Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

1

Introduction to Liquid Crystals: Chapter 1

Liquid Crystal Displays:Manipulating light with molecules.

by

Helen GleesonDepartment of Physics and Astronomy

University of ManchesterManchester, U.K.

INTRODUCTION

Liquid crystals are materials which haveunusual, fascinating and often beautifulproperties. Their discovery was credited toan Austrian biologist almost 110 years agowhen he noticed that the solid phase of aderivative of cholesterol melted to form aniridescent fluid before becoming a clearliquid. He realised that the classification ofmaterials into solids, liquids and gases wasnot adequate and the term 'liquid crystal'was coined. Many thousands of materialsare now known to be liquid crystalline,including synthetic systems (e.g. materialsfor displays, Kevlar, detergents and anti-asthmatic drugs) and naturally occurringmaterials (e.g. beetles' wings, DNA,spiders' webs, and even budgerigardroppings).

Liquid crystals are best known for their usein displays which became common in theearly 1980s and are now used extensively.Liquid crystal displays (LCDs) can be foundin objects as diverse as watches, calculators,information display boards, aeroplane

cockpits and lap top computers. Theyoperate by modulating light and simpleoptics and an understanding of what liquidcrystals are is all that is necessary tounderstand how a liquid crystal light valveworks. LCDs were invented in the 1950s,but only became common in the late 1970s.The main reason for this marketing delaywas the lack of good stable chemicals thatform liquid crystals. The materials that wereavailable in the 1950s degraded on exposureto light, something that is clearlyunacceptable in a device that manipulateslight! Professor George Gray from HullUniversity's Chemistry Department in theU.K. solved the materials problem in theearly 1970s by inventing a new series oflight stable molecules which formed liquidcrystal phases over a very wide temperaturerange. It didn't take long for industry to takeadvantage of this windfall of materials andby the late 1970s liquid crystal displays wereall the rage.

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

2

WHAT ARE LIQUID CRYS TALS?

To understand how liquid crystal deviceswork, it is necessary to explain what a liquidcrystal actually is. As the name suggests,liquid crystals are materials which haveproperties intermediate to the solid and liquidstates of matter. They are true fluids (i.e.they flow readily), but they retainorientational order on melting from the solidto the liquid crystal state. This is shownschematically in Figure 1 where a solid inthe form of a hexagonal crystal lattice meltsto form a liquid crystal before finally meltingto the 'normal' liquid state.

Crystal Liquid

Nematic LiquidCrystal

HEAT HEAT

Figure 1.On heating a crystal structure the moleculescan form an oriented liquid crystal structurebefore finally becoming totally disordered inthe high temperature liquid phase.

One of the prerequisites for a material toform a liquid crystalline state is that themolecules are geometrically anisotropic. Putmore simply, they must be different indifferent directions (e.g. cigar shaped asshown here). As the crystal melts, there is astate where the molecules are free to move,but still retain some order - the liquid crystalstate. In this case they all point in the samedirection and posses ORIENTATIONALORDER, but no positional order. Aspherical molecule cannot form a liquidcrystal state as there is no preferredorientation which it can adopt. It is thisorientational order which is characteristic ofthe liquid crystal state. When the

temperature is increased further, themolecules eventually have sufficient energyto move completely randomly and no longerpossess even orientational order.

There are several ways in which fluid,ordered states can form. The simplest is theone shown in figure 1 , and is known as a'nematic' phase. The name comes from theGreek word nematos meaning thread-likeand is due to the rather beautiful thread-likeappearance of these materials when viewedusing a polarising microscope. The onlytype of order that occurs in nematic liquidcrystals is orientational. All of the commonliquid crystal devices employ nematic liquidcrystals.

More ordered liquid crystal states can existwhere, in addition to pointing in a preferreddirection, the molecules form a layeredstructure. Such materials are known as'smectic' and their structure is shownschematically in figure 2. Smectic phasesoccur at lower temperatures than nematicphases (they have more order) and are farmore viscous as the easiest mode of flow isfor the layers to slip over one another.

Figure 2.A cartoon of a smectic phase where the cigarshaped molecules have orientational order

and form a layered structure.

