Liquid Crystals by imran aziz

8/8/2019 Liquid Crystals by imran aziz

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ContentsContents

Properties of liquidProperties of liquid

crystalscrystals

Types of liquid crystalsTypes of liquid crystals

ThermotropicThermotropic

nematicnematic

smecticsmectic cholestericcholesteric

LyotropicLyotropic

structuresstructures

applicationapplication

NIOSOMESNIOSOMES

[email protected][email protected]


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Synergistic Effects of Surfactants

Observed when surfactants having relatively similarstructure or ionic property are mixed

Resulted in the formation of liquid crystal structures or

complexes at the interface by intermolecular

interactions between surfactants

Examples

Anionic and nonionic in synthetic latex emulsion

polymerisation,

Mixture of a dispersant and a hydrating agent to

increase dispersion stability in agricultural chemicals



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STATES OF MATTER

Common states:

solid

liquid

gas

Matter can exist in

other states



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LIQUID CRYSTALS

A state that occurs between a solid & a liquid

Possess properties characteristics of both liquids

& crystalline solids

Also possess properties not found in either

liquids or solids

May response to external perturbations & some

changes colour with temperature



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A crystal is a highly ordered structure which possesseslong-range positional & orientational order

For many substances these two types of order are

destroyed simultaneously when the crystal melts to form

a liquid

For some substances, these orders are destroyed in

stages. These are liquid crystals. E.g. Slide 17

Crystals vs Liquid Crystals



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Liquid crystal can flow like a liquid, due to loss of positional order Liquid crystal is optically birefringent, due to its orientational order

Transition from crystalline solids to liquid crystals caused by a change of

temperature gives rise to THERMOTROPIC liquid crystals

substances that are most likely to form a liquid crystal phase at a certaintemperature are molecules that are ELONGATED & have some degree

of RIGIDITY. Try slide 29

Properties of liquid crystals



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Typical chemical structures

cholesterol ester

phenyl benzoates

surfactants such aspolyethylene-oxides, alkali

soaps, ammonium salts,lecithin

paraffins

glycolipids

cellulose derivatives



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Typical applications

LCD displays

dyes (cholesterics)

advanced materials (Kevlar)

membranes temperature measurement (by changing colours)

solvents for GC, NMR, reactions, etc.

Drug delivery



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Types of liquid crystals

Thermotropic Phase transition depends on temperature

Nematic

Smectic

Cholesteric

Lyotropic

Phase transition depends on temperature &

concentration



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As temperature increases..

The first liquid crystal phase is the smecticA, where thereis layer-like arrangement as well as translational and

rotational motion of the molecules.

A further increase in temperature leads to the nematic

phase, where the molecules rapidly diffuse out of the initial

lattice structure and from the layer-like arrangement as well.

At the highest temperatures, the material becomes an

isotropicliquid where the motion of the molecules changes

yet again.



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Nematic

Simplest form is anematic liquid crystal

i.e. long-rangeorientational order

but no positionalorder

The preferred directionis known as director



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NematicNematic

Despite the high degree of orientational order, nematic phaseas a whole is in disorder i.e. NO MACROSCOPIC ORDER

(orientation within a group is similar but not from one group to

another)

Structure of nematic phase can be altered in a number of ways.E.g. electric or magnetic field or treatment of surfaces of the

sample container

Thus, possible to have microscopic order & macroscopic order

Nematic liquid crystals are widely used in electro-optic displaydevices



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Cholesteric

The first liquid crystal that was observed through a polarising microscope is

cholesteryl benzoate. Thus, CHOLESTERIC liquid crystal OR chiral nematic liquid

crystal

E.g. cholesteryl benzoate: LC @ 147C, isotropic @ 186C

Cholesteric liquid crystals have great potential uses as

- sensors

- Thermometer

- fashion fabrics that change colour with temperature

- display devices



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- In CHOLESTERIC phase, there is

orientational order & no positional order,

BUT, director is in HELICAL ORDER.

