Transcript Slide 1

Liquid Crystals
Gavin Lawes
Wayne State University
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May 23rd, 2008
Liquid Crystal Displays
LGE.com
Amazon.com
casio.com
Apple.com
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LCI at Kent State
Outline
Liquid Crystals
What are liquid crystals?
Light and polarization filters
Demonstration
Types of liquid crystals
Nematic, smetic, chiral, and more
Controlling the liquid crystal transition
Demonstration
Twisted nematic displays
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Why does “liquid crystal” sound like an oxymoron?
Highly regular
array of atoms
with fixed
positions
Short length scales
(nm)
Crystals
Long length scales
(mm+)
Self-supporting,
often hard
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Random,
disordered array
of atoms that
move around.
Short length scales
(nm)
Liquids
Flow (unable to
support shear
forces)
Long length scales
(mm+)
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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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Example of a liquid crystal
5CB liquid crystal
(4-cyano-4′-n-pentyl-biphenyl)
liqcryst.chemie.uni-hamburg.de
T. Bouchouar et al, Polymer 42, 1663 (2000)
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director
Phase diagram for 5CB liquid crystal
Crystal
Liquid crystal
(nematic)
23 oC
Isotropic liquid
35 oC
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The amount of order is given by:
S=½<3cos2q-1>
Crystals:
Isotropic liquids:
Liquid crystals:
S=1
S=0
0.3<S<0.9
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q
Light polarization
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.
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Unpolarized light
Polarized light
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High intensity
Low intensity
“normal” filter
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½I0
I0
polarizing filter
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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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Image of a liquid crystal under crossed polarizers
Liquid crystal
ccmr.cornell.edu
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Type of liquid crystals
Orientational order
along director, but no
positional order.
Nematic
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Orientational order
along director,
molecules form planes,
but no positional order
within each plane.
Smectic A
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Orientational order along
director, molecules form
planes, but stacking
direction is not parallel to
director.
Smectic C
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pitch
Orientational order along
director, but direction of
director rotates (in xy
plane as you move along
z direction).
Cholesteric
(chiral nematic)
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Controlling the liquid crystal transition
Lyotropic 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
Bilayer
(low concentration)
(high concentration)
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Thermotropic liquid crystals
Show liquid crystal phase changes as a function of temperature.
Temperature
Crystalline
Liquid crystal
More crystalline
More isotropic
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Isotropic liquid
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.
Nematic liquid crystal
Polymer coating
on surface
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.
E
E
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Twisted nematic displays
Combine boundary effect control of director with electric field
effect control of director.
Glass
0o textured
surface
90o textured
surface
Glass
Unpolarized
light
Transparent Nematic liquid
electrode
crystal
0o polarizer
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Transparent
electrode
90o polarizer
Zero bias response
Director 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 response
Director in the nematic aligns with electric field.
Light polarization does not change when propagating through LC.
E
DARK
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Summary
Liquid 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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