Molecular structure can be very complex andmolecular engineers find new methods ofadding different functionality to molecules allthe time. One of the most common methodsof making a molecular structure morecomplex is to introduce chirality. Chiralitydescribes the lack of mirror symmetry in a

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

3

molecule, i.e. the fact that for somemolecules the mirror image can not besuperimposed on the original, somethingbest shown in a diagram (figure 3).Chirality is often referred to as handedness -the left and right hands are chiral as one isthe mirror image of the other, but theycannot be superimposed on one another (tryit!). The addition of chirality to molecules isvery important in liquid crystals and canresult in some fascinating fluids. Here, onlythe simplest liquid crystal phase that can beformed from chiral molecules is described,known as the cholesteric phase. The name 'cholesteric' was adopted becausemany of the materials first discovered thatshowed chiral liquid crystal structuresderived from cholesterol. In fact, manynaturally occurring liquid crystals are chiraland these are discussed later.

Left handed Right handed

Figure 3.Chirality and handedness.

The structure of the cholesteric phase isshown schematically in figure 4. A slice ofthis structure is identical to the nematicphase, i.e. all the molecules point in thesame direction. However, successive slicespoint in different directions and the overallstructure is helical - this can be seen bytracing out the positions tips of the moleculeson progressing from one slice to the nextthroughout the structure in the diagram

Figure 4.The arrangement of molecules in a

cholesteric liquid crystal.

. The helicoidal structure has fascinatingoptical properties which are described in thenext chapter.

THE CONSTRUCTION AND OPTICS OF AN LCD.

Having described liquid crystal phases, it isnow possible to explain how a liquid crystaldevice works. The device has a simpleconstruction - it is a sandwich of nematicliquid crystal between glass. The liquidcrystal layer is extremely thin - only about 10µm thick, which is less than the thickness ofa human hair. Although the nematic liquidcrystal has a structure in which the moleculesall point in a preferred direction, thisdirection must be defined in the device inorder for it to operate.

The method of doing this is surprisinglysimple. The glass surfaces are treated with arubbed polymer layer which forces the liquidcrystal molecules to lie in a specific

Figure 5.Alignment of nematic liquid crystals by

grooved surfaces.

direction. The rubbing forms micro-groovesin the polymer surface and the liquid crystalmolecules prefer to lie along rather thanacross the grooves, see figure 5.

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

4

The other important element in the device arethe transparent electrodes on the innersurfaces of the glass. These allows a voltageto be applied across the device, takingadvantage of another property of liquidcrystals, their response to an electric field.Nematic liquid crystal molecules possess asmall electronic dipole (a small positivecharge at one end of the molecule and a smallnegative one at the other). The dipoleensures that the molecule responds to anelectric field, as shown in figure 6. Onapplication of the field the molecules rotate tominimise their energy - this is achieved bythe positive ends of the molecules rotating topoint in the direction of the negativeelectrode and vice versa. The idea isidentical to magnetic dipoles rotating in amagnetic field.

The elements needed to form a liquid crystaldevice are almost all there. The moleculescan be made to lie pointing in a preferreddirection within the device, and, when avoltage is applied, that direction is changed.As the voltage is removed, the moleculeswill return to their original position lying flatagainst the glass plates. Now the way inwhich such a device interacts with light mustbe considered. The construction of the mostcommon device, the TWISTED NEMATIC

-+-+

-+-+

-+-+

-+

-+

-+

-+

-+-+

+ ++

+

++

+

+

+

-+

-+ -+

-+ -+

+

+

++

+

+

-

V-

+

-

+

-

+

-

+

-

+

-

+

-

+-

+

-

+

-

+

-

+

-

+

-

+

-

+

Figure 6.The re-orientation of nematic liquid crystal

molecules on application of a voltage.

(TN) device is shown in figure 7.Notice infigure 7(a) that the glass surfaces of thedevice have been treated so that themolecules on the top surface lie at rightangles to those on the bottom. Because theliquid crystal used is nematic, the moleculestry to all point in the same direction, thoughthe surface treatment prevents them fromdoing so. The molecular orientation in thedevice has to reach a compromise and themolecules are twisted through 90˚ (a quarterturn) between the top and bottom glasssurfaces, as shown in the figure. It is thistwist which gives the device its name and itis extremely important in the operation of thedevice.

Appliedvoltage

Polarizer

Glass

Figure 7The operation of a twisted nematic device in (a) the light state and (b) the dark state. The glasssurfaces are coated with alignment layers which allow the molecules to become preferentiallyoriented in specific directions on the surfaces. The glass is also coated with transparent IndiumTin Oxide electrodes which allows a voltage to be applied across the liquid crystal layer.

