Liquid crystals : The fourth state of matter Thierry Chuard, LLB, Universität Bern

Abstract  What is a liquid crystal ?  Some applications of liquid crystals being a liquid crystal ?  How can we identify a compound

What is a liquid crystal ? Crystal Highly organized solid Liquid Non-organized fluid Liquid crystal Organized fluid (mesophase) heat

The liquid crystalline state is a hybrid state between the liquid and and the solid phase. Liquid crystals possess both the fluidity of liquids and organization found in solids. This is an anisotropic phase.

Liquid crystals Depends on - the temperature - the solvent - the concentration Depends only on the temperature Thermotropic Lyotropic

Which kind of molecules can show liquid crystalline state ? Calamitics Rigid and rod-shaped Rigid and disc-shaped Discotics

A calamitic compound 4-methoxybenzilidene-4'-butylaniline CrystalLiquid crystal 20 °C 47 °C

How do calamitic molecules organize ?

The nematic phase Director Random orientation of the dipoles No polarity of the mesophase

The nematic phase The less organized mesophase is the most fluid 

The cholesteric phase A cholesteric phase is a chiral nematic phase

The cholesteric phase p The wavelength of the light reflected by a cholesteric material is proportional to the helical pitch p and to the mean refractive index n :  = p ∙ n Bragg’s law

Smectic ASmectic C The smectic (lamellar) phases

Smectic B The smectic (lamellar) phases hexagonal phase

N SASA SCSC The same compound can show different mesophases Crystal 63 °C60 °C80 °C Liquid 86 °C

A discotic compound Liquid crystal CrystalLiquid 69 °C 122 °C

How do discotic molecules organize ? HexagonalTetragonalRectangular  Nematic phase  Columnar phases

How can we identify a compound being a liquid crystal ?  X-Ray diffraction  Miscibility  Differential scanning calorimetry  Polarized transmitted-light microscopy

Temperature (°C) Temperature (°C) Differential scanning calorimetry Temperatur e Control Heat flow

Differential scanning calorimetry Temperature (°C) Thermogram = difference between heat flows

Temperature (°C) Differential scanning calorimetry Temperatur e Control 1-2 mg Heat flow

Differential scanning calorimetry Temperature (°C) Melting point Thermogram = difference between heat flows

Thermogram of a non liquid-crystalline compound LiquidSolid

Thermogram of a liquid-crystalline compound Solid Liquid Crystal Liquid

X-Ray diffraction  Determination of the layer thickness in smectic (lamellar) phases  mg samples  Identification of the mesophase  Determination of the angle in non orthogonal smectic phases

polarized light Polarized transmitted-light microscopy

The Schlieren texture Nematic phase

The focal conic fan texture Smectic A phase

The focal conic fan texture Columnar hexagonal phase

Some applications of liquid crystals  Displays  Materials (polymers)  Thermometry  Pigments (cars)  Specific oils  Adjustable Tinted Window

LC displays  different technologies  Switching time always faster  Switching voltage always lower  Screen size  Resolution  Colors quality and greyscale

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

TN cells switching Front glass Rear glass Electrode Counter-Electrode Alignment layer 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 

ITO electrodes

Conclusion