Transcript Slide 1
Liquid Crystals
Gavin Lawes Wayne State University
DCMST May 23 rd , 2008
Liquid Crystal Displays
LGE.com
Amazon.com
casio.com
Apple.com
DCMST May 23 rd , 2008 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 DCMST May 23 rd , 2008
Why does “liquid crystal” sound like an oxymoron?
Short length scales
(nm) Highly regular array of atoms with fixed positions
Crystals
Long length scales
(mm+) Self-supporting, often hard DCMST May 23 rd , 2008
Short length scales
(nm)
Liquids
Long length scales
(mm+) DCMST May 23 rd , 2008 Random, disordered array of atoms that move around.
Flow (unable to support shear forces)
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
DCMST May 23 rd , 2008 T. Bouchouar et al, Polymer 42, 1663 (2000)
Phase diagram for 5CB liquid crystal Crystal 23 o C Liquid crystal (nematic) 35 o C Isotropic liquid
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The amount of order is given by:
S=½<3cos
2 q
-1> Crystals: Isotropic liquids: Liquid crystals: S=1 S=0 0.3
q DCMST May 23 rd , 2008
nrc.gc.ca
Light polarization
polarization
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 DCMST May 23 rd , 2008
High intensity “normal” filter Low intensity DCMST May 23 rd , 2008
I 0 polarizing filter DCMST May 23 rd , 2008 ½I 0
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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.
DCMST May 23 rd , 2008
Image of a liquid crystal under crossed polarizers
Liquid crystal ccmr.cornell.edu DCMST May 23 rd , 2008
Type of liquid crystals
Orientational order along
director
, but no positional order.
Nematic
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Smectic A
DCMST May 23 rd , 2008 Orientational order along
director
, molecules form planes, but no positional order within each plane.
Smectic C
DCMST May 23 rd , 2008 Orientational order along
director
, molecules form planes, but stacking direction is not parallel to director.
pitch
Cholesteric (chiral nematic)
DCMST May 23 rd , 2008 Orientational order along
director
, but direction of director rotates (in
xy
plane as you move along
z
direction).
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
(low concentration)
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Bilayer
(high concentration)
Thermotropic liquid crystals
Show liquid crystal phase changes as a function of temperature.
Temperature Crystalline Liquid crystal More crystalline More isotropic Isotropic liquid DCMST May 23 rd , 2008
Thermochromic liquid crystals
Thermotropic liquid crystals can show a colour change as a function of temperature (caused by changes in the
pitch
).
Temperature
Red
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Blue
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 0 o textured surface 90 o textured surface Glass Unpolarized light 0 o polarizer Transparent electrode Nematic liquid crystal Transparent electrode 90 o polarizer DCMST May 23 rd , 2008
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.
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LIGHT
Biased response
Director in the nematic aligns with electric field.
Light polarization does not change when propagating through LC.
E
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DARK
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.
DCMST May 23 rd , 2008
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/ ) DCMST May 23 rd , 2008
END DCMST May 23 rd , 2008
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q DCMST May 23 rd , 2008
nrc.gc.ca
Light polarization
polarization
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.
DCMST May 23 rd , 2008
Unpolarized light Polarized light DCMST May 23 rd , 2008
High intensity “normal” filter Low intensity DCMST May 23 rd , 2008
I 0 polarizing filter DCMST May 23 rd , 2008 ½I 0
DCMST May 23 rd , 2008
DCMST May 23 rd , 2008
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.
DCMST May 23 rd , 2008
Image of a liquid crystal under crossed polarizers
Liquid crystal ccmr.cornell.edu DCMST May 23 rd , 2008
Type of liquid crystals
Orientational order along
director
, but no positional order.
Nematic
DCMST May 23 rd , 2008
Smectic A
DCMST May 23 rd , 2008 Orientational order along
director
, molecules form planes, but no positional order within each plane.
Smectic C
DCMST May 23 rd , 2008 Orientational order along
director
, molecules form planes, but stacking direction is not parallel to director.
pitch
Cholesteric (chiral nematic)
DCMST May 23 rd , 2008 Orientational order along
director
, but direction of director rotates (in
xy
plane as you move along
z
direction).
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
DCMST May 23 rd , 2008
Amphilic molecules dissolved in water form different liquid crystal structures depending on concentration.
Micelle
(low concentration)
DCMST May 23 rd , 2008
Bilayer
(high concentration)
Thermotropic liquid crystals
Show liquid crystal phase changes as a function of temperature.
Temperature Crystalline Liquid crystal More crystalline More isotropic Isotropic liquid DCMST May 23 rd , 2008
Thermochromic liquid crystals
Thermotropic liquid crystals can show a colour change as a function of temperature (caused by changes in the
pitch
).
Temperature
Red
DCMST May 23 rd , 2008
Blue
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.
DCMST May 23 rd , 2008
Electric field control
The director of a liquid crystal tends to align with an applied electric field.
E E
DCMST May 23 rd , 2008
Twisted nematic displays
Combine boundary effect control of director with electric field effect control of director.
Glass 0 o textured surface 90 o textured surface Glass Unpolarized light 0 o polarizer Transparent electrode Nematic liquid crystal Transparent electrode 90 o polarizer DCMST May 23 rd , 2008
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.
DCMST May 23 rd , 2008
LIGHT
Biased response
Director in the nematic aligns with electric field.
Light polarization does not change when propagating through LC.
E
DCMST May 23 rd , 2008
DARK
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.
DCMST May 23 rd , 2008
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/ ) DCMST May 23 rd , 2008
END DCMST May 23 rd , 2008
DCMST May 23 rd , 2008