Liquid Crystals

Type of physical science: Chemistry

Field of study: Chemistry of solids

Liquid crystals are substances in an intermediate state that exhibits properties of both liquids and solids. They are produced from chemicals with greatly ordered crystal arrangements and have applications in many aspects of science, medicine, and industry.

Overview

Liquid crystals are often defined as being a state form of a substance that possesses properties of matter in both liquid and solid states. Usually, matter is viewed as existing in one of four state forms—solid, liquid, gas, or plasma—depending on the conditions under which it is studied. In the solid state, substances are very orderly, with their particles (molecules, atoms, or ions) arranged in highly organized ways. Particles in solids can vibrate about fixed positions, but they cannot rotate. The liquid state is less orderly; particles are able to rotate as well as vibrate. Particles in liquids still possess some organization, however. The gas state is even more disordered, and its disorganized constituent particles are widely separated from one another. The plasma state is similar to the gas state but is mainly defined as containing charged particles (free electrons and ionized atoms or molecules) that interact with one another and add up to no net electrical charge.

Heating a solid material increases particle motion because energy is being added. Eventually, after the addition of enough heat energy, most substances become liquids as their particles leave the fixed positions necessary for retention of the solid state. Addition of still more heat energy to a sample of most liquids converts it to the gas state. Removal of the heat energy via cooling reverses the process. Gases can usually be caused to pass from the gas state to the liquid state to the solid state by appropriate lowering of the temperature at which they are being studied. The transition of gas to plasma and back to gas operates by a slightly different mechanism, namely ionization of the gas and then deionization of the plasma.

The arrangement of particles of any material in the gas, solid, liquid, and (to a certain extent) plasma states determines certain of its characteristic physical properties. For example, gases and plasmas, with their disorganized particle arrangement, have neither fixed volumes nor fixed shapes and tend to expand to fill any container into which they are placed, although plasma particles also exhibit organized collective behavior. Liquids, being more ordered, tend to have fixed volumes, but their level of particle organization is not sufficient to give them fixed shapes, and they can be poured into any container, taking on the shape of that container. Solids are so highly organized that they possess fixed shapes and cannot be poured.

A common, everyday example of a substance that is often converted from the solid to the liquid to the gas state is water. Solid water, or ice, has a fixed shape and cannot be poured. When it is melted, it forms liquid water, which has a fixed volume but no definite shape. When boiled, water is converted to steam, the gas form of water. Steam can be converted first to liquid water and then to ice by cooling it (removing heat energy).

Liquid crystals involve only the liquid and solid states. The particles of many solids are arranged in very regular, repeating patterns. These solids are called crystalline solids and are said to exist as crystals. Usually, crystals are converted into liquids very smoothly by heating. In many organic chemicals, however, and in a few inorganic ones, the tendency toward an ordered arrangement of particles is so great that their crystalline form does not melt directly to a liquid form. Rather, it first passes through an intermediate form that is neither a liquid nor a crystal but possesses properties of both these state forms of matter. When this happens, the substance is said to exist in the liquid crystal state.

Substances in the liquid crystal state have a paracrystalline structure. They may pour like liquids, but they can be shown to be in a semiordered state intermediate in organization between that of liquid disorder and crystal order. Because myriad organic chemicals and many chemicals of bioorganic chemical importance can be liquid crystals, the liquid crystal state is quite important.

Liquid crystals tend to occur in those chemical compounds whose molecules are very unsymmetrical and exist as rigid, rodlike entities. There are two main types of liquid crystals: thermotropic liquid crystals, produced by heating any substance capable of attaining a paracrystalline state; and lyotropic liquid crystals, produced by mixing several components, such as an appropriate organic molecule and a polar solvent, like water. Thermotropic liquid crystals, often considered the most important group of paracrystalline materials, are divided into three classes according to their structural characteristics: ordinary nematic, smectic, and cholesteric nematic. Lyotropic liquid crystals will not be discussed here.

Ordinary nematic liquid crystals are the least ordered type of liquid crystal, making them the most like a true liquid. Chemicals in the ordinary nematic state are arranged so that the long axes of all of their particles are parallel but are free to slide or roll around. Heating an ordinary nematic liquid crystal usually converts it to a liquid quite easily. The term "nematic" comes from the Greek word for "thread."

