types of LC

8/7/2019 types of LC

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y Liquid crystal is a term referring to substances that are not crystalline (solid) nor isotropic(liquid), but somewhere between the two. There are three main types, or what is scientifically

known as mesophases, of liquid crystals which can be identified by their varying amounts ofmolecular order and positioning. This arrangement of molecules is what makes the substance

either more solid or liquid.

Nematic

y The nematic phase is the simplest form of liquid crystal and is the phase in which the crystalmolecules have no orderly position and are free to move any which way. However, while they

have no specific order, during this phase the molecules do tend to point in the same direction,which is what differentiates it from a pure liquid. Liquid crystal in this phase can be

characterized by its thread-like appearance when looked at under a microscope. The use ofnematic liquid crystal is common in telescope lenses as it allows for a clear image when

researchers are confronted with atmospheric turbulence.

Smectic

y The smectic phase of liquid crystal, which is defined as being equivalent to the slippery, thick

residue found at the bottom of soap dishes, is characterized by a slight degree of translationalorder in the crystal molecules which is not found in the nematic phase. While keeping similar

orientation and pointing in the same direction as the molecules in nematic liquid crystal do, inthis phase the molecules tend to line themselves up into layers. While these layers as a whole

move freely, movement within the layers is restricted; therefore, it creates a slightly more solidsubstance. Smectic liquid crystal has been found to have fast electro-optical response time and

because of this is used, along with nematic liquid crystal, in producing liquid crystal display

(LCD) screens.

Cholesteric

y The cholesteric phase, also known as chiral nematic phase, is characterized by the molecules

being aligned and at a slight angle to one another, stacked within very thin layers it is the last

phase before a substance becomes crystalline, or solid. This type of liquid crystal also has the

characteristic of changing color when it is exposed to different temperatures. It is for this reason

that cholesteric liquid crystal is used in common household items such as thermometers and

mood rings

quid crystal - Definition

quid crystals are a class ofmolecules that, under some conditions, inhabit aphase in which they exhibit isotropic, f


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e behavior that is, with little long-range ordering but which under other conditions inhabit one or more phases wnificant anisotropicstructure and long-range ordering while still having an ability to flow.

quid crystals find wide use in liquid crystal displays, which rely on the optical properties of certain liquid crystalline

lecules in the presence or absence of an electric field. In the presence of electric field, these molecules align with th

ctric field, alteringpolarization of the light in a certain way.

e ordering of liquid crystalline phases is extensive on the molecular scale but does not extend to the macroscopic sc

ght be found in classical crystallinesolids. The ordering in a liquid crystal might extend along one dimension, but aother dimension it might have significant disorder.

quid crystals are divided into two groups depending on the shape of the molecules. Calamitic liquid crystals consistd-like molecules and have order in the direction of the longer axes of the molecules. In contrast, discotic liquid cryst

composed of flat-shaped molecules which align in the direction of the shorter axes of the molecules.

portant types of calamitic liquid crystals include

y nematics (most nematics are uniaxial but biaxial nematics are also known)y smectics (smectic A, smectic C, and hexatic)

portant types of discotic liquid crystals include

y discotic nematicsy columnar phases

ological membranes are a form of liquid crystal. Their rod-like molecules (e.g.,phospholipids) are organizedrpendicularly to the membrane surface, yet the membrane is fluid and elastic. It can also host important proteins suc

eptors freely "floating" inside, or partly outside, the membrane.

Contents [hide]

yotropic liquid crystals

ffect of chirality

External linkReferences

otropic liquid crystals

yotropic liquid crystal is a group of liquid-crystalline assemblies that consists of two or more components and exhi

uid-crystalline properties in certain concentration ranges. In the lyotropic phases, solvent molecules fill the space arcompounds to provide fluidity to the system. In contrast to thermotropic liquid crystals, these lyotropics have anot

gree of freedom of concentration that enables them to induce a variety of different phases.

en within the same phases, their self-assembled structures are tunable by the concentration: for example, in lamella


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ases, the layer distances increase with the solvent volume. Since lyotropic liquid crystals rely on a subtle balance ofermolecular interactions, it is more difficult to analyze their structures and properties than those of thermotropic liq

ystals.

