liquid crystals

Topics: Liquid crystal, Crystal, Matter Pages: 5 (1174 words) Published: October 19, 2014
Materials in nature can be divided into different phases, also called states of matter, depending on the mobility of the individual atoms or molecules. The obvious states of matter are the solid, the fluid and the gaseous state. In the solid state, intermolecular forces keep the molecules close together at a fixed position and orientation, so the material remains in a definite shape. In the fluid state, the molecules are still packed closely together, but they are able to move around. Hence a fluid does not have a rigid shape, but adapts to the contours of the container that holds it. Like a liquid a gas has no fixed shape, but it has little resistance to compression because there is enough empty space for the molecules to move closer. Whereas a liquid placed in a container will form a puddle at the bottom of the container, a gas will expand to fill the container.

Although the three categories seem very well defined, the borders between the different states are not always clear. Apart from the three familiar states, there exist a large number of other intermediate phases. A simple example is a gel. A gel is not quite solid, neither is it a liquid. Liquid crystals are another important intermediate phase which exhibits features from both the solid and the fluid state. Liquid crystals have the ordering properties of solids but they flow like liquids. Liquid crystalline materials have been observed for over a century but were not recognized as such until 1880s. In 1888, Friedrich Reinitzer (picture) is credited for the first systematic description of the liquid crystal phase and reported his observations when he prepared cholesteryl benzoate, the first liquid crystal. Ordinary fluids are isotropic in nature: they appear optically, magnetically, electrically, etc. to be the same from any direction in space. Although the molecules which comprise the fluid are generally anisometric in shape, this anisometry generally plays little role in macroscopic behavior. Nevertheless, there is a large class of highly anisometric molecules which gives rise to unusual, fascinating, and potentially technologically relevant behavior. There are many candidates for study including polymers, micelles, micro-emulsions and materials of biological significance, such as DNA and membranes. Although all of them are very interesting this introduction will focus only on liquid crystals. Liquid crystals are composed of moderate size organic molecules which tend to be elongated, like a cigar. At high temperatures, the molecules will be oriented arbitrarily, as shown in the figure below, forming an isotropic liquid. Because of their elongated shape, under appropriate conditions, the molecules exhibit orientational order such that all the axes line up and form a so-called nematic liquid crystal. The molecules are still able to move around in the fluid, but their orientation remains the same. Not only orientational order can appear, but also a positional order is possible. Liquid crystals exhibiting some positional order are called smectic liquid crystals. In smectics, the molecular centers of mass are arranged in layers and the movement is mainly limited inside the layers.

The nematic liquid crystal phase is by far the most important phase for applications. In the nematic phase all molecules are aligned approximately parallel to each other. In each point a unit vector can be defined, parallel to the average direction of the long axis of the molecules in the immediate neighborhood. This vector, known as the director, is not constant throughout the whole medium, but is a function of space. The figure below shows the molecular structure of a typical rod-like liquid crystal molecule. It consists of two or more ring systems connected by a central linkage group.

Typical shape of a liquid crystal molecule
The presence of the rings provides the short range molecular forces needed to form the nematic phase, but also affects...
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