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7/23/2019 I. Introduction to Rheology (1)
1/27
Introduction to Rheology
Part 1
Introduction to the Rheology of Complex
Fluids
1Dr. Aldo Acevedo - RC !"P!
7/23/2019 I. Introduction to Rheology (1)
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Rheology
!tudy of deformation and flo# of matter
A fluidis a su$stance that deforms continuously under
the action of a shearing force. Intuitively% a fluid flo#s&
In'uiry into the flo# $ehavior of complex fluids
Complex fluids do not follo#s (e#ton)s *a# or +oo,e)s
*a# of elasticity
/Dr. Aldo Acevedo - RC !"P!
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Reflected upon the resistance of li'uids to a cylinder
rotating in a vessel.
Newton (-Stokes) Law Deformation rate is expected to $e proportional to stress
and the constant coefficient of proportionality is called
viscosity.
0he study of simpler fluids have their o#n #ell-definedfield% called fluid mechanics.
Purely viscous fluid.
=
Newton and Simple Fluids
Dr. Aldo Acevedo - RC !"P!
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2hat is Rheology Any#ay3
An ans#er for your $affled family and friends. 4
5Rheology is the study of the flo# of materials that $ehave in an
interesting or unusual manner. "il and #ater flo# in familiar% normal
#ays% #hereas mayonnaise% peanut $utter% chocolate% $read dough%
and silly putty flo# in complex and unusual #ays. In rheology% #e
study the flo#s of unusual materials.6
57 all normal or (e#tonian fluids air% #ater% oil% honey follo# the
same scientific la#s. "n the other hand% there are also fluids that do
not follo# the (e#tonian flo# la#s. 0hese non-(e#tonian fluids% for
example mayo% paint% molten plastics% foams% clays% and many otherfluids% $ehave in a #ide variety of #ays. 0he science of studying
these types of unusual materials is called rheology6
4Faith 8orrison% 50he (e#s and Information Pu$lication of 0he !ociety of Rheology6% 9ol :1 ;an /
7/23/2019 I. Introduction to Rheology (1)
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Eamples of !omple Fluids
Foods mulsions mayonaisse% ice cream
Foams ice cream% #hipped cream !uspensions mustard% chocolate ?els cheese
@iofluids !uspension $lood ?el mucin
!olutions spittle Personal Care Products !uspensions nail polish% face scru$s !olutions?els shampoos% conditioners Foams shaving cream
lectronic and "ptical 8aterials
*i'uid Crystals 8onitor displays 8elts soldering paste
Pharmaceuticals ?els creams% particle precursors mulsions creams Aerosols nasal sprays
Polymers BDr. Aldo Acevedo - RC !"P!
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Rheology)s ?oals
1. sta$lishing the relationship $et#een appliedforces and geometrical effects induced $y
these forces at a point in a fluid.
0he mathematical form of this relationship is called
the rheological e'uation of state% or the
constituti"e e#uation$
0he constitutive e'uations are used to solvemacroscopic pro$lems related to continuum
mechanics of these materials.
Any e'uation is ust a model of physical reality.
Dr. Aldo Acevedo - RC !"P!
7/23/2019 I. Introduction to Rheology (1)
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Rheology)s ?oals
1. sta$lishing the relationship $et#eenrheological properties of material and itsmolecular structure composition.
Related toE stimating 'uality of materials nderstanding la#s of molecular movements Intermolecular interactions
Interested in #hat happens inside a point duringdeformation of the medium.
2hat happens inside a point3
:Dr. Aldo Acevedo - RC !"P!
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8aterial !tructure
8ore or less #ell-organiGed and regularly spaced shapes
Arrangements% organiGation or intermolecular interactions
!tructured 8aterials H properties change due to the influence of
applied of applied forces on the structure of matter
Rheology sometimes is referred to as mechanical
spectroscopy.
5!tructure 8echanisms6 are usually proposed% analogous to
reaction mechanisms in reaction ,inetics
!tructural pro$es are used to support rheological studies and
proposed mechanisms.
Does (e#tonian fluids suffer structural changes3
>Dr. Aldo Acevedo - RC !"P!
7/23/2019 I. Introduction to Rheology (1)
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Rheological analysis is $ased on the use of continuum
theories
meaning thatE 0here is no discontinuity in transition from one geometrical
point to another% and the mathematical analysis of
infinitesimal 'uantities can $e used discontinuities appear
only at $oundaries
Properties of materials may change in space due to
gradients $ut such changes occur gradually
changes are reflected in space dependencies of material
properties entering e'uations of continuum theories
Continuity theories may include an idea of anisotropy of
properties of material along different directions.
JDr. Aldo Acevedo - RC !"P!
7/23/2019 I. Introduction to Rheology (1)
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Rheology as an Interdisciplinary !cience
Rheologyof *i'uids
Physics Chemistry
xplanation and prediction
of rheological properties
Kmolecular physicsKstatistical physicsKthermodynamics% etc7
Direct correlation $et#een
chemical parameters and
rheological properties
Kmolecular massK82DKchemical structuresKintermolecular interactions
%aterial &esign
1
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Rheology as an Interdisciplinary !cience
Rheologyof *i'uids
8echanics
ofContinuum
0echnology
ngineering
Analysis of flo# pro$lems.
(e# applications
Rheological studies give $ac,ground for
formulation of $oundary pro$lems in dynamics ofli'uids governing e'uations and their solutions
to find numerical values of macro properties.
11Dr. Aldo Acevedo - RC !"P!
7/23/2019 I. Introduction to Rheology (1)
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Rheology as an Interdisciplinary !cience
Rheologyof *i'uids
Physics Chemistry
8echanics
ofContinuum
0echnology
ngineering
1/Dr. Aldo Acevedo - RC !"P!