compound which has two immiscible hydrophilic and hydrophobic parts within the same molecule is called an

phiphilic molecule. Many amphiphilic molecules show lyotropic liquid-crystalline phase sequences depending on tume balances between the hydrophilic part and hydrophobic part. These structures are formed through the micro-p

gregation of two incompatible components on a nanometer scale.

e content of water or other solvent molecules changes the self-assembled structures as follows:

y Discontinuous cubic phase (micellar phase)y Hexagonal columnar phase (middle phase)y Bicontinuous cubic phasey Lamellar phasey Bicontinuous cubic phasey Reverse hexagonal columnar phasey Inverse cubic phase (Inverse micellar phase)

e same characteristics can be observed in immiscible diblockcopolymers.

ese lyotropic liquid-crystalline nanostructures are abundant in living systems such as DNA,polypeptides, and cellmbranes. Accordingly, lyotropic liquid crystals attract particular attention in the field of biomimetic chemistry.

ect ofchirality

hen the molecules that form liquid crystals have asymmetric carbon atoms and when the system has not chirality bu

emic modification, the orientation vector of the molecular axis of the liquid crystals changes continuously and acroscopic spiral structure appears in the system as a result. The cycle of the spiral structure is different for each

lecule, but each molecule has the property that it reflects the light corresponding to its cycle. From this property, thuid crystals change color when the cycle of the spiral structure agrees with the visible rays of light. Some kinds of l

ystals change the cycle of their spiral structure when the temperature changes. This principle is applied in liquid cryrmometers.

matic liquid crystals, which have spiral structures, are called cholesteric liquid crystals. Cholesteric liquid crystals a

t distinguished from nematic liquid crystals thermodynamically; hence cholesteric liquid crystals are sometimes calral nematic liquid crystals.

though almost all chiral liquid crystals include asymmetric carbon atoms in their molecules, it has recently beencovered that macroscopic chirality appears in liquid crystals that consist of bent-core molecules which do not have

ymmetric carbon atoms. However, the appearance mechanism of this macroscopic chirality is not yet clear.

quid crystal - Definition

quid crystals are a class ofmolecules that, under some conditions, inhabit aphase in which they exhibit isotropic, f


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e behavior that is, with little long-range ordering but which under other conditions inhabit one or more phases wnificant anisotropicstructure and long-range ordering while still having an ability to flow.

quid crystals find wide use in liquid crystal displays, which rely on the optical properties of certain liquid crystalline

lecules in the presence or absence of an electric field. In the presence of electric field, these molecules align with th

ctric field, alteringpolarization of the light in a certain way.

e ordering of liquid crystalline phases is extensive on the molecular scale but does not extend to the macroscopic sc

ght be found in classical crystallinesolids. The ordering in a liquid crystal might extend along one dimension, but aother dimension it might have significant disorder.

quid crystals are divided into two groups depending on the shape of the molecules. Calamitic liquid crystals consistd-like molecules and have order in the direction of the longer axes of the molecules. In contrast, discotic liquid cryst

composed of flat-shaped molecules which align in the direction of the shorter axes of the molecules.

portant types of calamitic liquid crystals include

y nematics (most nematics are uniaxial but biaxial nematics are also known)y smectics (smectic A, smectic C, and hexatic)

portant types of discotic liquid crystals include

y discotic nematicsy columnar phases

ological membranes are a form of liquid crystal. Their rod-like molecules (e.g.,phospholipids) are organizedrpendicularly to the membrane surface, yet the membrane is fluid and elastic. It can also host important proteins suc

eptors freely "floating" inside, or partly outside, the membrane.

