Liquid CrystalsLiquid Crystals

Gavin LawesGavin LawesWayne State University

DCMSTMay 23rd, 2008

Liquid Crystal Displays

LGE.com

Amazon.com

LCI at Kent Statecasio.com

Apple.com

DCMSTMay 23rd, 2008

OutlineLiquid Crystals

What are liquid crystals?

Light and polarization filtersDemonstration

Types of liquid crystalsNematic, smetic, chiral, and more

Controlling the liquid crystal transitionDemonstration

Twisted nematic displays

DCMSTMay 23rd, 2008

Why does “liquid crystal” sound like an oxymoron?

Short length scalesHighly regular array of atoms

(nm)y

with fixed positions

Crystals

Long length scales( +) Self-supporting,(mm+) Self supporting,

often hard

DCMSTMay 23rd, 2008

Short length scalesRandom, disordered array

(nm) of atoms that move around.

Liquids

Long length scales( +)

Flow (unable to support shear(mm+) support shear forces)

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So what are liquid crystals?Liquid crystals have properties associated with both liquids and crystals.q y

Anisotropic rod-like particles in liquid crystals develop i i l d (lik l ) BUTorientational order (like crystals) BUT…

these particles can move around (like liquids)…these particles can move around (like liquids).

DCMSTMay 23rd, 2008

Example of a liquid crystal

5CB liquid crystal

liqcryst.chemie.uni-hamburg.de

(4-cyano-4′-n-pentyl-biphenyl)

T. Bouchouar et al, Polymer 42, 1663 (2000)

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Phase diagram for 5CB liquid crystal

ordi

rect

o

Crystal Liquid crystal Isotropic liquid

23 oC 35 oC

(nematic)p q

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Th t f d i i bThe amount of order is given by:

S ½ 3 2θ 1S=½<3cos2θ-1> θ

Crystals: S=1yIsotropic liquids: S=0Liquid crystals: 0.3<S<0.9Liquid crystals: 0.3 S 0.9

DCMSTMay 23rd, 2008

Light polarization

Light is an electromagnetic wave consisting of perpendicular

ill ti ti d l t i

polarization

oscillating magnetic and electric field.

Because light interacts with matter primarily through the electric field component the direction of the

nrc.gc.cacomponent, the direction of the electric field defines the polarization of the wave.

DCMSTMay 23rd, 2008

Unpolarized light Polarized light

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High intensity Low intensity

“normal” filter

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I0 ½I0

polarizing filter

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Li id t l t lik l i i filtLiquid crystals act like polarizing filters.

The orientationally ordered rod-like molecules in liquid crystals affect the polarization of the transmitted light.

Because the director of liquid crystals can be changed, these materials can be used as switchable light filters.

DCMSTMay 23rd, 2008

Image of a liquid crystal under crossed polarizers

Liquid crystal

ccmr.cornell.edu

DCMSTMay 23rd, 2008

Type of liquid crystals

Orientational order along director, but no g ,positional order.

N iNematic

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Orientational order along director,

l l f lmolecules form planes, but no positional order within each plane.

Smectic A

p

Smectic A

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Orientational order along director, molecules form ,planes, but stacking direction is not parallel to director

Smectic C

director.

Smectic C

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pitch

Orientational order along

p

Orientational order along director, but direction of director rotates (in xyplane as you move along z direction).

CholestericCholesteric(chiral nematic)

DCMSTMay 23rd, 2008

Controlling the liquid crystal transitionLyotropic liquid crystals

Liquid crystalline properties depend on concentration of molecules in solution.

Often associated with amphiphilic molecules.

Hydrophilic Hydrophobic

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Amphilic molecules dissolved in water form different liquid crystal structures depending on concentrationstructures depending on concentration.

ll lMicelle(low concentration)

Bilayer(high concentration)

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Thermotropic liquid crystals

Show liquid crystal phase changes as a function of temperature.

C t lli

Temperature

Crystalline Isotropic liquidLiquid crystalMore crystalline More isotropic

DCMSTMay 23rd, 2008

Thermochromic liquid crystals

Thermotropic liquid crystals can show a colour change as a function of temperature (caused by changes in the pitch).

TemperatureTemperature

Red BlueDCMST

May 23rd, 2008

Red Blue

Boundary constraints

The director in a liquid crystal system can be oriented by textures at the interface.

Nematic liquid crystalNematic liquid crystal

Polymer coating on surface  Director of nematic is lined up with polymer

coating on surface.

DCMSTMay 23rd, 2008

coating on surface.

Electric field control

The director of a liquid crystal tends to align with an applied electric field.

EEE

DCMSTMay 23rd, 2008

Twisted nematic displaysCombine boundary effect control of director with electric field effect control of director.

Unpolarizedlight

Glass Glass0o textured surface

90o textured surface

light

0o polarizer 90o polarizerNematic liquid crystal

Transparent electrode

Transparent electrode

DCMSTMay 23rd, 2008

Zero bias responseDirector in the nematic twists between bottom and top of the liquidDirector in the nematic twists between bottom and top of the liquid crystal.

Li ht l i ti f ll thi t i t h ti th h LCLight polarization follows this twist when propagating through LC.

LIGHTDCMST

May 23rd, 2008

LIGHT

Biased responseDirector in the nematic aligns with electric fieldDirector in the nematic aligns with electric field.

Light polarization does not change when propagating through LC.

E

DARKDCMST

May 23rd, 2008

SummaryLiquid crystals exhibit properties intermediate between a liquid and a crystal.

Rod-like molecules in liquid crystals show different types of orientational order.

Because the transmission of light through a liquid crystal depends on the polarization and director of the order, liquid crystals can be p , q yused for making switchable optical devices, including displays.

DCMSTMay 23rd, 2008

On-line resources for liquid crystals

1. Polymers and liquid crystals page at Case Western(h // l d /)(http://plc.cwru.edu/)

2. Liquid Crystal Institute at Kent Stateq y(http://www.lci.kent.edu/)

3 University of Cambridge page on liquid crystals3. University of Cambridge page on liquid crystals(http://www.doitpoms.ac.uk/tlplib/liquid_crystals/)

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DCMSTMay 23rd, 2008