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

5

The optical properties of this twisted nematicdevice rely on the use of polarised light(notice that in figure 7 it is placed betweencrossed polarisers). Two pieces of crossedpolariser alone they do not transmit light.However, if the twisted nematic device isplaced between crossed polarisers, light istransmitted. This is because the twistedstructure guides the direction of polarisationof the light so that it is rotated by 90˚ onpassing through the device. Light passingthrough the first polariser has the direction ofpolarisation rotated by the TN device suchthat it can pass through the second (crossed)polariser; the device appears light. Theeffect can be demonstrated by taking a glassmicroscope slide and building up a twist(quarter turn) by placing a few strips ofsellotape one on top of another. The glassalone will not allow light through crossedpolarisers, but the twist built up of sellotapestrips will. Both the sellotape and the liquidcrystal are examples of birefringent (ordoubly refracting) materials and it is thisproperty which allows the polarised light tobe guided.

The operation of the device is now clear. Inthe absence of a voltage, the liquid crystaladopts a twisted structure that guides lightand therefore appears light when placedbetween crossed polarisers. However whena sufficiently large voltage (usually about1.5V) is applied across the device themolecules rotate to point in the direction ofthe electric field and the twist is destroyed.In the absence of the twist light is not guidedthrough the crossed polarisers and the deviceappears dark. When the voltage is removed,the molecules relax back to the originaltwisted state under the influence of the

surface forces and the device again transmitslight.

The device is a light valve which transmitslight when no voltage is applied to thedevice, and appears dark when the voltage isapplied. Notice that all that liquid crystalscan do is allow light to be transmitted or not- they are known as passive devices, asopposed to active devices such as lightemitting diodes or electroluminescentdisplays which actually create light in someway. (Light emitting diodes work byelectrons an holes combining in thesemiconductor material which makes up thediode, producing a photon, whilst anelectroluminescent material is a phosphorwhich emits light when it is subjected to analternating electric field.) In the completedliquid crystal display device, a mirror isoften placed behind the TN display andambient light (from the room lights,windows or sunlight) is used forillumination. In more complex displays suchas computer screens, a flatelectroluminescent back light is placedbehind the TN device to provide the light.

By the way, because the device relies onpolarized light, several different effects caneasily be observed, all of which rely on basicoptics. For example, if you look at a liquidcrystal display whilst wearing Polaroid sunglasses, you will be able to see that thedevice uses polarised light by rotating thedisplay or turning your head. Alternatively,by using parallel polarisers in the devicerather than crossed ones, you can make thedisplay appear bright when the voltage isapplied and dark with the voltage off (theopposite to that described above).

WHAT ABOUT COLOUR?

In many cases, especially computer terminalsand television sets, the display must becoloured, not just black and white. Colouris achieved by putting small colour filters infront of the device. The colours which arechosen are the primary colours (red, greenand blue) from which any other colour canbe made. The filters are placed on extremelysmall picture elements known as pixels next

to one another so that the coloured picturecan be built up - the eye is tricked intocombining the colours from a small area ofthe display to give the composite colour.This technique is used in all colour TV setsand if you look closely at one you will beable to see the colour filters on the front ofthe display.

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

6

WHAT ABOUT COMPLEXIT Y AND SPEED?

The most common example of a TN deviceis probably in the digital watch. All that isnecessary in such an application is the abilityto display the digits from 0 - 9 and changebetween them faster than the eye responsetime (about 25ms). The electrode design istypically one such as that shown in figure8(a) as all the required digits can be made upfrom this pattern. The response time of theliquid crystal changing from the light to darkstate is approximately 10ms.

Front electrode Rear electrode

(a)

Top electrodes:(horizontal)

Bottom electrodes(vertical)

V2

V2

V2

V2

V2

V2

V2

Scanning

0 0 0 0

(b)Figure 8

Electrode patterns for (a) a 7 segment alpha-numeric display (front and rear surfaceelectrodes shown), and (b) a dot-matrixdisplay for more complex graphics. The

electrode connections are always made alongthe edges.