Contents [hide]

yotropic liquid crystals

ffect of chirality

External linkReferences

otropic liquid crystals

yotropic liquid crystal is a group of liquid-crystalline assemblies that consists of two or more components and exhi

uid-crystalline properties in certain concentration ranges. In the lyotropic phases, solvent molecules fill the space arcompounds to provide fluidity to the system. In contrast to thermotropic liquid crystals, these lyotropics have anot

gree of freedom of concentration that enables them to induce a variety of different phases.

en within the same phases, their self-assembled structures are tunable by the concentration: for example, in lamella


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ases, the layer distances increase with the solvent volume. Since lyotropic liquid crystals rely on a subtle balance ofermolecular interactions, it is more difficult to analyze their structures and properties than those of thermotropic liq

ystals.

compound which has two immiscible hydrophilic and hydrophobic parts within the same molecule is called an

phiphilic molecule. Many amphiphilic molecules show lyotropic liquid-crystalline phase sequences depending on tume balances between the hydrophilic part and hydrophobic part. These structures are formed through the micro-p

gregation of two incompatible components on a nanometer scale.

e content of water or other solvent molecules changes the self-assembled structures as follows:

y Discontinuous cubic phase (micellar phase)y Hexagonal columnar phase (middle phase)y Bicontinuous cubic phasey Lamellar phasey Bicontinuous cubic phasey Reverse hexagonal columnar phasey Inverse cubic phase (Inverse micellar phase)

e same characteristics can be observed in immiscible diblockcopolymers.

ese lyotropic liquid-crystalline nanostructures are abundant in living systems such as DNA,polypeptides, and cellmbranes. Accordingly, lyotropic liquid crystals attract particular attention in the field of biomimetic chemistry.

ect ofchirality

hen the molecules that form liquid crystals have asymmetric carbon atoms and when the system has not chirality bu

emic modification, the orientation vector of the molecular axis of the liquid crystals changes continuously and acroscopic spiral structure appears in the system as a result. The cycle of the spiral structure is different for each

lecule, but each molecule has the property that it reflects the light corresponding to its cycle. From this property, thuid crystals change color when the cycle of the spiral structure agrees with the visible rays of light. Some kinds of l

ystals change the cycle of their spiral structure when the temperature changes. This principle is applied in liquid cryrmometers.

matic liquid crystals, which have spiral structures, are called cholesteric liquid crystals. Cholesteric liquid crystals a

t distinguished from nematic liquid crystals thermodynamically; hence cholesteric liquid crystals are sometimes calral nematic liquid crystals.

though almost all chiral liquid crystals include asymmetric carbon atoms in their molecules, it has recently beencovered that macroscopic chirality appears in liquid crystals that consist of bent-core molecules which do not have

ymmetric carbon atoms. However, the appearance mechanism of this macroscopic chirality is not yet clear.


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The basics about Liquid Crystals

What are liquid crystals ?

Materials in nature can be divided into different phases, also called states of matter, dependingon the mobility of the individual atoms or molecules. The obvious states of matter are the solid,the fluid and the gaseous state. In the solid state, intermolecular forces keep the molecules close

together at a fixed position and orientation, so the material remains in a definite shape. In thefluid state, the molecules are still packed closely together, but they are able to move around.

Hence a fluid does not have a rigid shape, but adapts to the contours of the container that holdsit. Like a liquid a gas has no fixed shape, but it has little resistance to compression because there

is enough empty space for the molecules to move closer. Whereas a liquid placed in a containerwill form a puddle at the bottom of the container, a gas will expand to fill

the container.

Although the three categories seem very well defined, the borders betweenthe different states are not always clear. Apart from the three familiar states,

there exist a large number of other intermediate phases. A simple exampleis a gel. A gel is not quite solid, neither is it a liquid. Liquid crystals areanother important intermediate phase which exhibits features from both the

solid and the fluid state. Liquid crystals have the ordering properties ofsolids but they flow like liquids. Liquid crystalline materials have been

observed for over a century but were not recognized as such until 1880s. In1888, Friedrich Reinitzer (picture) is credited for the first systematic description of the liquid

crystal phase and reported his observations when he prepared cholesteryl benzoate, the firstliquid crystal.

Ordinary fluids are isotropic in nature: they appear optically, magnetically, electrically, etc. to be

the same from any direction in space. Although the molecules which comprise the fluid aregenerally anisometric in shape, this anisometry generally plays little role in macroscopic

behavior. Nevertheless, there is a large class of highly anisometric molecules which gives rise tounusual, fascinating, and potentially technologically relevant behavior. There are many

candidates for study including polymers, micelles, micro-emulsions and materials of biologicalsignificance, such as DNA and membranes. Although all of them are very interesting this

introduction will focus only on liquid crystals.

