Although they constitute only a small percentage of all organic compounds, the importance of liquid crystals is growing all the time. Their increasingly broad practical use and their occurrence in many systems, e.g. biological, make liquid crystal phases interesting not only to researchers from various fields. The result of the work of scientists carried out over the last decades is the synthesis of tens of thousands of liquid crystal compounds and the discovery of their extraordinary properties.
Characteristics of liquid crystals
The intermediate states between solids and liquids – mesophases – are called liquid crystals. Their particles have an elongated or disk-shaped form. Liquid crystals were discovered in 1888 by the German botanist F. Reinitzier. He was studying a compound called cholesterol benzoate. While heating this substance, he observed the transition of solid crystals into a cloudy liquid. Further increase in temperature resulted in a clear liquid. Subsequent research by Reinitzier and other researchers focused on the characteristics of the resulting transition state, i.e. liquid crystals.
Liquid crystals, due to the arrangement of their molecules, are divided into:
- Smectic phases, S – molecules are arranged in successive layers. Their axes are parallel to each other.
- Cholesteric phases, D – similarly to the smectic phase, the axes are parallel to each other. The particles are arranged in columns.
- Nematic phases, N – freely moving molecules, align themselves in specific directions in space. In the case of nematics, their centres of gravity are not ordered.
Liquid crystals combine the characteristics of both liquids (ability to flow) and solids (organization of the structure). This state is maintained in the case of a particular substance, only in a certain range of temperatures. Under the influence of even a slight change in electric current or temperature, their structure changes. Liquid crystal phases have very good optical properties. They exhibit linear and circular dichroism.
Methods of obtaining liquid crystals
Liquid crystal phases are possible only for substances whose molecules have the right structure – a strongly anisotropic shape, amphiphilicity. They are created in single- and multi-component systems. The main factor determining the obtaining of liquid crystals is a gradual change in temperature. It has great influence on the order of the phases appearing one after the other. This sequence can be presented as follows: crystal – smectic – nematic – cholesteric – isotropic liquid.
One way to obtain liquid crystals is to heat the solid crystalline form of a specific substance. The resulting crystals are called the thermotropic mesophase. Increasing the temperature does not melt the solid substance immediately, but it transforms its crystalline form into a liquid crystalline form. Further heating will lead to a liquid.
Another way is to dissolve molecules known to exist in liquid crystal phases in a suitable solvent. Such crystals are called the lyotropic mesophase.
The use of liquid crystals
Undoubtedly, liquid crystals are most commonly associated with displays. LCD (liquid crystal displays) use the phenomenon of optical birefringence. Cells in which liquid crystals are embedded are connected by electrodes. Controlling the resulting voltage allows the molecules to be arranged in order to obtain the effect of refracting light. Mixing liquid crystals with dyes results in (depending on the orientation of the molecules) the absorption of light of different wavelengths and it becomes possible to get a colour image. Compared to cathode ray tube models, an LCD screen can only operate at its maximum resolution in true resolution. This is forced by a fixed number of pixels. In addition, they do not have the flicker effect due to a lower refresh rate. The undoubted advantages of liquid crystal screens also include lower power consumption, production of a weaker magnetic field, and less harmful effects on the eyesight. LCD screens are also used in aviation instruments, calculators and electronic watches.
Other uses of liquid crystals:
- As additives to paints and emulsions they show the ability to change colour under the influence of temperature (tinted car windows, toys that change colour during bathing).
- Creams with liquid crystals are gaining popularity. They prevent water loss from the epidermis.
- Liquid crystals are used in some thermometers. They use the change of the colour of the reflected light depending on the temperature. A similar feature is used in temperature detectors.
- An interesting solution is the use of liquid crystals as logic gates. They are applied i.a. in optoelectronics, where so-called non-linear phenomena are used.