The structure of cholesteric depends onthe PITCH, the distance over which the

director makes one complete turn One pitch - several hundred nanometers

Pitch is affected by:-

Temperature

Pressure

Electric & magnetic fields



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Smectic SMECTIC phase occurs at temperature below

nematic or cholesteric

Molecules align themselves approx. parallel & tend

to arrange in layers

Not all positional order is destroyed when a

crystal melts to form a smectic liquid crystal

Chiral smectic C liquid crystals are useful in LCDS



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LIQUID CRYSTAL POLYMERS

Can form nematic, cholesteric, smectic

When liquid crystal polymers solidify, the liquid

crystal structure freeze in

This results in materials of high tensile strength

& in some cases unusual electro-optical

behaviour

E.g. Kevlar aramid fibre bullet-proof vest &airplane bodies (aromatic polyamide)



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Examples of phase changes

Cholesteryl myristate

solid smectic A71C

cholesteric isotropic79C 85C

4, 4-di-heptyloxyazoxybenxene

solid

74C 94C 124C

smectic C nematic isotropic



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Thermotropic vs Lyotropic

THERMOTROPIC Absence of solvent

Rigid organic molecules

Depends on Temperature

Structures: Smectic

Nematic

Cholesteric

LYOTROPIC In solvent

Surfactants Depends on Temperature,

Concentration, salt, alcohol

Structures: Lamellar

Hexagonal etc



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Structure formation in surfactant solution

micelle rodhexagonal

monolayer

bilayerReverse micelleFormation of MICROEMULSION

REVERSE

HEXAGONAL

Oil/alcohol



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Effect of temperature and concentration on the

structure of lyotropic liquid crystals



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SURFACTANT VESICLES

[A] Phospholipids (e.g. lecithin) + H2O ---->phospholipid vesicles or liposomes

[B] Liposomes + (long chain) stearylamine ------->

tve charge liposome (carriers for DNA)

[C] Liposomes + dicetyl phosphate -----

- ve charge liposome



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Vesicles

Bilayers that fold into a 3D structure

Vesicles form because they get rid of

the edges of bilayers, protecting the

hydrophobic chains from the water, but

they still allow for relatively small

layers.

Lipids found in biological membranes

spontaneously form vesicles insolution.



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Application of Liposomes

can encapsulate: drugs,

proteins,

enzymes

administered intravenously, orally or intramuscularly

decrease toxicity

increase specificity of drug uptake

enable slow release



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Problems with phospholipids

phospholipids undergo oxidative

degradation

handling & storage must be under nitrogenexpensive


f f


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Formation of liquid crystals using surfactants

[A] Anionic

e.g. alkane sulfonates

[B] Cationic

e.g. hexadecyl trimethyl ammonium bromide

[C] Amphoteric

e.g. alkyl betaines

Due to toxicity of ionic surfactants, the vesicles are not used for drug delivery

[D] Non-ionic

e.g. alcohol ethoxylates R-O-(CH2CH2O) m H m: 2-20, R :

mixed; alkyl group C8C18



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Non-ionic + cholesterol -> NIOSOMES

These vesicles prolong the circulation of entrapped

drug

Properties depend on

1. Composition of bilayer

2. Method of production

e.g. cholesterol & single alkyl-chain non-ionicsurfactant with a glyceril head group

Niosomes



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The three states of mater?The three states of mater?

solid liquid gas

Which state is a soap bubble in?



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The fourth state of materThe fourth state of mater

e.g.: ice -> water

e.g.: cholesteryl benzoate

heat

0oC

heat heat

145.5oC 178.5oC

Fredrich Reinitzer, Otto Lehmann, 1888

The new liquid state showed birefringence!

Orientational order but no positional order!


Is liquid crystal more like solid ors qu crys a more e so or


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Is liquid crystal more like solid ors qu crys a more e so or liquid?liquid?

Latent heat: the energy required to cause phase transition.

H2O

80 cal/g of ice -> water,

540 cal/g water -> steam.

Cholesteryl myristate:

65 cal/g of solid -> LC, 7 cal/g LC- liquid.