Liquid crystals are composed of moderate size organic molecules which tend to be elongated,like a cigar. At high temperatures, the molecules will be oriented arbitrarily, as shown in the

figure below, forming an isotropic liquid. Because of their elongated shape, under appropriateconditions, the molecules exhibit orientational order such that all the axes line up and form a so-

called nematic liquid crystal. The molecules are still able to move around in the fluid, but theirorientation remains the same. Not only orientational order can appear, but also a positional order

is possible. Liquid crystals exhibiting some positional order are called smectic liquid crystals. In


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smectics, the molecular centers of mass are arranged in layers and the movement is mainlylimited inside the layers.

Isotropic Nematic Smectic

The nematic liquid crystal phase is by far the most important phase for applications. In thenematic phase all molecules are aligned approximately parallel to each other. In each point a unit

vector can be defined, parallel to the average direction of the long axis of the molecules in theimmediate neighborhood. This vector, known as the director, is not constant throughout the

whole medium, but is a function of space.

The figure below shows the molecular structure of a typical rod-like liquid crystal molecule. It

consists of two or more ring systems connected by a central linkage group.

Typical shape of a liquid crystal molecule

The presence of the rings provides the short range molecular forces needed to form the nematicphase, but also affects the electrical and elastic properties. The chemical stability of liquidcrystals, their resistance to e.g. moisture or ultraviolet radiation, depends strongly on the central

linkage group. Compounds with a single bond in the center are among the most stable ones. At


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Discotic shaped liquid crystal molecule

Banana shaped liquid crystal molecule

One type of liquid crystal molecule can exhibit many different liquid crystal phases. The phase inwhich a pure liquid crystal (with only one type of molecule) exists depends on the temperature.

Pure liquid crystals, or mixtures of them, in which the phase is controlled by temperature arecalled thermotropic liquid crystals. The Brownian motion of the molecules increases with the

temperature, reducing the order in the material. At high temperature, orientational order is lostand the material changes to the isotropic phase. When decreasing the temperature, the materialchanges to the nematic phase. The temperature at which the phase transition occurs, is specific

for each material and is called the nematic-isotropic transition temperature or clearing point. By

further lowering the temperature, the phase can change to the smectic A phase, the smectic C andfinally to the solid state. Each of the phase transitions occurs at a specific temperature, butdepending on the material additional phases can appear or some can be missing.

Beside the thermotropic liquid crystals, a different class of liquid crystals is called lyotropic.

These are mixtures of rod-like molecules in an isotropic solvent and the concentration of thesolution is primarily responsible for the occurring phase. Lyotropic liquid crystals are mainly of


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interest in biological applications and exhibit a considerable number of different phases. In ourresearch, only thermotropic liquid crystal are examined.

The ordering of the liquid crystal molecules may seem strange, but in our daily environment

similar arrangements are common as the pictures in this linkillustrate. In the rest of the tutorial

pages some interesting physical and optical properties of liquid crystals are explained, limited tothe nematic liquid crystal phase. Finally the principle of a liquid crystal display will beexplained.


Why are liquid crystals so interesting ?

Nematic liquid crystal media have uniaxial symmetry, which means that in a homogeneous

liquid crystal medium a rotation around the director does not make a difference. The bulkordering has a profound influence on the way light and electric fields behave in the material.

Uniaxial anisotropy results in different electrical and optical parameters if considered along thedirector or in a plane perpendicular to it. This gives rise to interesting technological possibilities.

Two unusual phenomena are the following: the reorientation of the molecules in an electric fieldand optical birefringence of the molecules.

Reorientation of the molecules in electric fields

As a result of the uniaxial anisotropy, an electric field experiences a different dielectric constantwhen oscillating in a direction parallel or perpendicular to the director. The difference is called

the dielectric anisotropy. If the dielectric constant along the director is larger than in the directionperpendicular to it, one speaks of positive anisotropy.

Due to the anisotropy, the dielectric displacement and the induced dipole moment are not parallelto the electric field, except when the director is parallel or perpendicular to the electric field. As a

result, a torque is exerted on the director. For materials with positive anisotropy, the directorprefers to align parallel to the electric field. Liquid crystals with a negative anisotropy tend to

orient themselves perpendicularly to the electric field.

