liquid crystals and lcds

8/3/2019 Liquid Crystals and LCDs

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Liquid Crystals :Structure, Properties and

Applications

Jeremiah C. Millare


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Introduction to Liquid Crystals

D iscovery:began in 1888 when an Austrian botanist named

Friedrich Reinitzer observed that a material known ascholesteryl benzoate had two distinct melting points.


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Sturcture:

Crystalline materials demonstrate long range periodic order in threedimensions.

By definition, an isotropic liquid has no orientational order.

Substances that aren't as ordered as a solid, yet have some degree

of alignment are properly called liquid crystals .


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Characterizing Liquid Crystals:

Positional order refers to the extent to which anaverage molecule or group of molecules shows

translational symmetry (as crystalline material shows).

Orientational order represents a measure of thetendency of the molecules to align along the director ona long-range basis.

Bond Orientational Order describes a line joining thecenters of nearest-neighbor molecules without requiringa regular spacing along that line.


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T ransformation:

Most liquid crystal compounds exhibit polymor ph ism , or

a condition where more than one phase is observed inthe liquid crystalline state.

T he term meso ph ase is used to describe the"subphases" of liquid crystal materials. Mesophases are

formed by changing the amount of order in the sample,either by imposing order in only one or two dimensions,or by allowing the molecules to have a degree of translational motion.


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Liquid Crystal PhasesN ematic Phases

Molecules have no positional order but tend to point in thesame direction (along the director).


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Liquid Crystal PhasesS mectic Phases

T he word "smectic" is derived from the Greek word for soap.Molecules in this phase show a degree of translational order not

present in the nematic.


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Liquid Crystal Phases

S mectic C*The director makes a tilt angle with respect to thesmectic layer. The difference is that this angle

rotates from layer to layer forming a helix.


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Liquid Crystal Phases

Columnar PhasesColumnar liquid crystals are different from the previous typesbecause they are shaped like disks instead of long rods. T hismesophase is characterized by stacked columns of molecules.

T he arrangement of the molecules within the columns and the arrangement of thecolumns themselves leads to new mesophases.


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E xternal Influences on Liquid Crystals

S urface Preparations For example, when a thin polymer coating

(usually a polyimide) is spread on a glasssubstrate and rubbed in a single directionwith a cloth, it is observed that liquid crystal

molecules in contact with that surface alignwith the rubbing direction.

Freedericksz Transition

Deformation occurs where the director changes its orientation from one molecule

to the next because of an electric or magnetic field.


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Light and Polarization

Light can be thought of as a wave, that vibrates back and forth as itmoves. Individual light waves each have their own wavelength, aswell as direction of vibration.

Ordinary visible light is a mixture of m any kinds of waves. T he lightthat enters your eye is composed of waves of a m ultitude of different

wavelengths .


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E xample of Light Polarization

We perceive light waves that vibratehorizontally as 'glare'. As light hits aparticular portion of the truck'ssurface and returns to our eye, the

waves get polarized. T his meansthat a higher than normal number of them are vibrating horizontally.When this light enters our eye, thelarge number of horizontallyvibrating waves seem to overpower

the other waves, and everythingnear that part of the truck is hiddenby the 'glare' caused by thepolarized waves.

Whenever we change a bea m of

light waves so that m

any of thewaves are vibrating in the sa m edirection, we have 'polarized' thelight bea m .


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H ow can a beam of light be

polarized?In order to polarize light, we just need to pass it througha barrier that contains many very narrow slits.

What comes out the other side is called polarized light .


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B irefringence in Liquid Crystals

When light enters a birefringent material, such as a nematic liquidcrystal sample, the process is modeled in terms of the light beingbroken up into the fast (called the ordinary ray ) and slow (called the

extraordinary ray ) components.

Because the twocomponents travel atdifferent velocities, thewaves get out of phase .When the rays arerecombined as they exit thebirefringent material, the

polarization state haschanged because of thisphase difference.


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See Virtual T extbook


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A pplications of Liquid Crystals

Liquid Crystal DisplaysLiquid Crystal Thermometers

Optical Imaging


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LCD s

You probably use items containingan LCD (liquid crystal display )every day. T hey are all around us --in laptop computers , digital clocksand watches , microwave ovens , CD players and many other electronicdevices. LC D s are common becausethey offer some real advantages over other display technologies.

laptop computer display may act funny in cold weather or during a hot day at the beach.


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H ow LC D s Work?

As light strikes the first filter, it ispolarized.

As the light passes through theliquid crystal layers, the

molecules also change the light'splane of vibration to match their own angle.

When the light reaches the far sideof the liquid crystal substance, it

vibrates at the same angle as thefinal layer of molecules.

If we apply an electriccharge to liquid crystalmolecules, they untwist

Consequently, no light can pass through that areaof the LC D , which makes that area darker than thesurrounding areas.


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N ext Attraction

Colored LC D sLED s

OLED s

liquid crystals and lcds

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