Understanding liquid crystal

1
Understanding liquid crystal

• PHY0411A
• Jiang Han
• Fong King Sing

2
Contents

• Introduction
• Different phase
• Physical properties
• Application

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States of matter

• Solid
• Definite shape
• Particles are packed closely together to form
  crystal structure
• Particles cannot move freely, only vibration is
  possible

4
States of matter

• Liquid
• No definite shape but definite volume
• Particles can vibrate, move about, and slide past
  each other

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Phase change of matter

• Normal matter
• Temp?
• Single phase change
• crystalline solid ? isotropic liquid
• Some organic molecules
• Temp?
• Multiple phase change
• crystalline solid ?mesophase(s) ? isotropic
  liquid

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Liquid Crystals

• Some organic molecules were found to exhibit one
  or more mesophases
• Such mesophases were classified into two types
• Disordered Crystal Mesophases
• Ordered Fluid Mesophases ? Liquid Crystal

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Liquid Crystals

• In ordered fluid mesophases, matters have
  properties between those of a conventional
  liquid, and those of a solid crystal. For
  instance, a liquid crystal can flow like a
  liquid, but have the molecules in the liquid
  arranged and/or oriented in a crystal-like way

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Classification of LC phase

• Liquid crystals can be divided into two types
  according to how they are formed
• Lyotropic LCs
• exhibit phase transitions as a function of
  concentration of the mesogen in a solvent as well
  as temperature
• Thermotropic LCs
• exhibit a phase transition into the LC phase as
  temperature is changed
• We will only discuss thermotropic LCs in this
  presentation

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Phases of Thermotropic LCs

• Nematic phases
• Smectic phases
• Chiral phases

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Nematic phase

• Molecules have no positional order, but they have
  long-range orientational order
• Molecules flow and their center of mass positions
  are randomly distributed as in a liquid, but they
  all point in the same direction

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Smectic phase

• At temperatures below the nematic phase, the
  liquid crystal material may gain an amount of
  positional order it transforms into smectic
  phase
• In such phase, molecules still form a fluid, but
  prefer to lie on average in layers
• Within each layer the liquid crystal is
  essentially a 2-dimensional nematic liquid crystal

Smectic A
Smectic C
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Smectic phase

• There are over 10 types of smectic phase, we name
  it Smectic A, B, CThe most common ones are A and
  C
• When the nematic-like director in each layer is
  parallel to the layer normal the material is
  Smectic A
• If this director tilts away from the layer normal
  the Smectic C phase is formed

Smectic A
Smectic C
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Smectic phase

• The positional ordering may be described in terms
  of the density of the centers of mass of the
  molecules, where z is the coordinate parallel
  to the layer normal, the average density of the
  fluid is ?0 , d is the distance between layers
  and is the order parameter.

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Strength of the smectic order

• ? measures the strength of the smectic order
• When ?0 there is no layering and the material
  is nematic
• If ?gt0 then some amount of sinusoidal layering
  exists and the material is smectic

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Physical Properties of LC

• To understand a material
• Electronic response
• Dielectric permittivity
• Refractive index
• Mechanical response
• Elastic constant

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Physical Properties of LC

• Director of a LC
• Liquid crystal are composed of rod-like molecules
• Long axes are aligned to one another
• Represent the direction of preferred orientation
  of molecules

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Physical Properties of LC

• Order parameter
• In long range, liquid crystals have no position
  order, but still have orientation order
• Molecules share the same director, but
  distribution of angle can be different

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Physical Properties of LC

• Order parameter
• Define an order parameter to quantify the
  orientational order
• Typical value 0.30.9
• Reduce to 0 at liquid crystal-liquid phase
  transition

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Physical Properties of LC

• Elastic continuum theory
• The material is treated as a continuum
• Ignore molecular details
• Useful in studies of their mechanical properties,
  eg. response to twist, bend ...
• Assume order parameter is constant

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Electric Properties

• Electric displacement
• Energy density depends on
• Larger e , larger induce dipole
• Difference in e give reason for rotation of LC

LC director n
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Electric Properties

• Consider non-polar compound
• Torque is provided
• Useful in LCD

E
??(?-??) gt 0
?gt??
?gt ??
??(?-??) lt 0
E
?lt??
?lt ??
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Electric Properties

• Typical relation of dielectric constant on
  temperature
• Decrease with temperature
• Recall order parameter

4-n-pentyl-4'-cianobiphenyl
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Optical Properties

• Review
• If EM wave Poynting vector in
  z-directionVector in x-direction
• Refractive index depends on permittivity
• It is easier to polarize materials along their
  anisotropy axis
• Dielectric displacement can be nonparallel to
  electric field , i.e. polarization also

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Optical Properties

• Nematic phase
• Uniaxial LC has two refractive indices
• Ordinary refractive index
• Electric vector vibrates perpendicular to the
  optical axis
• Extraordinary refractive index
• Electric vector vibrates parallel to the optical
  axis
• Practically,
• Positive birefringence effect

LC director n
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Optical Properties

• Chiral nematic phase
• The optical axis of chiral nematic is the helix
  axis
• Circular orientated liquid crystal
• Light exhibits circular polarization

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Mechanical Properties

• A liquid crystal experiences 3 distortions
• Response to the distortion is described by 3
  elastic constant, K11, K22, K33

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Mechanical Properties

• Splay
• Mathematical description

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Mechanical Properties

• Twist
• Mathematical description

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Mechanical Properties

• Bend
• Mathematical description

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On the surface

• Liquid crystals may have different surface
  alignment
• Energy is needed to move the director from its
  easy axis
• The magnitude depends on the anchoring angle

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• What so important about the above parameters
• Determine the free energy of liquid crystal
• Useful in application

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Energy of liquid crystal

• Free energy density

Energy due to external field (E, M, EM field)
Energy due to distortion
Coupling energy of LC to the surface
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Energy of liquid crystal

• We have the equation for free energy
• In principle, we can minimize it to calculate the
  equilibrium state of the liquid crystal (static)

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Application of LCs

• Liquid Crystal Display (LCD)
• Thermometers
• Some LCs change its color at different
  temperature, used in aquarium and pool
• Liquid crystal sheets are often used in industry
  to look for hot spots

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Liquid Crystal Display (LCD)

• Widely used display technology nowadays
• Calculators
• Digital clock, watches
• Laptop computers
• PDAs
• Mobile phones
• TVs
• etc

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Four facts that make LCD works

• Light can be polarized
• Liquid crystals can transmit and change polarized
  light
• The structure of liquid crystals can be changed
  by electric current
• There are transparent substances that can conduct
  electricity

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Twisted nematic field effect

• First discovered in 1970

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Principle of LCD
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Summary

• Liquid crystal is a mesophase
• Different phases of liquid crystal
• Highly anisotropic, its response to physical
  processes depend on orientation
• Special optical property
• Various application

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END