Author: Nauka v Sibiri (Science in Siberia), 1999 Prepared by Arkady MALTSEV
Liquid crystals are anisotropic, or variable in their properties (in their optical characteristics in particular) because of the orderly orientation of their molecules. The useful potential of these substances is but little studied-this is a real Klondike for researchers specializing in this field. There are several types of these crystals such as nematic, cholesteric and smectic ones which differ by the position and orientation of their molecules. We shall now consider the first of these three, nematic crystals.
Back in 1888 the Czech chemist F. Reinitzer and his German colleague 0. Lehman took notice of an unusual state or condition of organic derivatives of cholesterol. The crystals of some of them possess two melting points and, accordingly, two liquid states-turbid and transparent. And later on it was Lehman who introduced a new term, "liquid crystals", because these substances possess the main characteristics of their solid counterparts-the anisotropy (variability) of different physical properties (thermal and electrical conductivity, optical parameters, etc.).
In the beginning, however, the discovery caused no sensation among experts and even aroused opposition among some prominent physicists and chemists. The reason for that was fairly simple-the so-called liquid crystals did not fit into the well-established classification according to which all materials had to fall into the three categories, or classes, of liquids, solids and gases. Nevertheless, the enthusiasts went on with their investigations so that the list of newly discovered wonder substances continued to grow at a rapid pace, and by the late 1920s there were more than one thousand of them. And it was then that an attempt was made to formulate a theory of liquid crystals.
An important role in this respect belonged to the Swedish researcher C. Ozeen (who laid the foundation of the present-day physics of liquid crystals) and the German scientists L. Homstein and G. Zoher. The theory formulated by the latter serves to explain practically all of the phenomena occurring with these substances.
A tangible contribution to the studies of liquid crystals was provided by Leningrad physicist V. Frederiks who conducted his studies at the Physico-Technical Institute under the guidance of Academician Abram Joffe (1880-1960). He demonstrated, both in theory and experimentally, that the orientation of nematic liquid crystals in a magnetic field occurs due to diamagnetic(*) anisotropy. And at some critical (threshold) value of tension all of their molecules turn around simultaneously, transforming a liquid crystal film from an opaque to a transparent state.
This work has since been regarded as a classical one and had a strong impact on the progress of research in this branch of physics. Subsequently the processes of reorientation of liquid crystals in magnetic and electric fields came to be known as Frederiks transitions. And the effect itself provided the basis for the development of data-display structures. In other words, if a "driving", or control, field is displayed on a screen in the form of digits, letters or other symbols, when the set-up is switched on, its liquid-crystal array will
* Diamagnetism-property of material to magnetize in an external magnetic field in a direction opposite to that of the field.- Ed.
generate an exact image of what "was seen".
The studies of Prof. Frederiks laid the foundation of what came to be known as the Leningrad school of liquid-crystal studies. In the subsequent years work in this field was continued in many cities of the former Soviet Union, including Moscow, Kiev, Kharkov, Novosibirsk and Krasnoyarsk. And the 1980s saw a real boom in industrial applications of these substances. They gained preference over all other methods of data display thanks to the small size of such devices, low energy consumption, ease of handling and the low costs involved. Liquid crystal data displays became the chief rival of the clumsy cathode-ray tubes in television and computer technology.
Specialists working in this field came up with a range of new electro-optical materials-what are called liquid-crystal composites which are fine polymer films with droplets of liquid crystals (several microns in size) dispersed within them. This innovation opened up the prospect of producing a new generation of "rapid-action" displays and, later on, TV screens.
What we here call the Krasnoyarsk school of liquid-crystal research originated in the Molecular Spectroscopy Lab of the Kirensky Institute of Physics of the Siberian Branch of the USSR Academy of Sciences in 1957. In the late 1980s local experts working on the development of rapid-action optical electronic devices launched research into new composites. In 1991 they produced what were called ferroelectric(*) liquid-crystal materials (these pioneering studies still retain their priority status among the international scientific community(*)).
Another important contribution came from a team of young scientists at the Krasnoyarsk Research Center of the Siberian Branch of the Russian Academy of Sciences. Thus a research work of Vladimir Presnyakov and Alexander Shabanov - Frederiks Transitions in Bipolar Droplets of Nematics-has received high appraisal. The two scientists studied Frederiks transitions in a nematic material designated as 5 TSB dispersed in polyvinyl butyral(**). Since the molecules of the former have a rod- like form, their reorientation can be easily observed under a polarized microscope, and microphotos and video recordings thereof can also be made if need be. In their experiments the Krasnoyarsk researchers placed a fine polymer film sample between two glass plates with transparent electrodes located inside. The experimental set-up made it possible to change magnetic field strength, thickness of the composite and the technique of its preparation.
These studies have shown: in the composite thus observed threshold reorientation does not occur in its pure form. According to the classical model of Frederiks transition in liquid-crystal drops a simultaneous turn-around of all molecules can take place only under critical stress. And in the given case this was observed only where molecule axes were perpendicular to the direction of the electric field, while in the rest of the droplet volume there was random reorientation.
The above experiment also revealed the peculiarities of the passage of light through a composite film under the effect of the stress applied. In the classical version the characteristics of this parameter are sigmoid. But the Krasnoyarsk researchers saw a distinctly oscillating form thereof. They think this effect to be of purely interferential nature. And so they have made theoretical calculations of this phenomenon and obtained its numerical values which are in good agreement with the experimental data.
All of the above, however, applies to 10-30 u droplets. For thinner samples (1-3 u), even if they are stacked, no interference effects are observed.
The Krasnoyarsk studies have made it possible not only to obtain the main characteristics of the composite material, but also find a way of enhancing its contrast range (here: the ratio of the optical parameters of the material, such as translucence in the "activated" and "deactivated" condition of the material). This can be done by varying film thickness and the size of liquid-crystal droplets so as to improve the quality of the image on displays and TV screens.
The research done by the young Siberian physicists is another step forward in the investigations of what seem to be well-known materials. And since what we call highly organized biological matter, such as cell membranes, has a lot in common with liquid crystals, further research in this field should ultimately bring us closer to unlocking the mysteries of living nature.
* Ferroelectrics- substances which, in the absence of an electric field, possess spontaneous electrical polarization, strongly dependent on external conditions, in a definite temperature range. - Ed.
* See more in: S. Pshirkov, "Science Center in Krasnoyarsk", Science in Russia, No. 2, 1999. - Ed.
** Polyvinyl butyral-synthetic polymer product of interaction of polyvinyl alcohol with butyraldehyde (solid colorless material).-Ed.
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