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LIQUID CYRSTALS

1- INTRODUCTION2- CLASSIFICATION OF LIQUID CRYSTALS3- APPLICATIONS

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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.

1.INTRODUCTION

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.

4Liquid Crystalline PhasesMesophase: 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).

52. CLASSIFICATION OF LIQUID CRYSTALS

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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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The transitions to the liquid crystalline state are induced thermally

82.1. 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:

9Nematic 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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PHOTOGRAPHS OF LIQUID CRYSTALS PHASES BY POLORIZED OPTICAL MICROSCOPE (NEMATIC PHASES)

11Smectic 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.

12Chiral 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.

13Nematic vs. Chiral Nematic

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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)

OTHER PHASES

152.2. Lyotropic Liquid Crystals Lyotropic LCs are two-component systems where an amphiphile is dissolved in a solvent. Lyotropic mesophases are concentration and solvent dependent.

16A compound that has two immiscible hydrophilic and hydrophobic parts within the same molecule is called an amphiphilic molecule.

BILAYER MICELLE

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Soap is an everyday example of a lyotropic liquid crystal.

182.3. 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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20There'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.

LCDs

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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.

BIOLOGICAL LIQUID CRYSTALS

22MINERAL LIQUID CRYSTALS- 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