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The structure of LCThe structure of LC

solid liquidliquid crystal

Director

If random, = 57o



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Types of liquid crystalsTypes of liquid crystals

Discotic

Chiral nematic



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LC in an electric fieldLC in an electric field

Orientation of a dipole inan electric field

LC film in an electric field



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Light & polarizationLight & polarization



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Interaction of light with LCInteraction of light with LC

Ex Ey

Ex Ey Ex EyReflected

beam

Incoming

beam

Refracted beam

ExEy

Ex Ey Ex EyReflected

beam

Incoming

beam

Refracted beam

Birefringence


LCs under a polarizationLCs under a polarization


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LCs under a polarizationLCs under a polarization

microcopemicrocope



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Aligning liquid crystalsAligning liquid crystals

rubbing

polishing



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Liquid crystal displayLiquid crystal display



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The operation of a pixel in LCDThe operation of a pixel in LCD


O h l d f LC


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Other cool gadgets from LCsOther cool gadgets from LCsThermochromic Ink

Polymeric LC

Interactive art using

iridescent coloringOptoelectronic computers



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Oth li id t l i tOth li id t l i t


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Other liquid crystals in natureOther liquid crystals in nature

Iridescent color


I t d tiI t d ti


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IntroductionIntroduction

Liquid crystals are very important in the study of optics,Liquid crystals are very important in the study of optics,chemistry and polymer science. The discovery of the liquidchemistry and polymer science. The discovery of the liquid

crystal happened over a century and since this time thecrystal happened over a century and since this time the

phenomena has been applied to many products in ourphenomena has been applied to many products in our

society. This presentation will cover the properties, differentsociety. This presentation will cover the properties, different

phases and uses of liquid crystals. It will also give illustrationsphases and uses of liquid crystals. It will also give illustrationsof liquid crystals from the micro perspective.of liquid crystals from the micro perspective.


What are liquid crystals ?


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What are liquid crystals ?Liquid crystals form from organic compounds and is thought of

as the phase of matter between the solid and liquid state of acrystal.This phenomena was discovered in 1888 by Austrian

chemist Frederich Reinitzer.


PhPh


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PhasesPhases

http://en.wikipedia.org/wiki/Image:Shilirren_texture.jpg


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Nematic PhaseNematic Phase

Molecules in this phaseMolecules in this phase

are long and rod-like inare long and rod-like in

shape. They are free toshape. They are free to

move in space.move in space.


Chi l N tiChiral Nematic
http://en.wikipedia.org/wiki/Image:Shilirren_texture.jpg


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Chiral NematicChiral Nematic

This phase is composedThis phase is composed

of nematic molecules inof nematic molecules in

a helical structure abouta helical structure about

the layer normal.the layer normal.

The distance that it takesThe distance that it takes

for the molecules tofor the molecules to

complete one turn iscomplete one turn is

called the pitch.called the pitch.

= n*p*[email protected][email protected]

Smectic PhaseSmectic Phase


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Smectic PhaseSmectic Phase

This phase can beThis phase can bereached at lowerreached at lowertemperatures than thetemperatures than thenematic phase.nematic phase.

Molecules alignMolecules alignthemselves in layers.themselves in layers.(They are restricted to(They are restricted totheir plane.)their plane.)

More order and higherMore order and higher

viscosityviscosity


Columnar PhaseColumnar Phase


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Columnar PhaseColumnar Phase

Characterized by its stacked columns of molecules.Characterized by its stacked columns of molecules. Discovered in 1977 by ChandrasekharDiscovered in 1977 by Chandrasekhar



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Order ParameterOrder Parameter

S = S =

The order parameter in a liquid crystalThe order parameter in a liquid crystal

range from 0.3 to 0.9range from 0.3 to 0.9


Electric field EffectsElectric field Effects


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Electric field EffectsElectric field Effects

If an electric field isIf an electric field isapplied to a liquidapplied to a liquid

crystal the moleculescrystal the molecules

will align in the samewill align in the same

direction as the field.direction as the field.


Twisted NematicTwisted Nematic


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Twisted NematicTwisted Nematic

This is called theThis is called thetwisted nematic liquidtwisted nematic liquid

crystal and the spacingcrystal and the spacing

between the planesbetween the planes

change withchange withtemperature. Thetemperature. The

spacing is associatedspacing is associated

with the wavelengths ofwith the wavelengths of

light.light.



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Uses of Liquid CrystalsUses of Liquid Crystals


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Uses of Liquid CrystalsUses of Liquid Crystals

Flat screen televisionFlat screen televisionWristwatchesWristwatches

Laptop screensLaptop screens

Digital clocksDigital clocksThermometersThermometers

Switchable windowsSwitchable windows


ConclusionConclusion
http://shopper.howstuffworks.com/products/Hewlett+Packard+f2105+%28Black%2C+Silver%29+21+inch+LCD+Monitor/SF-1/PID-25727430


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We know today that many chemical compounds can existWe know today that many chemical compounds can existin the liquid crystal state, such as cholesteryl benzoate.in the liquid crystal state, such as cholesteryl benzoate.