In order to make more complex displays,arrays of electrodes are arranged in a criss-cross pattern (vertical electrodes on the topsurface, say, and horizontal ones on thebottom surface), figure 8(b). As this device

is just a complex arrangement of the twistednematic device described, the whole devicewill transmit light if it is placed betweencrossed polarisers. A picture, graph ormessage can be made up of dark dots if thepixels (picture elements or dots) at thecrossover points of the top and bottomelectrodes can be switched to the dark stateindividually. A liquid crystal device requiresa certain voltage to be applied to theelectrodes in order to rotate the moleculesand cause the display to appear dark. If halfthe voltage which is required to switch theliquid crystal is supplied along the bottomsurface electrode, and the other half to thetop surface electrodes, only the crossoverpoint will have a sufficiently large potentialdifference across it to turn it dark. Theremainder of the liquid crystal will not beaffected.

Graphics may be built up from these dots inmuch the same way as newspaper printmakes up pictures. This type of display isknown as 'dot matrix' as the picture is madeup of a matrix of dark dots. The wholepicture is built up dynamically by scanningthe voltage applied to one set of electrodes asshown. Very simple graphics such as thoseon programmable calculators can besuccessfully displayed in this way.

Of course, life is rarely as simple as it mightbe. The response time of the liquid crystal isactually the major restriction in makingdisplays increasingly more complex. Evento make up a picture with very basicresolution typically 100x100 pixels (or 100horizontal and 100 vertical lines) are needed.The speed with which the picture can be builtup is a severe restriction. If a display hasonly 10 lines and liquid crystal a responsetime of 10ms, the picture will take 100ms (=0.1s) to build up - this is slow enough forthe eye to see it. What is more, the firstrows will have relaxed back by the time thescan is back round to re-address them.There are some electronic tricks which canbe played to make this scan time for a picturefaster, but the basic problem remains.Because the liquid crystal will relax back tothe original state soon after the electric fieldis removed, the refresh rate must be muchfaster than a simple scan will allow.

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

7

The problem is far worse if a high resolutioncolour display is required, because threeseparate pixels are needed to make up one'dot' of the display (one red, one green andone blue). In some black and white flatpanel liquid crystal displays (such as areused in some computer games and lap top

computers), the display is indeed rather slowand the information update can be seeneasily. In fact, the technology is really beingpushed to its limits in these cases and fullcolour complex displays cannot be made inthis simple way.

A SOLUTION: THIN FIL M TRANSISTOR DISPLAY S .

The problem of combining speed andcomplexity which was discussed above isovercome in a very ingenious way by the useof high technology. For full colour flatpanel, highly complex displays, silicontransistors are fabricated directly onto theglass at the corner of each pixel, as shown infigure 10. The technology needed to do thisis both advanced and expensive since for ahigh resolution colour display ~1,500,000transistors must be fabricated onto the glass.However, having individual transistors oneach pixel means that each pixel canessentially be addressed individually ratherthan having to scan the display line by line.As a result the whole display is much faster

and high resolution can readily be achieved.The resulting displays are superb and someof the top class lap tops now sport thistechnology. In fact, in Japan, where thetechnical engineering expertise wasdeveloped to produce these displays on alarge scale, small, full colour, flat panel TVsets can be bought. These are really highquality devices - their only drawback is theirprice. The production of silicon chips onglass is difficult and expensive, and in somecases the devices have cost more to producethan they can actually be sold for (~$1,000)!Clearly, though the technologicalachievement was amazing, something else isneeded.

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AA

AAAA

Liquid crystal

Bottom glass

Top glass

Top polarizer

Bottom polarizer

TFT

Gate line

Drain bus

Commonelectrode

Figure 10.A diagram showingthe construction of a thin film transitor device (redrawn from ‘Liquid

Crystals: applications and uses, Edited by B Bahadur)

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

8

THE FUTURE!

Liquid crystals are extremely versatilematerials. Over the last few years materialsknown as ferroelectric liquid crystals havebeen developed which look set to take overfrom the TN device which has really reachedits technological and economic limits. (Aferroelectric material is one which not onlyswitches between two states extremelyquickly when an electric field is appliedacross it, but will remain stable in one of thestates until a reverse field is applied to switchit back.) Ferroelectric liquid crystals haveresponse times of only 10µs - 1,000 timesfaster than nematic materials. Further, theydon't need expensive deposition technologyto switch each pixel individually. Theferroelectric materials have the property thatthey are as stable in the light state as the darkstate. Because they will automatically stay inone state there is no need to re-address themas often as the nematic materials.Ferroelectric liquid crystal devices should bein the market place within the next few yearsas the next generation of liquid crystaldevices.

