ABSTRACT
The study of liquid crystals (LCs) began in 1888 when an Austrian botanist named Friedrich Reinitzer observed that a material known as cholesteryl benzoate exhibited two distinct melting points [1]. In his experiments, Reinitzer increased the temperature of a solid sample and watched the
crystal change into a hazy liquid. As he further increased the temperature, the material changed into a clear, transparent liquid. Because of this early work, Reinitzer is often credited with discovering a new phase of matter-the liquid crystal phase also known as liquid crystalline or mesophase. Most people are familiar with the fact that matter can exist in three states: solid, liquid, and gas (vapor). However, this is a simplifi cation, and under extreme conditions, other forms of matter can exist, e.g., plasma at very high temperatures or superfl uid helium at very low temperatures. The difference between these states of matter is the degree of order in the material, which is directly related to the surrounding temperature and pressure. If the temperature is raised, more energy is transferred into the system, leading to increasingly stronger vibrations. Finally, at the transition temperatures between the solid and liquid states, the long range positional order is broken and the constituents may move in a random fashion (Figure 25.1), constantly bumping into one another and abruptly changing direction of motion. The thermal energy is not high enough to completely overcome the attractive forces between the constituents, so there is still some positional order at short range. Because of the remaining cohesion, the density of the liquid is constant even though the liquid takes the shape of its container, as opposed to a solid. The liquid and solid phases are called condensed phases. If we continuously increase the temperature until the next phase change, the substance enters its gas (or vapor) state and its constituents are no longer bound to each other (Figure 25.1).