The effect of an electric field on a liquid crystal medium with positive anisotropy is illustrated inthe pictures below. Originally the orientation is almost horizontal. When an electric field with

direction along the blue arrow is applied, a torque (represented in green) rising from thedielectric anisotropy, acts on the molecule. The torque tends to align the molecule parallel to the

field. When the field strength is increased, the molecule will reorient parallel to the field.


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Original orientation Situation in electric field

Result electric field Result strong electric field

The technological importance of the reorientation is obvious: it gives a switchable medium bysimply varying the applied electric field in the liquid crystal medium. In most applications aliquid crystal is used in a thin layer between two glass surfaces. To generate the electric field,

thin electrodes layers are deposited on the bottom and/or top glass surface. For optical devicestransparent electrodes are used, made from Indium Tin Oxide (ITO). If the generated field is

strong enough, the molecules will reorient to follow its direction.

Optical birefringence

Applications of liquid crystals almost always involve optics. Optical waves also involve electricfields, but the associated frequencies are much higher than those of the fields generated by theapplied voltages. Therefore the dielectric constants, which arise from the electronic response of

the molecules to the externally applied fields, are different. To make the distinction, therefractive index is usually given for optical waves instead of the dielectric constant.

Optical waves can also reorient the liquid crystal director in an analogue manner as the

electrically applied fields. In a display this can be neglected, since both the optical dielectricanisotropy and the intensity of the optical fields are typically much lower than those used in the

static case. Therefore the optical transmission is mostly independent of the director calculations.

To understand the influence of birefringence on the propagation of light through a liquid crystal,the light must be represented by an electric field. This electric field is described by a wave vector


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in each point. At a certain time and location, the direction and the length of the vector correspondwith the direction and magnitude of the electric field. For a plane wave propagating in a specific

direction, the electric field vector in an isotropic medium describes an ellipse in the planeperpendicular to the propagation direction. This ellipse represents the polarization of the light.

Some special cases are the linear polarization and the circular polarization where this ellipse is

distorted to a straight line or a perfect circle. Generally each ellipsoidal polarization can bedecomposed as a superposition of linear polarizations along two perpendicular axes. In anisotropic medium, both linear polarizations move with the same speed. The speed of the wave is

determined by the refractive index of the medium.

Light propagation in an isotropic medium

For the uniaxial liquid crystal medium, an electric field feels a different refractive index when itoscillates in the plane perpendicular to the director or along the director. This uniaxial anisotropy

of the refractive index is called birefringence. Birefringence allows to manipulate the

polarization of the light propagating through the medium.

The elliptical polarization of light entering a liquid crystal medium must be decomposed into twolinear polarizations called the ordinary and the extra-ordinary mode. Along these two directions,

the two linearly polarized modes feel a different refractive index. Therefore, they propagatethrough the liquid crystal with a different speed as illustrated in the picture below.

Light propagation in a birefringent medium


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In the isotropic medium, the two parts propagated with the same speed. Combining them backtogether will result in the same polarization ellipse as the original. In birefringent media, the

different speed of the ordinary and extra-ordinary waves results in a phase difference between

the two modes (= retardation). At the end of the medium this phase difference between the twooscillations will result in a different polarization ellipse.

Switchable birefringence

To observe the influence of birefringence, polarized light must be used. Most light sources suchas a light bulb or a fluorescent lamp produce unpolarized light. Optical applications oftenrequired polarized light with a known oscillation direction of the light. To obtain polarized light,

ordinary light sources can be used in combination with polarizers.

A polarizer is a special type of birefringent layer. The ordinary wave propagates unmodifiedthrough the medium, whereas the extra-ordinary wave is absorbed in the medium. An arbitrarily

polarized wave entering such a medium will result in a linearly polarized wave at the back of themedium. In the picture above the effect of a polarizer is illustrated for two different orientations

of the absorbing direction.

Polarizer with vertical transmission axis

Polarizer with horizontal transmission axis


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If two polarizers with orthogonal absorption direction are used, all light emitted by the lightsource is absorbed. This is typically referred to as a set of crossed polarizers.

Crossed polarizers

Birefringence is important for modifying and controlling the polarization of light propagatingthrough the medium. A liquid crystal layer inserted between crossed polarizers can change the

polarization of the light propagating through, which results in light transmission after the crossedpolarizers.

