But, soft! What nymphs are these?

8. LIQUID CRYSTALS AND But, soft! What nymphs are these?

DEVICESSourceNo Spec: TextW: Wikipedia

Liquid CrystalsLiquid CrystalsProperties of LCFluidityAnisotropyE & M optic property

1st order transition (melting)

E & M – optic propertyStir Opalescence

Crystal Liquid Crystal Liquid1st order transition

1st order transition

3Dorder

1D or 2Dorder

zeroorder

Long range Positional &

(Orientational) None“Mesophase”Positional &

Orientationalp

Cf. (Positional) Plastic crystal

C H CNC6H13 CN

LC CategoriesLC Categories

Nematic PhaseNematic Phase2

Oder Parameter (S)1S 3cos 1θ

WS=0 for no orientational order to S=1 for perfect orientational order

S= 3cos 12

θ −

(Typically 0.3-0.9 for nematic )CL

OF

OCC4H9 C8H17

Smectic Phase• In addition to the orientational order, smectic phase possess 1-D

translational ordering into layer.g y• 12 different types: SA, SB(hexatic SB and Crystal B) etc up to SK

Cholesteric Phase• Cholesteric phase = Chiral nematic phase• Pitch; selective reflection of lightPitch; selective reflection of light

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Discotic LC PhasesDiscotic LC Phases

Lyotropic LCy p• LC transition via influence of solvent• Amphiphilic in nature; surfactants; • Hollow spheres, rods, and sheets• Critical micelle concentration• Closely packed structures filling 74% (sphere), cylinder (91%) and bilayer sheet (100%)• Soap: lowering the surface tension of water and allowing more material to enter it and

be dissolved.• Nanoreactor; Nanoparticle synthesis

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Liquid Crystalline PolymersLiquid Crystalline Polymers

Vectra® polymer

Birefringence of LCg• Ordinary and extraordinary refractive index: o-ray and e-ray• As the radiation propagates through the length of the sample, phase change is brought about when

it gets out of the sample– ‘phase retarder’-- generating elliptical polarization• Crossed polarizers : transmission through the analyzer shown due to the elliptical polarizationCrossed polarizers : transmission through the analyzer shown due to the elliptical polarization• Rubbing and alignment• Freedericksz Transition: Occurrence of a change from an aligned to a deformed state produced by

the E or M field application: threshod 4x102 V/cm or 0.2 T

Twisted Nematic (TN) DisplayTwisted Nematic (TN) Display

• Reflective System (calculator and watches) and Back Lighting System (display and TV)• Reflective System (calculator and watches) and Back Lighting System (display and TV)• Seven segment, Pixel (for XGA 768x1024 =8x105 pixels)• Passive matrix and Active matrix• Scanning principle: time to scan the entire matrix is shorter than the turn-off time

C l filt• Color filters• Super-Twisted Nematic (STN): improved multiplexing capability with steeper tilt vs.

voltage characteristics but color compensation is needed

Ferroelectric LCFerroelectric LC• Fast response, smaller pixel size• Bistability is often detrimental to the accidental state 30

40

Ps ychange due to the external mechanical stress ; no recovery happens

DOBAMBC10

20

(nC/㎠)

C10H21O C N C CHCO2CH2CHC3H7

CH3

Tc-20 Tc

T

C10H21O CH

N CH

CHCO2CH2CHC3H7

*

Sc*SSFLC (surface-stabilized FLC)

Polymer Dispersed LCPolymer-Dispersed LC• Switchable windows, projection display

Liquid Crystal LensesLiquid Crystal Lenses• Consists of a number of pixels in the form of rectangular grid or• Consists of a number of pixels in the form of rectangular grid or

concentric rings: modulate the refractive index of the individual pixel

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