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Rheology At the completion of this section the student will be able to: Rheology At the completion of this section the student will be able to:

Rheology At the completion of this section the student will be able to: - PowerPoint Presentation

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Rheology At the completion of this section the student will be able to: - PPT Presentation

describe Newtonian behaviour illustrate and explain 3 different kinds of nonNewtonian flow illustrate and explain time dependent nonNewtonian flow describe different ways to measure viscosity and different viscometers ID: 698286

stress shear viscosity rate shear stress rate viscosity newtonian time shearing flow liquids materials behaviour strain material force dilatant

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Slide1

Rheology

At the completion of this section the student will be able to:

describe Newtonian

behaviour

;

illustrate and explain

3

different kinds of non-Newtonian flow;

illustrate and explain time dependent non-Newtonian flow;

describe different ways to measure viscosity and different viscometersSlide2

Questions and FeedbackSlide3

t

shear stress [F’/A]

Nm

-2

g strain [dx/dr] no unitsHooke’s Law (F=kx):t = G gwhere G is storage modulus (Pa)

F’

A

dx

dr

Hookean

materials

For an elastic or

Hookean

material, stress is proportional to strain ,

if you double the tension you double the extension

.

NB. (shear) strain is defined as the flow deformation per unit lengthSlide4

e.g. a rubber band : instantaneous deformation, deformed state lasts as long as the stress is applied, work done is recoverable.

Before

F

During

AfterSlide5

Newtonian Liquids

Newtonian liquids are inelastic liquids in which stress is proportional to the rate

of strain. If you double the force you double the velocity gradient

t

shear stress [F’/A]

Nm-2

g

rate of strain [dv/dr] sec-1Newtonian fluid:

t = h gwhere

h is viscosity Nm-2sec

F’

A

dv

dr

NB.

dv = dx/dt

Velocity gradient= rate of strain is also called the

shear rate

.

.

.Slide6

Work done is dissipated as heat and cannot be recovered.

F

Before

During

AfterSlide7

ViscoelasticityViscoelastic materials: a viscoelastic material exhibits viscous as well as elastic behaviour

Note: in the rheological sense water is a "viscous" fluid. Normally, however, the term "viscous" is used for fluids with high viscositySlide8

Only partial elastic recovery is observed when the force is removed.

A portion of the work done is recoverable and the remainder is lost as heat.

F

Before

During

AfterSlide9

Stress = force

Strain rate and rate of shear = speed

therefore, you are really plotting force v speed or vice-versa

Viscosity = 1/gradient.

Force (shear stress)

Speed (shear rate)Slide10

An increase in temperature gives a decrease in viscosity according to the equation:

h = Ae

(E

v

/RT)

0

0.0004

0.0008

0.0012

0.0016

0.002

0

10

20

30

40

Temperature [

°

C]

Viscosity [kg/(m*s)]

Viscosity of water as a function of temp.

“Nothing changes viscosity like temperature”Slide11

Newtonian Flow

Newtonian liquids have constant viscosity. There is no change in viscosity with either changing shear stress or shear rate.Pure liquids are examples of Newtonian liquids eg

water, castor oil, etc.

Shearing stress

Rate of shear

NewtonianSlide12

Non–Newtonian Flow

Shearing stress

Rate of shear

dilatant

plastic

pseudoplastic

Newtonian

Materials in which there is not a simple relation between shearing stress and rate of shear are termed

Non–Newtonian

.

Examples of non–

newtonian

samples are liquid and solid

heterogenous

dispersions such as colloidal solutions, emulsions, liquid suspensions, and ointments.Slide13

Plastic FlowPlastic materials do not flow until a

yield stress is exceeded, e.g. thick tomato sauce, toothpaste*.The physical behaviour of fluids with a yield stress is usually explained in terms of an internal structure in three dimensions which is capable of preventing deformation for values of stress less than the yield value.

Shearing stress

Rate of shear

Plastic

Newtonian

Plug flow: Movement of a material as a unit without shearing within the mass

yield stress

plastic viscositySlide14

an example of plastic flow optimisation

Same formulation, but have changed the solution conditions so that yield stress changes. Note that plastic viscosity does not change.

Shear stress,

t,

Pa

Shear rate, , s

-1

g

.Slide15

Pseudoplastic FlowPseudoplastic materials always flow like a liquid but viscosity decreases as shear rate increases, e.g. mayonnaise.

Decrease in viscosity with shear rate may be due to: orientation and disentaglement increasing with shear rate; or solvating layers being sheared away resulting in decreased particle size.

Shearing stress

Rate of shear

Pseudoplastic

NewtonianSlide16

Dilatant FlowDilatant fluids are characterised by increasing viscosity with increasing shear rate.

Dilatant behaviour is not nearly as common as pseudoplastic behaviour.

Dilatant behaviour is sometimes observed in suspensions at high solids content.

Shearing stress

Rate of shear

Dilatant

NewtonianSlide17

Time Dependent Non–Newtonian Flow

If the viscosity decreases with time of shear the materials is thixotropic, if it increases with time of shear it is called rheopectic/dilatant-thixotropic/anti-thixotropic.NB: changes may not be linear as in schematics

thixotropic

Viscosity

Time

rheopectic

Viscosity

TimeSlide18

Thixotropy

Arises from structural breakdown and reaggregation in complex materials in which a loose network connects the sample. At rest or very low shear rates, the 3-D structure provides the system with some rigidity and the material behaves as a gel. Increased stress disrupts the structure and the particles start to align. The material commences flowing and its consistency decreases as shear rate and stress increase.

When the stress is decreased or removed, the internal structure starts to reform but with a time lag, as the particles which build the network need time in which to contact each other.Slide19

Determination of Rheological Properties

If measuring Newtonian fluid, i.e., one that the viscosity does not change with rate of shear, then can use instrument that operates and only one rate of shear (or shear stress), eg, capillary viscometer.

But if measuring non–Newtonian fluid then need to use instrument with range of shear rates (or shear stresses), eg

, cup and bob, and cone and plate.Slide20

Single Point MeasurementsA measurement made at this shear stress could show many materials to have the same viscosity even though they possess very different properties and behaviour

Use for Newtonians only!

Shearing stress

ViscositySlide21

Capillary Viscometer – only force is gravity

The viscosity can be determined by measuring the time required for the fluid to flow between the two marks, A and B, as it flows by gravity through the vertical capillary tube.This time is compared to that for liquid of known viscosity:

where

h

is viscosity,

r is density and t is the measured time for each of the liquids, 1 and 2.

2

2

1

1

2

1

t

t

r

r

h

h

=

A

BSlide22

Cup and Bob Viscometer

Stormer instrument: known weights cause bob to turn with known torque (shearing stress) and speed of rotation (rate of shear) is measured. Plot rpm versus mass added. Need to be calibrated with liquids of known viscosity for quantitative use.

wSlide23

Lets see if you can…

determine the rheological behaviour of a material from a graph of viscosity, shear rate, or shear stress (y-axis) versus shear rate, shear stress, or time (x-axis).

Shearing stress

Rate of shear

Time

Viscosity

Rate of shear

Shearing stress