Just as a final comment, you might besurprised that one state of matter, the liquidcrystal state, is responsible for all of the

technology described above. However, it iseven more far reaching than that. There aremore than 25 different ways in which liquidcrystal phases can be formed and only onehas been considered here. Each differentliquid crystal is a fluid state of matter inwhich the molecules can move freely, butwhere there is some orientational orpositional order. Liquid crystalline materialscan also change colour with temperature,form optical data storage media and detectpressure and heat. In all cases the system isfluid, ordered and may have some explicitfunctionality which may be a property whichchanges in response to electrical, optical, orthermal stimulus. Liquid crystals are actuallyof fundamental importance to life itself -DNA and cell membranes form liquidcrystal phases. Even the brain isapproximately 70% liquid crystalline, sothey really are important to our lives! Thenext chapter describes how the cholestericliquid crystals are responsible for someamazing optical effects in nature.

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

9

Introduction to Liquid Crystals: Chapter 2

Interference in natureand spiralling liquid crystals.

by

Helen GleesonDepartment of Physics and Astronomy

University of ManchesterManchester, U.K.

INTRODUCTION.

The last chapter discussed the nature ofliquid crystals, described their structures andconsidered the application of nematic phasesin devices. In this chapter only thecholesteric phase (with the helical structure)is consideredand particular emphasis isplaced on how it is responsible for somespectacular optical effects in nature.

We begin with a description of interferenceof light caused by periodic (repeating)structures. Interference is a term that refersto a consequence of the addition of wavesand, as light is an electromagnetic wave,applies to light in the same way as it appliesto any other sort of wave.

CONSTRUCTIVE

DESTRUCTIVE

Figure 1Waves adding resulting in interference.

There are two limiting cases of interference:constructive interference, where the twowaves adding up are in phase (i.e. the peaks

of each wave coincide in space and time)resulting in a large amplitude wave, figure1(a); and destructive interference in whichthe two waves are out of phase, such that thepeak of one coincides with the trough of theother and the amplitudes cancel, figure 1(b).

The interfering waves can be from anysource, BUT for constructive interference tooccur the waves must be in phase. One ofthe most common ways in which two lightwaves combine to give constructiveinterference is when light is incident on athin film, as shown in figure 2.

d

Incident light Reflected rays

Thin film, refractive index n

Airθi

tθ

Figure 2.In thin film interference, light reflecting from

the upper and lower surfaces interfere.

Part of the incident light wave is reflectedfrom the first boundary and part from thesecond. If the path difference between thesetwo reflected waves is a whole number ofwavelengths, then they will still be in phase

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

10

and the reflected waves will add to giveconstructive interference. The condition forconstructive interference for light which isincident at right angles to the surface is:

m ndλ = 2 ,

where m is an integer (i.e. 0, 1, 2,...), λ isthe wavelength of light, n is the refractiveindex of the film and d is its thickness and θis the angle of incidence. The refractiveindex must be included in this equationbecause λ is the wavelength of light in airand this becomes smaller in a medium ofrefractive index n by a factor λ/n. Usually mis taken to be 1, and n is about 1.5. The keything to notice about this equation is that itmeans that the wavelength of light that willbe reflected most strongly (i.e. undergoconstructive interference) is about the samesize as the film thickness. It is easy to seethat the soap layer in blue soap bubbles mustbe thinner than green or red ones becausewavelength of blue light is shorter than greenor red light. Notice that if the angle ofincidence increases (θ in the diagram), theeffective thickness of the layer becomesgreater so that the colour of the lightexperiencing constructive interference shiftstowards the red end of the spectrum. Youcan see this on a soap bubble if you moveyour head around whilst looking at thebubble.

An interesting technological application ofthin film interference is in compact discplayers. The digital information is read off aCD using light. Because the information is

laser beam scansas CD rotates

polymer filmn = 1.5

Air

Pit in track(corresponds to '1 ')

No pit in track(corresponds to '0')

Laser wavelength

in IR

Laser wavelength visible (red)

Figure 3.Thin film interference in a CD. The diagram

shows a section through the CD.

stored digitally, it consists only of 0s or 1sand these correspond to the light reflectedfrom the CD undergoing destructive orconstructive interference respectively. Oneform of operation is shown schematically infigure 3. Information is recorded in the trackof the CD by forming pits so the filmthickness is different at the placescorresponding to 0s and 1s on the disc. Theincident light has a long wavelength in air(the laser diode emits radiation in the infra-red region of the spectrum), but because thepolymer film on the CD has a refractiveindex of about 1.5, the wavelength of thelight in the thin film is visible and at the redend of the spectrum (about 7 x10-9mThedepth of the pits is half of the wavelength oflight, i.e. they are about 3.5 x 10-9m deep sothat the light reflected back from the pitsundergoes constructive interference and isrecorded as a 1. The thickness of the filmaway from the pits is designed so that thereflected light undergoes destructiveinterference and is recorded as a zero. Thepits in CD tracks can be seen using a highmagnification microscope.