A liquid crystal layer between crossed polarizers

Because the director can be controlled using an electric field, a liquid crystal is a controllablebirefringent medium. Therefore, the polarization state of the light after the liquid crystal layercan be changed and hence the intensity of the transmission through the crossed polarizers is

adapted.

Choosing the preferential direction of the molecules


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In a glass vessel a liquid crystal appears as an opaque milky fluid. The random variation of thedirector in the material on a micrometer scale is the main cause of the light scattering.

For applications, it is important to obtain a region free of defects with a known director

distribution. Therefore, liquid crystals are usually handled in thin layers between two substrates.

Control of the director at the surfaces allows reproducible director orientations as illustratedbelow. The fixed orientation of the surface director forces the director in the bulk to follow thisdirection. Two commonly used types of alignment are planar and homeotropic alignment. In

planar alignment the surface director is oriented parallel to the surface, for homeotropicalignment it is oriented perpendicular to the surface.

Planar alignment Homeotropic alignment Alignment by rubbing

Another simple and widely used process to achieve planar alignment is rubbing. A polymer layer(e.g. polyimide, nylon or polyvinylalcohol) is deposited on the surface and rubbed with a soft

tissue. A liquid crystal deposited on the rubbed polymer surface will exhibit a surface directorparallel to the direction of rubbing. One could say that microscopic grooves are created in the

surface which align the director. The direction of rubbing and the resulting surface director areshown below.

Alignment along the rubbing direction

The alignment with the surface is not perfect, there is a small angle of1 or 2 degrees between thesurface and the molecule director which is called the pretilt. The pretilt depends on the strengthof the rubbing.


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Pretilt originating from the rubbing process

Liquid crystals are generally used in thin layers between two glass parallel substrates. Thedistance, between the top and bottom substrate in a liquid crystal cell ranges typically from 1 to

100 m, depending on the used liquid crystal and the intended application. The two surfaces arekept parallel at a constant distance by spacers: microscopic spheres or rods made of polymer or

glass. The spacers are mixed in the glue that holds the two substrates together and if necessaryalso spread on the whole substrate surface by spinning.

The figure below shows three examples of liquid crystal layers sandwiched between twosubstrates. On the surfaces in contact with the liquid crystal, rubbed alignment layers are

deposited. In the left picture, the alignment layers of both surfaces were rubbed in the samedirection. This is called a -cell or splay cell. The obtained director distribution is

inhomogeneous and shows a splay distortion.

Splay-cell Anti-parallel rubbed Twisted nematic

A homogeneous director distribution is obtained when the top and bottom substrate are rubbed in

opposite direction as illustrated in the middle picture. An angle of 90 between the rubbing at the

top and bottom substrate results in a linear variation of the twist angle along the surface normal.

This is referred to as a twisted nematic cell.

Calculation of the director pattern in a liquid crystal medium


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A liquid crystal medium prefers a uniform director distribution. A variation of the director inspace induces an increase of the free energy. According to the elastic theory for liquid crystals,

the elastic energy related to the variation of the director 'n' in space can be written as

with the three elastic constants k11, k22 and k33. This equation is known as the Oseen-Frank

distortion energy. The three terms in the equation are related to distortion due to splay, twist andbend respectively as illustrated in the figure below. General deformations are a mixture of these

three types.

Calculations of the equilibrium director distribution involve minimizing the total free energy ofthe volume. The total energy of a liquid crystal is made up of three components: the elastic

energy density (as described above), the interface energy

related to the alignment of the director at the surfaces of the considered volume and the electric

energy density

related to the interaction of the applied electric field and the director of the liquid crystalmolecule.


How can we build a display withliquid crystals ?


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All the interesting characteristics explained in the previous sections make liquid crystalsattractive for science to seek for useful applications. A well-known application of liquid crystals

is the ubiquitous liquid crystal display, now comprising a world-wide billion dollar industry. All

ingredients necessary for building Liquid Crystal Display (LCD) were treated and this sectionwill briefly explain how LCD's work.

