, hoThe data are plotted as M/Ω vs h and extrapolation is made toa value of ho at M/Ω = 0.The quantity (h + ho) is substituted for h in the variousequations.
Likewise we can find out shear rate and torque:
Cone-plate viscometerIn a cone–plate viscometer (Fig. 25), a low angle (≤3 )cone rotates against a flat plate with the fluid samplebetween them.With careful calibration and good temperaturecontrol it can be a very effective research andViscosity can be measured through this formula
Parallel plate viscometerIn parallel plate viscometers the gap width is usually larger andcan be varied freelyThe wide gap means that there is less sensitivity to temperaturechangeswith the plate–plate instrument, the velocity, and therefore theshear rate, varies with the distance from the center of the plate.This makes viscosity data more difficult to evaluate. Rp= radius of plate H= distance between two plates
CHARACTERISTICS•more efficient than capillary viscometer•They can be used with a wide range of materials becauseopacity, settling, and non-Newtonian behavior do not causedifficulties.•shear rates as a function of time can be measured.Therefore, they are useful Viscosities over a range of forcharacterizing shear thinning and time-dependent behavior .
Moving body viscometerIn moving body viscometers, the motion of aball, bubble, plate, needle, or rod through a material ismonitored.The Stokes’ equation relating viscosity to the fall of a solidbody through a liquid may be written as equation 34, where r is the radius of the sphere; ds and dl are the density of the sphere and the liquid, respectively; g is the gravitational force; and v is the. velocity of the sphere
Ball viscometerBall is fall in the fluidTravel through the fluidSpeed of ball in fluid determines the viscosity of fluidUsed for suspension and polymer meltsASTM D3121
Rod viscometerThe falling rod viscometer, sis based on themovement of a rod rather than a plate throughthe fluid.In the 1990s, the Laray falling rod viscometerbecame a standard test instrument in the inkindustry (ASTM D4040),and more recent versions of the falling rodviscometers are capable of precisemeasurements of polymer melts and solutions
Needle viscometer In the falling needle viscometer (ASTM D5478), themoving body is a glass or stainless steel needle thatfalls vertically through the fluid. The viscousproperties and density of the fluid are derived fromthe velocity of the needle.technique is useful for the characterization of polymermelts and concentrated solutions.
• A rheometer is an instrument for measuring the rheological properties:1. It can apply a deformation mode to the material andmeasure the subsequent force generated.2. It can apply a force mode to a material and measurethe subsequent deformation.• Rheometers used for determining the material functions of thermoplastic melts can be divided into two broad categories: 1. rotational type and 2. capillary type
ROTATIONAL VISCOMETERSFor thermoplastic melt studies, rotational viscometers witheither the cone-n plate or parallel-disk configuration are used.B: cone-and-plate viscometer. C: parallel disk viscometer.The basic limitation in rotational viscometers is that they arerestricted in their use only to low shear rates for unidirectionalshear and low-frequency oscillations during oscillatory shear.
1. CONE-N-PLATE VISCOMETERThe sample, is trapped between the circular conical disk at the bottomand the circular horizontal plate at the top. The cone is connected tothe drive motor which rotates the disk at various constantspeeds, whereas the plate is connected to the torque-measuring devicein order to evaluate the resistance of the sample to the motion.It can be used to measure shear rate, shear stress, normal stressdifference, oscillatory shear .
2. PARALLEL-DISK VISCOMETERThe parallel-disk viscometer used for measuring the shearstress and normal stress difference of moltenthermoplastics is similar in principle to the cone-n plateviscometer except that the lower cone is replaced by asmooth circular disk.This type of viscometer was initially developed formeasuring the rheological properties of rubber. It can beused for polymer melts of extremely high viscosity andelasticity.
CAPILLARY RHEOMETERSThey are used for determining the rheological properties of polymermelts.1. Constant Plunger Speed Circular Orifice Capillary Rheometer:It extrudes the polymer melt through a capillary with a circularorifice using a plunger at constant speeds.The major advantage of this type of capillary rheometer is thathigher-shear rate levels than those attainable in rotationalviscometers can be achieved.
2. Constant Plunger Speed Slit Orifice Capillary Rheometer:This rheometer has a slit orifice cross section rather than acircular one. It extrudes the polymer melt through a capillary with a slitorifice using a plunger at constant speeds
3. Constant Speed Screw-Extrusion-type CapillaryRheometers:These type of capillary rheometers are capable ofgenerating rheological data from medium-to-highshear rates. These rheometers have been used forrheological studies of polymer melts but have notbecome as popular as the plunger type capillaryrheometers because they need a much larger quantityof polymer feed.
4. Constant Pressure Circular Orifice CapillaryRheometer (Melt Flow Indexer):This rheometer is also similar to Constant Plunger SpeedCircular Orifice Capillary Rheometer except for twodifferences.First, the capillary used is of very short length, andsecond, the polymer melt is extruded by the use of deadweights (i.e., constant pressure) rather than constantplunger speed.
• Rheological measurements are often used as an effective tool for1. Quality control of raw materials, manufacturingprocess/final product2. Predicting material performance•Melt rheology is concerned with the description ofthe deformation of the material under the influence ofstresses. Deformation and flow naturally exist whenthe thermoplastics are melted and then reformed intosolid products of various shapes.
• All polymer melts are viscoelastic materials; that is, theirresponse to external load lies in varying extent betweenthat of a viscous liquid and an elastic solid.• A polymer melt represents a cluster ofentangled, flexible strings of varying lengths. It is theseentanglements that provide the resistance to deformationand, therefore, with increasing molecular weight, the meltviscosity goes up, processibility worsens although, ofcourse, mechanical properties improve.