Constructive interference can also take placewhen waves are reflected off periodicstructures, as shown in Figure 4. Part of theincident light is reflected at each layer andagain one wavelength is strongly reflected -the condition is the same as that describedabove. Constructive interference of lightfrom periodic structures is often known asBragg reflection in honour of Sir WilliamBragg and his son Lawrence, who pioneeredthe study of crystal structure with x-rays (thewavelengths of x-rays are short enough to beof the same order as the spacing betweenatoms in crystals).

d

Incident lig ht Reflected rays

Layers of material

Figure 4Constructive interference from many layers.

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

11

BRAGG REFLECTION FRO M CHOLESTERIC LIQUID CRYSTALS.

The structure of the cholesteric phase wasdescribed in the last chapter. It is helicalwith many more turns than was shown in thediagram. Since the helix is repeatedthroughout the cholesteric liquid crystalstructure the structure is periodic and therepeat distance is the pitch of the helix (therepeat distance in a screw thread is the pitchof the screw), see figure 5. Most cholestericliquid crystals have a helical pitch which isabout the same size as the wavelength oflight, so that constructive interference oflight where the wavelength that matches thecholesteric structure will take place. Thecholesteric liquid crystal will appear brightlycoloured (or iridescent) and the interferencephenomenon is known as selective or Braggreflection. To a first approximation, theselective reflection of light from cholestericliquid crystals can be thought of in exactlythe same way as the Bragg reflection of lightfrom the much simpler periodic structureshown in figure 4. However, the helicalstructure in fact has very complex opticalproperties and the light that is selectivelyreflected from a cholesteric liquid crystal iscircularly polarized. In order to explain whatcircular polarisation is, it's necessary tointroduce a small diversion to recall what ismeant by polarisation.

pitch

left hand thread right hand thread

Figure 5.Screw threads and helical pitch.

Polarisation is just a term used to describethe directions in which the electric (andmagnetic) fields in an electromagnetic wave

are oscillating. Most light sources produceunpolarised light, where. the electric field ofthe light wave is distributed randomlyaround, but at right angles, to the direction inwhich the light is travelling. Linearlypolarised light is light in which the electricfield has been constrained to oscillate in asingle plane. For example, if a light wavewas travelling from left to right across thispage and the electric field was confined tothe plane of the page, it might be describedas vertically polarised (provided the pagewas held upright!), see figure 6(a). Theterm linear is used to describe such lightbecause the tips of the electric field vectorsmay be joined using a straight line. Acircularly polarised light wave is shown infigure 6(b). The tip of the electric fieldtraces out a circle as viewed along the ray.Clearly, the circle could be traced outclockwise or anticlockwise, and circularlypolarised light is described as being lefthanded or right handed. In a cholestericliquid crystal, the helical structure selectivelyreflects circularly polarised light of thehandedness that matches the structure. Theother handedness of circular polarisation istransmitted by the structure, even thoughthe wavelength might be correct toexperience Bragg reflection, because thehandedness of polarisation doesn't match thestructure. Thus, a cholesteric structure willselectively reflect light of a particularwavelength (or colour), but only half of thelight that falls on the structure will beselectively reflected (that of the correcthandedness), and the rest will pass throughthe cholesteric material unaffected.

Electric field oscillation - Linearly polarised light

Light propagation direction

End view

Electric field oscillation -circularly polarised light

Figure 6(a) Linear and (b) circularly polarised light.

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

12

Usually, the fact that the light reflected froma cholesteric material is circularly polarised isunimportant, and the material's iridescence isit's key feature. However, certain beetles

make use of polarisation to obtainspectacular optical effects so this point isimportant for discussion later in the chapter.

THE THERMOCHROMIC EF FECT.

