What are Liquid Crystals – Introduction to Liquid Crystals



What are Liquid Crystals – Introduction to Liquid Crystals

HISTORY

The study of liquid crystals began in 1888 when an Austrian botanist named Friedrich Reinitzer observed that a material known as cholesteryl benzoate had two distinct melting points. In his experiments, Reinitzer increased the temperature of a solid sample and watched the crystal change into a milky liquid at 145°C. As he increased the temperature further, the material changed again into a dear, transparent liquid at 179°C. Upon cooling, the liquid turned blue before finally crystallizing. Because of this early work. Reinitzcr is often credited with discovering a new phase of matter- the liquid crystal phase.

WHAT ARE LIQUID CRYSTALS?

There are three common states of matter: solid, liquid and gas. Solid molecules always maintain their orientation and stay in the same position with respect to one another. The molecules in liquids can change their orientation and move anywhere in the liquid. However, the molecules of liquid crystals tend to maintain their orientation, like the molecules in a solid, but also move around to different positions, like the molecules, in a liquid. It turns out that liquid crystals are closer to a liquid state than a solid. It takes a fair amount of heat to change a suitable substance from a solid into a liquid crystal, and it only takes .a little more heat to turn that same liquid crystal into a real liquid. This explains why liquid crystals are very sensitive to temperature and why they are used to make thermometers and mood rings. It also explains why a laptop computer display may act funny in cold weather or during a hot day.

Liquid crystals are a state of matter that combines a kind of long-range order (in a sense of a solid) with the ability to form droplets and to pour (in the sense of water like liquids). They also exhibit properties of their own such as the ability to form monocrystals with the application of a normal magnetic or electric filed; an optical activity of a magnitude without parallel in either solids or liquids; and a temperature sensitivity which results in a color change in certain liquid crystals. The compounds which have liquid crystalline properties must undergo the process of inciting in steps. In such substances the three dimensional order of the crystal must break down in steps. Many thousands organic substances and some polymers exhibit liquid crystallinity.

The distinguishing characteristic of the liquid crystalline state is the tendency of the molecules (mesogens) to point along a common axis, called the director. This is in contrast to molecules in the liquid phase, which have no intrinsic order. In the solid state, molecules are highly ordered and have little translational freedom. The characteristic orientational order of the liquid crystal state in between the traditional solid and liquid phases and this is the origin of the term mesogenic state, used synonymously with liquid crystal state.

It is sometimes difficult to determine whether a material is in a crystal or liquid crystal state. Crystalline materials demonstrate long range periodic order in three dimensions. By definition, an isotropic liquid has no orientational orden Substances that are not as ordered as a solid, yet have some degree of alignment are .properly called liquid crystals.

To quantify just how much order is present in a material, an order parameter (S) is defined. Traditionally, the order parameter is given as below

S = 1/2 3cos2 ( θ -1)

where 0 is the angle between the director and the long axis of each molecule. The brackets denote an average over all of the molecules in the sample. In an isotropic liquid, the average of the cosine terms is zero, and therefore the order parameter is equal to zero. For a perfect crystal, the order parameter evaluates to one. Parameterof a liquid crystal range between 0.3 and 0.9, with the exact value a function of temperature, as a result of kinetic molecular motion. The molecular structures of typical liquid crystals are rather long, rigid and permit alignment and ordering of molecules with respect to one another. Therefore, the liquid crystal may be described as condensed fluid state with spontaneous anisotropy.They have the crystal properties as birefringence and exhibit interference patterns in polarized light. Liquid crystal materials generally have several common characteristics. Among these are rod­like molecular structures, rigidness of the long axis, and strong dipoles. The most widely studied liquid crystal-forming molecules are like grains of rice in shape (but far smaller in size). A popular example is the molecule p-azoxyanisole (PAA).

Liquid crystals are anisotropic materials, and the physical properties of the system vary with the average alignment with the director. If the alignment is large, the material is very anisotropic. Similarly, if the alignment is small, the material is almost isotropic.

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