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)
DCMSTMay 23rd, 2008
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)
DCMSTMay 23rd, 2008
Phase diagram for 5CB liquid crystal
ordi
rect
o
Crystal Liquid crystal Isotropic liquid
23 oC 35 oC
(nematic)p q
DCMSTMay 23rd, 2008
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
DCMSTMay 23rd, 2008
High intensity Low intensity
“normal” filter
DCMSTMay 23rd, 2008
I0 ½I0
polarizing filter
DCMSTMay 23rd, 2008
DCMSTMay 23rd, 2008
DCMSTMay 23rd, 2008
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
DCMSTMay 23rd, 2008
Orientational order along director,
l l f lmolecules form planes, but no positional order within each plane.
Smectic A
p
Smectic A
DCMSTMay 23rd, 2008
Orientational order along director, molecules form ,planes, but stacking direction is not parallel to director
Smectic C
director.
Smectic C
DCMSTMay 23rd, 2008
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
DCMSTMay 23rd, 2008
Amphilic molecules dissolved in water form different liquid crystal structures depending on concentrationstructures depending on concentration.
ll lMicelle(low concentration)
Bilayer(high concentration)
DCMSTMay 23rd, 2008
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/)
DCMSTMay 23rd, 2008
ENDEND
DCMSTMay 23rd, 2008
DCMSTMay 23rd, 2008
Top Related