What are liquid crystals ?

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.

Friedrich Reinitzer
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.

Isotropic phase of a liquid crystal Nematic phase of a liquid crystal Smectic phase of a liquid crystal
Isotropic Nematic Smectic

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
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 the electrical and elastic properties. The chemical stability of liquid crystals, their resistance to e.g. moisture or ultraviolet radiation, depends strongly on the central linkage group. Compounds with a single bond in the center are among the most stable ones. At one side of the rings there is a long side chain which strongly influences the elastic constants and the transition temperature of the liquid crystal phases. At the other end, a terminal group is connected, which determines the dielectric constant and its anisotropy. A few examples of molecules that exhibit a liquid crystal phase are shown below.

Example of a typical liquid crystal molecule
Example of a typical liquid crystal molecule

Beside these simple examples, more complicated ways of stacking are possible, giving rise to many other types of liquid crystals. Chiral molecules, molecules without mirror symmetry, can give rise to helices comprising cholesteric liquid crystal phases. In ferroelectric or antiferroelectric liquid crystals the smectic layers possess a permanent polarization which is constant or alternating between successive layers respectively. Apart from the rod-like molecules, more advanced-shaped liquid crystals are possible such as disk-like or banana-shaped liquid crystals which can give rise to other types of ordering. Discotic liquid crystals can be stacked a columnar phase, the bottom picture illustrates a possibility for stacking banana shaped liquid crystals.

Discotic shaped liquid crystal molecules
Discotic shaped liquid crystals
Banana shaped liquid crystal molecules
Banana shaped liquid crystals

Examples of molecules which give rise to discotic and banana shaped liquid crystal phases are shown below.

Discotic shaped liquid crystal molecule
Discotic shaped liquid crystal molecule
Banana shaped liquid crystal molecule
Banana shaped liquid crystal molecule

One type of liquid crystal molecule can exhibit many different liquid crystal phases. The phase in which a pure liquid crystal (with only one type of molecule) exists depends on the temperature. Pure liquid crystals, or mixtures of them, in which the phase is controlled by temperature are called thermotropic liquid crystals. The Brownian motion of the molecules increases with the temperature, reducing the order in the material. At high temperature, orientational order is lost and the material changes to the isotropic phase. When decreasing the temperature, the material changes to the nematic phase. The temperature at which the phase transition occurs, is specific for each material and is called the nematic-isotropic transition temperature or clearing point. By further lowering the temperature, the phase can change to the smectic A phase, the smectic C and finally to the solid state. Each of the phase transitions occurs at a specific temperature, but depending on the material additional phases can appear or some can be missing.

Beside the thermotropic liquid crystals, a different class of liquid crystals is called lyotropic. These are mixtures of rod-like molecules in an isotropic solvent and the concentration of the solution is primarily responsible for the occurring phase. Lyotropic liquid crystals are mainly of interest in biological applications and exhibit a considerable number of different phases. In our research, only thermotropic liquid crystal are examined.

The ordering of the liquid crystal molecules may seem strange, but in our daily environment similar arrangements are common as the pictures in this link illustrate. In the rest of the tutorial pages some interesting physical and optical properties of liquid crystals are explained, limited to the nematic liquid crystal phase. Finally the principle of a liquid crystal display will be explained.

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Liquid crystal tutorial written by Chris Desimpel, based on the introduction of his Ph.D. thesis and literature.