Liquid crystals : The fourth state
of matter
Thierry Chuard, Universität Bern
What is a liquid crystal ?
Crystal
Highly organized solid
Liquid
Non-organized fluid
Liquid crystal
Organized fluid
heat
heat heat
Example for a liquid crystal
4-methoxybenzilidene-4'-butylaniline
Crystal Liquid Liquid
crystal
20 °C 47 °C
OCH3 CH
N
How do liquid crystals
organize ?
The nematic phase
Directo r
+ +
+ +
+ +
+ +
+ -
- -
- -
- - -
-
+ - + +- +
+ + +
+ +
+ - -
-
- - - -
-
Random orientation
of the
dipoles
Heating a nematic phase
The less organized material is the most fluid
The cholesteric phase
A cholesteric phase is a chiral nematic
phase
Twisted nematic (TN) cell
Front glass
Rear glass Contacts
Electrode
Counter-electrode
Alignment layers
LC
Twisted nematic (TN) cell
Front glass
Rear glass LC Polarizer
Polarizer
TN cells switching
Front glass
Rear glass Electrode
Counter-Electrode Alignment layer
Alignment layer
Polarizer
Polarizer
The plane of the polarized light is turned by 90°
by the LC The light
can
go through the
2nd polarizer
Only the polarized Light in the
plane of the polarizer goes through
The plane of the polarized light
is not turned by the LC
The light cannot go through the
2nd polarizer
How can we obtain a white pixel on a black
background with the
same disposal ?
The plane of the polarized light is turned by 90°
by the LC
The light cannot go through the
2nd polarizer
The plane of the polarized light
is not turned by the LC The light can go through the
2nd polarizer