One of the most noticeable properties ofcholesteric liquid crystals is not just that theyselectively reflect light of a particularwavelength and are therefore stronglycoloured, but that the colour that is reflecteddepends on temperature. Such a material isknown as thermochromic (thermo meansheat and chromic means colour). The helicalstructure of a cholesteric liquid crystalsexpands and contracts as the temperaturechanges, so that the repeat distance of thestructure and therefore the selectivelyreflected wavelength is temperaturedependent. The typical dependence of theselective reflection wavelength withtemperature is shown in figure 7. Lowertemperatures correspond to a longer pitch(and reflect red light), whilst at highertemperatures the pitch is shorter and thereflected light is blue. This may seemwrong. Usually materials are thought toexpand as the temperature increases so itmay be expected that the longer pitch state(red) will occur at higher temperatures. Thisdoesn't happen in these cases because influids the density usually stays constant sothe desire for molecules to move furtherapart as they are given more thermal energycannot be achieved by expansion. In fact,adjacent molecules twist to higher angles inan attempt to move further apart so thestructure becomes more highly twisted as thetemperature increases. The more highlytwisted the structure is, the shorter is itsrepeat distance and therefore the more theselective reflection colour will move towardsthe blue end of the spectrum.

The use of cholesteric liquid crystals asthermochromic materials is widespread andthey particularly common in medicalapplications. In the simplest example, liquidcrystals are used in forehead thermometers(also known as 'fever indicators'). Thesedevices are based on a strip printed withseveral different thermochromic materials,each operating at a slightly differenttemperature around the normal body

temperature. The segment that appearsbright green is that which displays thecorrect temperature (green appears brightestto the human eye).

BLUE

GREEN

RED

700

400

500

600

Temperature.

long pitch

short pitchWav

elen

gth

(nm

)

Figure 7The usual temperature dependence of the

colour of a cholesteric liquid crystal.

Thermal mapping of various areas of thebody has been used as a diagnostic techniquefor a wide ranging group of medicalconditions in which a temperature differentialnear the skin surface may be related to thedisorder. Medical applications of liquidcrystals are widespread; material seems tohave been applied to almost every possiblearea of the body! Examples include theevaluation of deep vein thrombosis and theprediction of foot ulceration in diabeticpatients through thermal mapping. The useof liquid crystal thermography to screen forcancer, especially breast cancer, is common.Subcutaneous and intracutaneous malignanttumours are usually between 0.9˚C and3.3˚C warmer than the surrounding tissue,making thermography a useful candidate forcancer screening. A thermochromic sheetplaced over the affected area will be coloureddifferently around the tumour site than in theregion where the tissue is healthy. Liquidcrystals are particularly useful as theyprovide a safe, non-invasive imagingtechnique using visible light, in contrast to,say, x-rays, which carry their own risks.

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

13

Industrial applications of thermochromicliquid crystals are also well known, fromthermometers in refrigerators, cats eyes thatchange colour in freezing conditions,through to paints which when applied tosilicon chips indicate if they are over heating.At the extreme of applications, engineershave been known to paint aircraft wings withthermochromic liquid crystal to image theflow of air in flight!

The less serious uses of thermochromicliquid crystals are very well known. Moodrings change colour as the hand changestemperature, supposedly giving information

on the wearer's mood. Stress cards (aboutthe size of a credit card) can be held betweenthe finger tips to give a reading of howstressed the user is. If stress is related to thetemperature of the fingertips, then this is agood way to measure it! It is even possibleto purchase 'love-o-meters' which, whenclutched in the hand or elsewhere indicateswhether the holder is frigid, warm, steamyor passionate! Some textiles have been madethat incorporate thermochromic liquidcrystals which is an interesting idea, but notone which necessarily results in tastefulfashion (liquid crystal covered swimsuits arean example of this!).

CHOLESTERIC LIQUID C RYSTALS IN NATU RE.

Almost all living things exhibiting anisotropyin their organic tissues either are or oncewere liquid crystalline. There are manyexamples of liquid crystals in living thingsand a surprisingly large number of themexhibit cholesteric phases.