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Rheological 'roperties
!tress !hear stress (ormal stress (ormal !tress differences
9iscosity
!teady-state i.e. shear xtensional Complex
9iscoelastic 8odulus ?) H storage modulus
?6 H loss modulus Creep% Compliance% Decay Relaxation times and many more 7
most commonly sought
rheological 'uantity
1Dr. Aldo Acevedo - RC !"P!
7/23/2019 I. Introduction to Rheology (1)
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orlds Longest Running La*oratory Eperiment +
,he 'itch &rop Eperiment
Pitch H derivative of tar
Lroom temperature feels solid and can $e shattered #ith a $lo#
of a hammer
0his experiment sho#s that in fact at room temperature pitch is a
fluid&
1=Dr. Aldo Acevedo - RC !"P!
7/23/2019 I. Introduction to Rheology (1)
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orlds Longest Running La*oratory Eperiment +
,he 'itch &rop Eperiment
1J/: H Prof Parnell in niv. of Mueensland
Australia heated a sample of pitch and
poured it into a glass funnel #ith a sealed
stem. 0hree years #here allo#ed for it tosettle% after #hich the stem #as cut.
xamine the viscosity of the pitch $y the
speed at #hich it flo#s from a funnel into a
ar.
"nly eigth drops has fallen in >< years.
0he viscosity is approximated as 1
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!ommon Non-Newtonian eha"ior
shear thinning
shear thic,ening
yield stress
viscoelastic effects 2eissen$erg effect
Fluid memory
Die !#ell
1Dr. Aldo Acevedo - RC !"P!
7/23/2019 I. Introduction to Rheology (1)
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Shear ,hinning and Shear ,hickening
shear thinning H tendency of some materials to decrease in"iscosity#hen driven to flo# at high shear rates% such as $y
higher pressure drops
Increasing shear rate
1:Dr. Aldo Acevedo - RC !"P!
7/23/2019 I. Introduction to Rheology (1)
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Shear ,hickening
shear thic,ening H tendency of some materials toincrease in "iscosity#hen driven to flo# at high
shear rates
1>Dr. Aldo Acevedo - RC !"P!
7/23/2019 I. Introduction to Rheology (1)
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Rheological Eperiments from .Li#uid ody /rmor0 +
Silica suspensions in 'E1 (From N$2$ agner - 3ni" &elaware)
1JDr. Aldo Acevedo - RC !"P!
7/23/2019 I. Introduction to Rheology (1)
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4uicksand + / Non-Newtonian Fluid
Muic,sand is a colloid hydrogel sand% clay and salt #ater.
2hen undistur$ed $ehaves as a solid gel% $ut minor changes in thestress #ill cause a sudden decrease in its viscosity
After the initial pertur$ation% #ater and sand separate and dense
regions of sand sediment +igh volume fraction regions -N viscosity increases
!ufficient pressure must $e applied to reintroduced #ater into thecompacted sand.
0he forces re'uired to remove a foot from 'uic,sand at a speed of 1cms are a$out the same as 5that needed to lift a medium-siGed car.644
44 Ohaldoun% A.% . iser% ?.+. 2egdam and D. @onn% 5RheologyE *i'uefaction of Muic,sand nder !tress6%
(ature =: pp B /
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'henomenological %odeling of Shear ,hinning and
,hickening ?eneraliGed (e#tonian 'uationE
Po#er *a# 8odelE
m n 1 (e#tonian
m n N 1 !hear 0hic,ening% Dilatant
mn Q 1 !hear 0hinning
!lope of log vs log is constant AdvantagesE simple% success at predicting M vs P
DisadvantagesE does not descri$e (e#tonian Plateau at small
shear rates
)(=
1= nm
/1Dr. Aldo Acevedo - RC !"P!
% d li f Sh ,hi i d ,hi k i
7/23/2019 I. Introduction to Rheology (1)
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%odeling of Shear ,hinning and ,hickening
Carreau-asuda 8odel
a H affects the shape of the transition region
H time constant determines #here it changes from constant to po#er
la#
n H descri$es the slope of the po#er la#
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5ield Stress
0endency of a material to flo# only #hen stresses are
a$ove a treshold stress
@ingham 8odelE
y yield stress% al#ays positive
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Elastic and 6iscoelastic Effects
2eissen$erg ffect Rod Clim$ing ffect does not flo# out#ard #hen stirred at high speeds
/=Dr. Aldo Acevedo - RC !"P!
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Elastic and 6iscoelastic Effects
Fluid 8emory Conserve their shape over time periods or seconds or
minutes
lastic li,e ru$$er
Can $ounce or partially retract
xampleE clay plasticina
/BDr. Aldo Acevedo - RC !"P!
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Elastic and 6iscoelastic Effects
9iscoelastic fluids su$ected to a stress deform #hen the stress is removed% it does not instantly vanish
internal structure of material can sustain stress for some
time
this time is ,no#n as the relaxation time% varies #ith
materials due to the internal stress% the fluid #ill deform on its o#n%
even #hen external stresses are removed
important for processing of polymer melts% casting% etc..
/Dr. Aldo Acevedo - RC !"P!
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Elastic and 6iscoelastic Effects + &ie Swell
as a polymer exits a die% the diameter of li'uid stream
increases $y up to an order of magnitude
caused $y relaxation of extended polymer coils% as stress is
reduced from high flo# producing stresses present #ithin the
die to lo# stresses% associated #ith the extruded stream
moving through am$ient air
/:Dr. Aldo Acevedo - RC !"P!