Technical Developments at ISOLDE ISCC Meeting 3 rd November 2008
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Transcript of DCMST May 23 rd , 2008
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Liquid Crystals
Gavin LawesWayne State University
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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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