liquid crystals

LIQUID CRYSTALS

Dr A. Chandra Bose

INTRODUCTION A liquid crystal is a thermodynamic stable

phase characterized by anisotropy of properties.

No three-dimensional crystal lattice. Lying in the temperature range between the

solid and isotropic liquid phase. They are also known as mesophase

materials. In other words liquid crystal exist in a state

that is intermediate to the liquid and solid states and exhibit intermediate properties.

INTRODUCTION-COND. At lower temperatures

the materials become crystalline solid and at higher temperatures they turn in to clear isotropic liquid.

INTRODUCTION-COND. Liquid crystal materials generally have

several common characteristics. Among these are a rod like molecular

structure, long axis, and strong dipole and/or easily polarizable substituents.

A dipole is present when we have two equal electric or magnetic charges of opposite sign, separated by a small distance.

The distinguishing characteristic of the liquid crystalline state is the tendency of the molecules to point along a common axis, called the director

INTRODUCTION-COND. The study of liquid crystals began in 1888. Austrian botanist named Friedrich Reinitzer

observed that a material known as cholesteryl benzoate had two distinct melting points.

Reinitzer increased the temperature of a solid sample and watched the crystal change into a hazy liquid. As the temperature is increased further, the material changed again into a clear, transparent liquid.

Because of this early work, Reinitzer is often credited with discovering a new phase of matter - the liquid crystal phase.

EXAMPLES OF LIQUID CRYSTALS lecithin DNAcellulosecholesterol estersparaffinGraphiteSoap solutionSaliva

TYPES OF LIQUID CRYSTALS

Thermotropic Phase transition depends on temperature

NematicSmecticCholesteric

Lyotropic Phase transition depends on temperature

& concentration

I) NEMATIC LIQUID CRYSTALS The nematic phase is

characterized by long-range orientational order, i. e. the long axes of the molecules tend to align along a preferred direction.

In nematic ordering the molecules are aligned parallel to each other but apart from this the molecules are free to move relative to each other so that the material has liquid properties.

NEMATIC LIQUID CRYSTALS

T < TC1 TC1 < T < TC2 T > TC2TC1 Melting point TC2 Isotropic Transition Point

Nematic crystal changes the director direction when comes in contact with solid (p-azoxyanisole).

SMECTIC PHASES Smectic phases have

orientational order, and some degree of positional order. These phases are distinguished by the presence of layers perpendicular to the director. The figure to the right shows the chemical structure of a species with several smectic phases.

 The Smectic A phase has layers oriented at 90 degrees to the director. The Smectic C phase has a director tilted with respect to the layers.

SMECTIC Chain molecules

of liquid crystals aligned parallel

Form layer structures

Lateral forces are strong – interaction between the layers is weak

Found in soaps

CHOLESTRIC PHASE

Twisted nematic or chiral nematic

Nematic sheets stacked The orientation of the

molecules in each sheet is twisted

Director- directions of the sheet

Optically Active- rotate the plane of polarization

Pitch- temperature dependent

CHOLESTRIC PHASE Phase was first discovered in cholesterols

Colour effects

Wavelength to pitch ratio-

reflection, rotation,

polarization etc

Small Wavelength to pitch

ratio-polarized light rotates in

the same manner the director

does

LIQUID CRYSTAL STRUCTURES- COMPARISON

N-(p- Methoxy Benzylidene) p-n-Butylanaline – NematicN-(p- Eethoxy Benzylidene) p-n-Butylanaline – NematicCholesterylerucate – cholestericCholesteryl Nonanoate – cholesteric

Electro- Optic Properties (Modification of Optical Properties

by electric field) Modes of LCD operations

Dynamic Scattering Twisted Nematic

LIQUID CRYSTALS FOR DISPLAY

DYNAMIC SCATTERING

E < Ec E > Ec E >>>>Ec

Nematic – No electric Field – transparent - OFF E < Ec – No change in the molecular ordering E>>Ec - molecules lie parallel to the direction field Above Ec the current is carried by the ions Moving ions produce movement of the liquid Causes scattering of incident light Gives frosted appearance to the material – ON Poor contrast, Short life time, More power

consumption - disadvantages

TWISTED NEMATIC MODE

Liquid crystal cell ~ 9 microns thickness Transparent Conducting

coating – Indium tin oxide Surface treatment –

Polarizer & Analyzer

COLOUR LIQUID CRYSTAL DISPLAY For a LCD to show colours must have three

subpixels with red, green and blue color filters. Colour filters are used to create each color pixel. In LCD monitors thin "sandwiches" of glass

contain liquid-crystal filled cells (red, green and blue cells ) that make up a pixel.

Arrays of TFTs (thin film transistors) provide the voltage power causing the crystals to untwist and realign so that varying amounts of light can shine through each, creating images.

This particular sensitivity to light makes LCD technology very useful in projection (such as LCD front projectors), where light is focused through LCD chips

MERITS & DEMERITS OF LCD Passive display -

Requires No light Consumes less

power than LED No harmful

radiations like in CRT

Very small in size

- thin films Cheap Liquid crystals can

be Temperature Sensor

•Display fades away slowly when power is switched off-Dead pixels•Angle of view Is limited• Poor contrast• Temperature range 30 – 100 C•Highly sensitive to environmental conditions