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
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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