One of the most striking examples of liquidcrystals in biological systems is in beetlesand other insects where cholesteric liquidcrystals are responsible for the iridescentouter coats. If a collection of beetles isexamined with a polariser absorbing leftcircularly polarized light, the colours ofmany species appear black. This opticaleffect occurs because the beetles' backs wereformed from cholesteric liquid crystalswhich selectively reflect left handedcircularly polarised light. Only certainbrightly coloured beetles have cholestericliquid crystal films on their backs and otherssimply use thin film interference to producetheir brightly coloured outer coatings. Inorder to discriminate between selectivereflection of light and thin film interference,a poor dead beetle may be immersed in aliquid of refractive index close to that of itsouter coating (which is made of a materialknown as chitin). The colours willdisappear if the phenomenon responsible forthem is thin film interference. This isbecause if the refractive index of the thincoating of chitin is the same as that of thefluid, light will not distinguish between thetwo layers - the layers are identical to theprobing light. Because the thin chitin layeris now indistinguishable from the fluid, thin

film interference will not occur. In beetleswhere cholesteric liquid crystals areresponsible for the iridescence, they remaincoloured when immersed in the liquid.

The colour producing layer in beetles isprobably created in the late chrysalis stagefrom a liquid crystalline glandular secretionwhich rapidly hardens on their backs.Mixed colours such as bronze are created bychanging the pitch throughout the liquidcrystal layer. In one beetle, the PlusiotisGloriana, the optical system is even moresophisticated. The reflecting layer of thisscarab beetle has an anticlockwise helicoidalarchitecture and interferes only with leftcircularly polarised light. Below the firstreflecting layer is another thin, non-twistedlayer 1.8µm thick that transforms thetransmitted right circularly polarised light toleft circular which is reflected by a secondcholesteric layer. Thus the reflectivity of thisbeetle is twice that of beetles with only onecholesteric layer, nearing the optimum 100%value where both right and left circularlypolarised light are reflected.

Some liquid crystals phases occur whencertain materials are dissolved in water, avery common phenomenon in biologicalsystems. Biological materials that formliquid crystals include bile acid salts, longchain fatty acids, retinols and vitamins A, Eand K. The deposits that clog arteries arealso in a liquid crystalline state, the maincomponents being cholesteryl esters.Myelin, that forms the sheath around nerve

Introduction to liquid crystals by Helen Gleeson, Manchester University (1998)

14

cells and is prominent in the transmission ofelectrical impulses by the nerve, is alsoliquid crystalline. Studies show that most ofthe cholesterol that occurs in myelin and inthe brain, is synthesised in situ and forms aliquid crystal phase with water. Anotherimportant example of a biological liquidcrystal is DNA, the molecule in all cells thatcontains genetic information. DNA forms acholesteric liquid crystal in water, and it isinteresting that it will form this ordered fluidphase even if the sugar phosphate back boneto which the nucleic acids are connected isremoved. The nucleic acids themselves willform an ordered fluid, but the sugarphosphate back bone of the molecule is

necessary to get the genetic order right.Blood can in some circumstances form aliquid crystal phase, a situation which israther dangerous since the viscosityincreases, hindering the flow. The conditionwhere this occurs is known as sickle cellanaemia. Indeed it is not surprising that somany biological systems exhibit liquidcrystallinity. All require some sort of orderand a retention of fluidity. What is perhapssurprising is that it took so long for suchsystems to be recognised sinceunderstanding biological systems clearlyrequires some understanding of the liquidcrystal state.

SUMMARY

Thin film interference is the cause of some ofthe most spectacular optical effects that occurin nature. Cholesteric liquid crystals areunique materials which spontaneously adopta periodic structure in which the repeatdistance is correct to selectively reflect lightat visible wavelengths. When used by man,their applications span from the important(e.g. medical thermography) to theridiculous! In nature, liquid crystals play anextremely important role as ordered fluids

are the building blocks of cells. It is thoughtthat the liquid crystal state even takes part inthe process of cell division where the fluidwithin the cell must become ordered beforethe division of the cell into equal halves cantake place. The study of liquid crystalsallows us to understand ordered fluids andas well as being important with respect totechnological applications, it is playing anincreasingly important role in understandingmany biological processes.

FURTHER READING.

It is difficult to find good descriptions of liquid crystals that aren't at very specialised level. Thebest is perhaps 'Liquid Crystals: Nature's delicate phase of matter' by P J Collings, which isrelatively inexpensive and gives broad coverage of the subject. Articles about liquid crystaldevices often appear in magazines such as 'New Scientist'. There is also a virtual liquid crystaltextbook on the web site:

http://abalone.cwru.edu/tutorial/enhanced/files/textbook.htm

or look at our web site:

http://reynolds.ph.man.ac.uk