DCMSTMay 23rd, 2008

Liquid Crystals

Gavin LawesWayne State University

DCMSTMay 23rd, 2008

Liquid Crystal Displays

LCI at Kent Statecasio.com

Amazon.com

Apple.com

LGE.com

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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

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Why does “liquid crystal” sound like an oxymoron?

Crystals

Short length scales(nm)

Highly regular array of atoms with fixed positions

Long length scales(mm+) Self-supporting,

often hard

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Liquids

Short length scales(nm)

Random, disordered array of atoms that move around.

Long length scales(mm+)

Flow (unable to support shear forces)

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

Anisotropic rod-like particles in liquid crystals develop orientational order (like crystals) BUT…

…these particles can move around (like liquids).

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liqcryst.chemie.uni-hamburg.de

Example of a liquid crystal

5CB liquid crystal(4-cyano-4′-n-pentyl-biphenyl)

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

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23 oC 35 oC

Crystal Liquid crystal(nematic)

Isotropic liquid

Phase diagram for 5CB liquid crystal

dire

ctor

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The amount of order is given by:

S=½<3cos2q-1> q

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

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Light polarization

nrc.gc.ca

Light is an electromagnetic wave consisting of perpendicular oscillating magnetic and electric field.

Because light interacts with matter primarily through the electric field component, the direction of the electric field defines the polarization of the wave.

polarization

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Unpolarized light Polarized light

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“normal” filter

High intensity Low intensity

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polarizing filter

I0½I0

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Liquid 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.

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ccmr.cornell.edu

Image of a liquid crystal under crossed polarizers

Liquid crystal

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Type of liquid crystals

Nematic

Orientational order along director, but no positional order.

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Smectic A

Orientational order along director, molecules form planes, but no positional order within each plane.

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Smectic C

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

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Cholesteric(chiral nematic)

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

pitch

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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 concentration.

Micelle(low concentration)

Bilayer(high concentration)

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Thermotropic liquid crystals

Show liquid crystal phase changes as a function of temperature.

Crystalline Isotropic liquidLiquid crystal

Temperature

More crystalline More isotropic

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Thermochromic liquid crystals

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

Temperature

Red Blue

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Boundary constraints

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

Polymer coating on surface

Nematic liquid crystal

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

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Electric field control

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

EE

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

0o polarizer 90o polarizerNematic liquid crystal

Transparent electrode

Transparent electrode

Unpolarized light

Glass Glass0o textured surface

90o textured surface

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

Light polarization follows this twist when propagating through LC.

LIGHT

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Biased responseDirector in the nematic aligns with electric field.

Light polarization does not change when propagating through LC.

DARK

E

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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 used for making switchable optical devices, including displays.

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On-line resources for liquid crystals

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

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

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

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END

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