Thanks to the scientists that worked so diligently towardThanks to the scientists that worked so diligently towardunderstanding this phenomenon, the world can focus onunderstanding this phenomenon, the world can focus onways to make this product useful in society. Over the lastways to make this product useful in society. Over the lastcentury many applications such as the detection of hotcentury many applications such as the detection of hotpoints in microcircuits, the findings of fractures or tumorspoints in microcircuits, the findings of fractures or tumorsin humans and the conversion of infared images havein humans and the conversion of infared images have

become accessible due to the understanding of pitch in abecome accessible due to the understanding of pitch in aliquid crystal.liquid crystal.


Liquid Crystal Materials


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y

CN


Broad Classification


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Lyotropics Thermotropicsamphiphilic molecules, polar and non-polar

parts form liquid crystal phases over certain

concentration ranges when mixed with a

solvent

molecules consisting of a rigid core and

flexible tail(s) form liquid crystal phases

over certain temperature ranges.

+

-hydrophilic

polar head

hydrophobic

non-polar tail flexible tail

rigid [email protected][email protected]

The LyotropicPhases


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micelle

reversemicelle

cross section

cross [email protected][email protected]

The ThermotropicLiquid Crystal Molecule


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CN

Chemists

View

Physicists

EngineersView

Shape Anisotropy

Length > Width

The molecule above (5CB) is ~2 nm 0.5 [email protected][email protected]

Geometrical Structures of

M i M l l


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Mesogenic Molecules

Low Molecular Weight High Molecular Weight

(polymers)

( ) n

( )n

disk-like

rod-like

most practical [email protected][email protected]

The Liquid Crystal Phase


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n

TemperatureCrystal Nematic LC Isotropic


The Nematic Director n


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Long

Molecular

Axis

H H

H H

H H

H H

C NO

C CH

H

HH

C C

HH

H

H

H

n

The local average axis

of the long molecular axis

director


Other Liquid Crystal Phases


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n

Temperature Smectic C Smectic A Nematic

nz

n


Chirality


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left-handed right-handed

mirror images

non-superimposable

H-C-C-C-C-C C N

H H H H H

H H H H H

H-C-C-C-C-C C N

H H HCH3

H

H H H

H

H

non-chiral

chiral (RH)

The methyl group on the 2nd carbon atom

on the alkyl chain of the molecules extends

out of the plane of the paper and the hydro-

gen atom extends into the plane of the paper.Therefore the 2nd carbon can be thought of

as a right or left handed coordinate system


O di N ti

The Chiral Nematic


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CN

pitc

P

CN

Ordinary Nematic Chiral Nematic

director

n



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The Chiral Smectic: TGB

T i t d G i B d (TGB)


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Twisted Grain Boundary (TGB)

A twisted grain boundary smectic A phase (frustrated) - TGBA*



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Discotics Liquid Crystals

n


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Columnar, columns of molecules

in hexagonal lattice

Nematic discotic phase

n

n


Polymer Liquid Crystals

Combining the properties of liquid crystals and polymers


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Combining the properties of liquid crystals and polymers

Main Chain Side Chain

mesogenic moieties are

connected head-to-tail

mesogenic moieties

attached as side chains

on the polymer backbone

rigid

semi-flexible


Polymer Liquid Crystals

forming nematic liquid crystal phases


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o g e a c qu d c ys a p ases

n

main-chainside-chain


Example of Side-Chain Polymer LCs


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O C-O-(CH2)n-O R2C-O

O-(-CH2-C-)X-

R1

Too slow for display applications (switching times ~ 0.5-1 s Useful for other applications such as: Optical filters Optical memory Alignment layers for low molecular weight LCs Non-linear optic devices (optical computing)


n

The Order Parameter

1


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n

n

2

2

1(cos ) (3 cos 1)

2

= = S P

2

2

2

cos1

cos3

cos ( 0 ) 1

= =

= =

o

d

dno order

perfect order

2

2

(cos ) 1

(cos ) 0

= =

= =

S P

S P

perfect crystal

isotropic [email protected][email protected]

