Soft Condensed Matter Physics. Liquid Crystals. Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid , and those of a solid crystal . Hence LC show anisotropy. . Note: Mesogen:
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Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solidcrystal. Hence LC show anisotropy.
It is the fundamental unit of a liquid crystal that induces structural order in the crystals.
I. THERMOTROPIC LC
i. NEMATIC LC
ii. CHOLESTRIC LC
iii. SMECTIC LC
Liquid crystals are said to be thermotropic if there liquid crystalline properties depend on the temperature.
One of the most common LC phases is the nematic, where the molecules (mesogens) have no positional order, but they have long-range orientational order.(Most nematics are uniaxial: they have one axis that is longer and preferred, with the other two being equivalent (can be approximated as cylinders)
In Greek ‘nematic’ means thread. And hence the thread like structure of the nematic crystals.
Nematics have fluidity similar to that of ordinary (isotropic) liquids but they can be easily aligned by an external magnetic or electric field. An aligned nematic has the optical properties of a uniaxial crystal and this makes them extremely useful in liquid crystal displays (LCD).
In the case of Smectic type LC, the mesogens have both positional order and orientational order. The smectic phases, which are found at lower temperatures than the nematic, form well-defined layers that can slide over one another like soap.
The cholestric phase can be defined as a special type of nematic LC in which the thin layers of the parallel mesogens have their longitudinal axes rotated in adjacent layers at certain angle.
Hydrophilic end of the mesogen
II. Lyotropic LIQUID CRYSTALS
Liquid crystals which are prepared by mixing two or more substances, of which one is a polar molecule, are known as lyotropic liquid crystals.
Eg. Soap in water.
Hexagonal phase (hexagonal columnar phase) (middle phase)
Bicontinuous cubic phase
Bicontinuous cubic phase
Reverse hexagonal columnar phase
Inverse cubic phase (Inverse micellar phase)
1. Liquid Crystal Displays:Used in display devices (LCDs) such as Laptops, watches, calculators, clocks, etc.
2. Liquid Crystal Thermometers: Chiral nematic (cholesteric) liquid crystals reflect light and the color reflected also is dependent upon temperature.
3. Optical Imaging: An application of liquid crystals that is only now being explored is optical imaging and recording.
3. Some of the liquid crystals are used in hydraulic break/clutch system due to their high viscosity values.
Liquid crystal technology has had a major effect many areas of science and engineering, as well as device technology. Applications for this special kind of material are still being discovered and continue to provide effective solutions to many different problems.
Liquid Crystal Displays
The most common application of liquid crystal technology is liquid crystal displays (LCDs.) This field has grown into a multi-billion dollar industry, and many significant scientific and engineering discoveries have been made. Please refer to the LCD chapter for more detail.
Liquid Crystal Thermometers
As demonstrated earlier, chiral nematic (cholesteric) liquid crystals reflect light with a wavelength equal to the pitch. Because the pitch is dependent upon temperature, the color reflected also is dependent upon temperature. Liquid crystals make it possible to accurately gauge temperature just by looking at the color of the thermometer. By mixing different compounds, a device for practically any temperature range can be built.
The "mood ring", a popular novelty a few years ago, took advantage of the unique ability of the chiral nematic liquid crystal. More important and practical applications have been developed in such diverse areas as medicine and electronics. Special liquid crystal devices can be attached to the skin to show a "map" of temperatures. This is useful because often physical problems, such as tumors, have a different temperature than the surrounding tissue. Liquid crystal temperature sensors can also be used to find bad connections on a circuit board by detecting the characteristic higher temperature. [Collings, 140-142]
An application of liquid crystals that is only now being explored is optical imaging and recording. In this technology, a liquid crystal cell is placed between two layers of photoconductor. Light is applied to the photoconductor, which increases the material's conductivity. This causes an electric field to develop in the liquid crystal corresponding to the intensity of the light. The electric pattern can be transmitted by an electrode, which enables the image to be recorded. This technology is still being developed and is one of the most promising areas of liquid crystal research.
Other Liquid Crystal Applications
Liquid crystals have a multitude of other uses. They are used for nondestructive mechanical testing of materials under stress. This technique is also used for the visualization of RF (radio frequency) waves in waveguides. They are used in medical applications where, for example, transient pressure transmitted by a walking foot on the ground is measured. Low molar mass (LMM) liquid crystals have applications including erasable optical disks, full color "electronic slides" for computer-aided drawing (CAD), and light modulators for color electronic imaging.
As new properties and types of liquid crystals are investigated and researched, these materials are sure to gain increasing importance in industrial and scientific applications.
It is a colloidal system in which both the dispersed phase and the dispersion medium are liquids.
Eg. Milk is a emulsion of liquid fat in water, emulsion paint, cod liver oil, blood, etc.,
Water in Oil type
Oil in Water type
Milk, Most of the creams and lotions
Emulsifieris a substance used to make a emulsion stable. Eg, Soap is an emulsifier.
Milk, Ice cream, Artificial beverages, coffee, medicinal syrup, Cod-liver oil, castor oil, petroleum oil, most of the cosmetics, hair creams, cold creams, many of the paints, adhesives, etc., are all emulsions.
It is a colloidal system in which a liquid is suspended in a solid.
Eg. Hair gel, gel tooth paste, lipsticks, etc.,
It is a colloidal system in which a gas is dispersed in a liquid or solid.
Eg. Shaving foam (gas in liquid)
Sponge (gas in solid)