Construction of a liquid crystal display

On the picture below, a schematical drawing of anLCD is shown. The display consists of a thin liquidcrystal layer (thickness 5 m) sandwiched between

two glass substrates. For control of the reorientation of

the director, transparent electrodes are deposited on theglass substrates (white and red). In the picture, a so-called passive matrix is used. On the top and bottom

glass substrates, row and column electrodes arerespectively deposited. These are long parallel lines of

electrodes, with a perpendicular direction on the topand bottom glass surface. The overlap between a row

and a column electrode forms a single pixel (= pictureelement) of the display. In this simple example, 1 row

electrode is used in combination with 2 columnelectrodes. This gives us a two-pixel display.

The left column electrode is at the same potential level

as the row electrode. To the right column electrode(red), a different voltage is applied. In this way, an

electric field is generated in the right pixel orientedperpendicular to the glass surfaces.

On the picture one can see that the rubbing direction ofthe alignment layers (green) on top and bottom

substrate are chosen perpendicular to each other. Dueto this choice, the director in the left pixel makes a

homogeneous turn of 90 from bottom to top.Therefore, this type of LCD is called a 'Twisted

Nematic LCD' (TN-LCD). If a voltage is applied to the electrode, the director reorients tobecome perpendicular to the surfaces (right pixel).

In order to control the intensity of the transmitted light, the whole stack is sandwiched between

crossed polarizers (yellow). If unpolarized light enters the structure from below, the lightbecomes linearly polarized at the bottom polarizer. The light enters the liquid crystal layer with a


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polarization parallel to the bottom director. One can prove that if the thickness of the layer andthe liquid crystal parameters are chosen appropriate, the linear polarization of the entering light

will roughly follow the rotation of the director (= Mauguin regime). As a result the light istransmitted through the top polarizer and the pixel is in its bright state.

If we apply a voltage between the two electrode layers, the liquid crystal director is reorientedand the polarization of the light will no longer rotate through the liquid crystal layer. As a result,the light is absorbed at the top polarizer (= the analyzer) and the pixel becomes dark.

Adding the appropriate color filters makes a complete full color display as used in many liquid

crystal displays nowadays. Each pixel is on its turn subdivided into three pixels for controllingrespectively the red, green and blue components of the light.

Of course this is only one of the possibilities to build LCD's. Many other configurations existusing other types of liquid crystals, other molecule orientations or other manners to switch the

molecules. The most common ones are the 'Super Twisted Nematic LCD' (with a director

rotation in the layer > 90

) and the 'In-Plane Switching LCD' (molecules rotate parallel to theglass surface due to a horizontal electrical field).

Applications

Originally, LCD's were used in calculators or digital watches and had only a few black-and-white pixels. Nowadays, LCD's are widespread in all kinds of applications such as flat panel

displays for desktop applications or notebooks, mobile phones, projectors, ... A few examples ofdevices using LCD-technology are illustrated in the pictures below.


Other interesting applications ?

Of course not only display applications are possible with liquid crystals. Researchers investigatemore exotic and new applications of liquid crystals. Use of the non-linear properties of liquid

crystal mediums (the material parameters are not only direction depended, but also depend on theintensity of the entering light and the wavelength) one tries to find new applications. E.g.:

y Solitary wave propagation in liquid crystalsA high intensity laser beam injected in a liquid crystal can produce a local reorientationof the director molecules. In this way the light produces it's own waveguide and the laser

light will not diffract but stays confined in a narrow beam. The soliton application can


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lead to an addressable liquid crystal waveguide to switch light between several opticalfibers.

y Hollow liquid crystal fibersHollow optical fibers have already proved their use. If we fill them with liquid crystalsgives they can give interesting controllable behavior to the optical fibers.

y Optically addressed liquid crystal displaysIn optically addressed liquid crystal displays, the strong electric field of a non-visiblewavelength with high intensity is used to switch the molecules in stead of an externally

applied voltage.

y Liquid crystal solar cellA new and promising application using liquid crystals is the liquid crystal semiconductor.Liquid crystals are organic molecules similar to polymers. In polymers containing

conjugated systems (alternating single and double bond) the creation of a higher andlower pi-bond leads to the creation of a band gap simular to semiconductors. The use of

such a liquid crystal in a device simular to the Grtzel cell can lead to new types of solarcells.


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