I t ti b t i di id l l l t d b

Maier-Saupe Theory - Mean Field Approach


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Interactions between individual molecules are represented by a

potential of average force

( ) ( )2 2cos cosV vP P = < >

From Statistical Mechanics (Self Consistency)

( ) ( ) ( )( )

( )( ) ( )

1

2 2 2

02 1

2 2

0

cos exp cos ) cos

exp cos cos

< >

< >=< >

P vP P d

P

vP P d

{V: minimum} when phase is ordered (-P2(cos ))

{V: V=0} when phase is disordered ()

factor for intermolecular strength ( )

=(kT)-1

n



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Landau-de Gennes TheoryLandau-de Gennes Theory

2 3 4 21 1 1 1


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2 3 4 2

0

1 1 1 1( ) ( )

2 3 4 2

= + + + f f aS bS cS L S GS z

a= (T-T*), , b, c, T*, L are phenomenological constants

G is a surface interaction strength

OrderPara

meter,S

Temperature

Good near NI transition

surfac

e

Predicts order near


Optical Anisotropy: Birefringence

ordinary ray (no, ordinary index of refraction)


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y y ( o, y )

extraordinary ray (ne, extraordinary index

of refraction)


Optical Anisotropy: Birefringenceordinary wave

on n=


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extraordinary wave

o

2 2

2 2 2

1 cos sin

o en n n

= +

For propagation along the optic

axis, both modes are no

optic

axis


Birefringence Example: 1/4 Wave Plate

circular polarizedWhat is minimum d for


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Unpolarized

linear polarized

polarizer

LC: n=0.05d

liquid crystal 1/4 wave plate ?

( )

1

4

1

41 589

2,950 2.954 4 0.05

e o

e o

N N

n d n d

nmd nm m

n

= +

= += = = =

Takes greater number of e-waves

than o-waves to span d, use

n=0.05


Creating Deformations with aField and Surface - Bend Deformation


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E orB


Creating Deformations with aField and Surface - Splay Deformation


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E orB



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Elastic Constant K22 :Temperature Dependence

2


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7

6

5

4

3

2

-30 -20 -10 0T-TNI (C)

K22

(x10-12N

ewton)

P-azoxyphenetole

P-azoxyanisole(PAA)

2( )K S T


y

n

Effects of an Electric Field


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x

yE

n

sin cos

oE

= +=

n x

y

y

E

( )

( )

( ) ( ) ( )

2 2 2

2

22 12 2 2 6 2

1 1cos

2 2

1sin 2

2

1 18.85 10 / 5 0.5 10 / 5.5 /

2 2

e o o o

e

e o o

o o

f E

dfE

d

E C N m V m N m

= =

= =

= =

E n Electric Free Energy Density

Electric Torque Density

Using = 5 and E=0.5 V/ m


Freedericksz Transition -The Threshold I


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Ec

z

y

E

xAt some critical E

field, the director

rotates, before Ecnothing happens

n

y

x

nE

( ) ( )( )

( ) ( ) ( ){ }2 2 2

11 22 33

cos ,sin ,0

1

2dVOL

z z

F K K K dV

=

= + +

n

n n n n n

0 02

22

dK

dz

d


Freedericksz Transition -The Threshold II


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( )

2 221 1

sin2 2e o o

VOL VOLF dV E dV = = E n

2

2 2

220

1sin

2

0

d

d e o

dF F F K E dz

dz

F d F

ddzdz

= + = =

E-field

free energy

total

free energy

Minimize free energy with

Euler Equation


Freedericksz Transition -The Threshold III


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( )

( )

2

222 2

11

22

6 12 2 2

sin cos 0

10

5 10 8.85 10 / (10)

200,000 0.2

0.2 5 1

o

TH

o

TH TH

dK E

dz

K NE

d m C Nm

V V

m m

VV E d m volt

m

+ =

= =

=

= = =

1.0 E/Ec

mid-lay

ertilt(deg)

differential equation

soln.

small

threshold


Defects


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s=+1 s=+1 s=+1

s=1/2s=-1/2

s=-1

s=3/2 s=+2

The singular line

(disclination) is pointing

out of the page, and directororientation changes by

2 s on going around the

line (s is the strength)[email protected][email protected]


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Liquid Crystals by imran aziz