i. introduction to rheology (1)

7/23/2019 I. Introduction to Rheology (1)

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Introduction to Rheology

Part 1

Introduction to the Rheology of Complex

Fluids

1Dr. Aldo Acevedo - RC !"P!


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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!


3/27

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!


4/27

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 /


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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!


6/27

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!


7/27

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!


8/27

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!


9/27

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!


10/27

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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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.



12/27


Rheologyof *i'uids

Physics Chemistry

8echanics

ofContinuum

0echnology

ngineering

1/Dr. Aldo Acevedo - RC !"P!


13/27

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



14/27

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!


15/27

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


16/27

!ommon Non-Newtonian eha"ior

shear thinning

shear thic,ening

yield stress

viscoelastic effects 2eissen$erg effect

Fluid memory

Die !#ell



17/27

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!


18/27

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!


19/27

Rheological Eperiments from .Li#uid ody /rmor0 +

Silica suspensions in 'E1 (From N$2$ agner - 3ni" &elaware)

1JDr. Aldo Acevedo - RC !"P!


20/27

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 /


21/27

'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


22/27

%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#


23/27

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


24/27

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!


25/27


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!


26/27


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!


27/27

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!

i. introduction to rheology (1)

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