Who would have thought, more than a hundred years ago when these molecules were first studied, that this strange state of matter would become as critically important as it is today? Most households have calculators, cell phones, digital cameras and perhaps computer monitors containing an LCD or liquid crystal display. The compounds used in its creation are almost a contradiction in terms. Liquids flow, while crystals are solid! Liquid crystals are unique, in that they combine the physical and optical properties of both states of matter.

The molecules that make up liquid crystals are attracted to one another. However, these intermolecular forces have different strengths in different directions, because of the tubular shape of the molecules. At low temperatures, there is little thermal motion, and all the forces hold the molecules in a rigid lattice arrangement called a solid crystal . The crystal is composed of many layers of molecules, some of which are represented in the graphic below. Notice that the molecules are in definite positions and are oriented in a definite direction. Thus a crystal has positional as well as orientational order. (This and the next two illustrations were produced using the 'Bryce ' ray-tracer.)

As the substance is heated, thermal motion increases, and at a particular temperature there is enough energy to overcome the weak forces while still leaving the stronger forces in place. This sometimes results in the molecules taking up random positions within a layer, while still maintaining the overall structure. Here, the molecules have maintained most of their positional order, but have lost much of their orientational order. This is the liquid crystal state (image below). In this situation, the molecules within a layer can slide around one another, and the layers themselves may be able to slide over one another. These motions give this state some of the mobility that a liquid possesses.

As still more heat is added, there comes a point where the thermal motion is sufficient to break the remaining strong bonds between the molecules. Since there are now no long term attractions to hold the molecules in place, they bounce into completely random positions with neither positional nor orientational order. This is called the liquid state (image below).

All of the compounds photographed in this article are referred to as 'thermotropic', because they alter their properties when the temperature changes. There are many such compounds available. I have chosen cholesteryl benzoate (the first liquid crystal to be studied in 1888), cholesteryl stearate, cholesteryl laurate and three compounds labelled liquid crystal A, B and C. Unfortunately the last three were obtained twenty-five years ago, and there is no way of easily determining their structure.

The compounds are all white crystalline solids at room temperature, and have melting temperatures less than 200 degrees Celsius. In order to prepare a specimen, a very small quantity of the substance is placed on a slide and covered with a cover glass. An alcohol lamp is used to melt the solid. For many liquid crystal compounds it is important to study and photograph the results quickly. The liquid crystal state exists for a relatively short time (minutes) as the compound cools back to the solid crystal state. Initially, while in the liquid state, the 'crossed polar' observation results in a black field. (The liquid is of course isotropic.) As the liquid crystal state forms, extraordinary crystal forms are observed. (These are anisotropic.) After a few minutes, it is fascinating to watch as the solid crystal state slowly takes hold, resulting in a completely different crystal structure. (These crystals are also anisotropic.)

The next four images show the process of changing from the liquid crystal state to the solid crystal state. In each, you will observe circular forms which are growing larger at an alarming rate as I attempt to photograph the field. (Within several minutes, each field is entirely covered by these solid crystal circles which eventually grow together, obliterating the liquid crystal formations which can be seen outside of the circles.) The circular solid crystal formations are maintained until the slide is reheated. Researchers use specially adapted heating stages on their microscope to maintain the temperature within the liquid crystal range.

By using higher magnifications it is possible to observe interesting detail in the liquid crystal formations. The three images below show strange circular growths that frequently form in this phase. (You will have to study the edge of the last image carefully to see the beginnings of loops.)

Crossed polars were used in all photographs, but in some, compensators (l /4 and l) were used to alter the visual appearance of the crystal forms. Occasionally this results in the field having an almost three-dimensional appearance.

Liquid crystals produce patterns that are unlike any that I have observed with other compounds. The following photomicrographs show the diversity of forms produced by this unusual state of matter. Although the first is not colourful, it is one of my favorites. Many distinctive forms keep showing up as one looks at a multitude of slides, but this one was truly unique!

A Leitz SM-Pol microscope and Nikon Coolpix 4500 digital camera were used to take all of the photomicrographs in the article.

The low melting points of most liquid crystals make them easy to study under the polarizing microscope. I hope that the images shown in the article prompt you to have a look for yourself. These compounds are a truly fascinating field of study!

All comments to the author Brian Johnston are welcomed.

Link to further information about liquid crystals:

University of Wisconsin-Madison, 'Science is Fun in the Lab of Shakhashiri', Liquid Crystals.