Liquid crystals have properties of both liquids and crystals. They can be classified as thermotropic, lyotropic, or metallotropic based on what triggers their liquid crystalline phase. Thermotropic liquid crystals form phases based on temperature, while lyotropic phases depend on concentration in a solvent. Metallotropic phases are influenced by both inorganic-organic composition and temperature. Common liquid crystal phases include nematic, smectic, and cholesteric. Liquid crystals have many technological and natural applications, with most displays using liquid crystals and biological structures like membranes being forms of liquid crystals.

1. LIQUID CYRSTALS
1- INTRODUCTION
2- CLASSIFICATION OF LIQUID CRYSTALS
3- APPLICATIONS
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2.  A liquid crystal may flow like a liquid, but its
molecules may be oriented in a crystal-like way.
 There are many different types of liquid-crystal
phases, which can be distinguished by their
different optical properties such as birefringence.
 When viewed under a microscope using a
polarized light source, different liquid crystal
phases will appear to have distinct textures.
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3.  Liquid crystals can be divided into
thermotropic, lyotropic and metallotropic
phases.
 Metallotropic LCs are composed of both
organic and inorganic molecules; their liquid-
crystal transition depends not only on
temperature and concentration, but also on the
inorganic-organic composition ratio.
 Examples of liquid crystals can be found both
in the natural world and in technological
applications.
 Most contemporary electronic displays use
liquid crystals.
 For example, many proteins and cell
membranes are liquid crystals.
 Other well-known examples of liquid
crystals are solutions of soap and various
related detergents, as well as the tobacco
mosaic virus, and some clays.
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4. Liquid Crystalline Phases
Mesophase: a phase lying between solid (crystal)
and isotropic (liquid) states.
Liquid crystals: fluid (l) but also show birefringence (c);
have properties associated with both crystals and liquids.
Thermotropic: liquid crystalline phase is formed
when the pure compound is heated.
Lyotropic: liquid crystalline phase forms
when the molecules are mixed with a solvent (solution).
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5. 2. CLASSIFICATION OF LIQUID CRYSTALS
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6. Thermotropic
liquid crystals
phase forms as a function of temperature
Lyotropic
liquid crystals
Phase forms as a function of concentration
in a solvent
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7.  The transitions to the liquid crystalline state are induced thermally
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8. 2.1. Thermotropic Liquid Crystals
 The essential requirement for a molecule to be a
thermotropic LC is a structure consisting of a central
rigid core (often aromatic) and a flexible peripheral
moiety (generally aliphatic groups). This structural
requirement leads to two general classes of LCs:
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9. Nematic Phase (No translational order)
 The word “Nematic" is derived from the Greek word for
thread-like structure.
 It is the only liquid crystal phase with no long range
translational order.
 Preferred Orientation is denoted by the ‘Director’ n.
 The nematic phase is one of the most common LC
phases.
 It has thread like structure when seen under polarizing
microsope.
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10. PHOTOGRAPHS OF LIQUID CRYSTALS
PHASES BY POLORIZED OPTICAL
MICROSCOPE
(NEMATIC PHASES)
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11. Smectic Phase (One-dimensional translational)
 The word "Smectic" is derived from the Greek word for
soap
 Liquid-like motion of the rods in each layer
 No correlation of the molecular positions from one layer
to the next
 The layers can easily slide
 In the smectic A phase, molecules tend to be
perpendicular to the smectic layers
 In the smectic C phase, the molecules in the layers are
parallel and tilted in arrangement with respect to the
normal of the layers by a tilt angle θ.
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12. Chiral Phase (Cholesteric Phase)
 Also known as “Chiral nematic”
 Molecules have non-symmetrical carbon atoms and thus
lose mirror symmetry
 Shows a helical structure.
 In general the helical pitch of cholesteric liquid crystals is
of the order of visible light’s wavelength—about a few
hundreds nm and so shows different color.
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14. Blue phases can exist between the chrial nematic phase and the isotropic liquid phase of liquid crystalline materials
with high chirality.
Disk-shaped LC molecules can orient themselves in a layer-like fashion known as the discotic nematic phase.
Bowl-shaped LC molecules, like in discotics, can form columnar phases. (Bowlic Phase)
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15. 2.2. Lyotropic Liquid Crystals
 Lyotropic LCs are two-component systems where an amphiphile is dissolved in a
solvent.
 Lyotropic mesophases are concentration and solvent dependent.
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16. A compound that has two immiscible hydrophilic and hydrophobic parts within the
same molecule is called an amphiphilic molecule.
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17. Soap is an everyday example of a lyotropic liquid crystal.
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18. 2.3. Metallotropic Liquid Crystals
The addition of long chain soap-like molecules leads to a series of
new phases that show a variety of liquid crystalline behavior both as a
function of the inorganic-organic composition ratio and of
temperature.
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20. There's more to building an LCD than simply
creating a sheet of liquid crystals. The
combination of four facts makes LCDs possible:
- Light can be polarized. (See How Sunglasses
Work for some fascinating information on
polarization!)
- 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.
An LCD is a device that uses these four facts in a
surprising way.
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21. In particular, biological membranes and cell membranes are a form of liquid
crystal. Their constituent molecules (e.g. phospholipids) are perpendicular to
the membrane surface, yet the membrane is flexible. These lipids vary in shape.
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22. - Examples of liquid crystals can also be found in the mineral world, most of them being
lyotropics. The first discovered was V2O5, by Zocher in 1925.
- Zeolite
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