Smectic liquid crystals are more ordered than ordinary nematic crystals in that their particles are arranged in individual layers whose long axes are parallel to one another and perpendicular or otherwise inclined to the plane of the layer. The layers of smectic liquid crystals can slide past one another, but individual particles cannot move out of the plane of their layer. Because of this, smectic liquid crystals tend to be more like solids than ordinary nematic liquid crystals. This makes them more opaque and viscous than they would be in the ordinary nematic state. The term "smectic" comes from the Greek word for "cleansing," due to its soaplike consistency.

In cholesteric nematic liquid crystals, the particles are arranged in layers with their long axes parallel to one another and to the plane of the layer. The alignment of each layer is rotated by a fixed angle out of line with the layers immediately above and below it, giving cholesteric liquid crystals a long-range spiral arrangement. The term "cholesteric" comes from the Greek words for "stiff bile," and the first such liquid crystals were derived from the biochemical cholesterol.

The electrical and optical properties of liquid crystals make them quite useful. For example, an electrical current can realign the particles of a nematic liquid crystal and make it appear opaque. This effect is used in watches and other electronic instruments that possess a liquid crystal display. In such cases, an electrically generated symbol is seen in black against a cloudy ordinary nematic background that has not been realigned in this way. Another interesting property of liquid crystals is due to the fact that the spiral nature of cholesteric nematic liquid crystals is changed by heating them. This makes the crystals change color when heated, leading to many useful practical applications.

Applications

As has already been stated, the ordinary nematic liquid crystals have many very useful applications as liquid crystal displays (LCDs) in watches, calculators, televisions, computer screens, and many other manufactured items. LCDs are popular largely due to their low power consumption and their clarity when viewed in bright light.

LCDs are made by sealing a thin layer of the appropriate liquid crystal between two supporting layers of a transparent solid, often glass, on the surfaces of which thin, transparent metal electrodes have been formed. The display is operated by applying an electrical charge to a given area of the electrodes, causing the liquid crystal particles in that area to align in such a way that they look darker. The images that appear in LCDs are created with an oscillating circuit that sends pulses to a segmented pattern etched into the surfaces of the electrodes. The image seen depends on which of the pattern segments are activated at any given instant. LCDs are monochromatic, but they can be made to display color by adding red, green, and blue filters.

The visualizations of many LCDs use light polarization. Transparent plates on either side of the liquid crystal are polarized, or polaroid, plates like those in polarized sunglasses. The plates are crossed (placed at right angles to one another), and the whole assembly is situated in front of a mirror. Without the liquid crystal used, the display would be dark everywhere, because none of the light reflected from the mirror would be able to pass through the plates. However, the liquid crystal is arranged in the display so that it changes the direction of light polarization and allows reflected light to reach the eye of the viewer.

There are two main types of LCD: passive matrix and active matrix. Passive-matrix LCDs were used in the earliest laptop computer displays and are still used in less powerful devices, such as watches and calculators. Active-matrix LCDs were developed in the 1970s and feature thin-film transistors attached to each electrode, one for each pixel in the display. The primary difference between passive-matrix and active-matrix LCDs is that in the former, only one row of pixels can be addressed at a time, meaning that each pixel must maintain its current state until the display is refreshed and addressing one pixel tends to affect nearby pixels as well; in the latter, the transistors allow each pixel to be controlled directly, meaning that individual pixels can be refreshed as needed without affecting the others. This results in active-matrix LCDs having a much faster response time and better image quality than passive-matrix LCDs, but they are also more expensive and time-consuming to construct.

Another fascinating application of liquid crystals is related to the changes of color seen when the temperature of cholesteric nematic liquid crystals is increased or decreased. The color of any such paracrystalline form at a given temperature results from the fact that it behaves like a diffraction grating, reflecting light of a particular color because of the spiral nature of its particle arrangement. As the temperature of such a liquid crystal is changed, the shape of the spiral changes, producing diffraction-grating-like arrangements that will reflect light of different colors. This property of cholesteric liquid crystals has led to their use in adhesive-backed temperature-indicator tapes, used to identify body temperature or to monitor the temperature of machines, which provides an early indication of whether the machine is beginning to overheat dangerously. Cholesteric nematic liquid crystals are also used in so-called mood rings. These rings contain thin layers of appropriate paracrystalline chemicals that change color depending on skin temperature, purportedly an indicator of a person's moods (for example, cold when frightened and warm when excited). Such liquid crystals are also used in the film thermometers often used to take human body temperature and for monitoring the temperature of aquariums in homes and pet shops.

Context

The first liquid crystal was discovered in 1888 by the German scientist Friedrich Reinitzer. Reinitzer observed that an organic chemical, cholesteryl benzoate, changed from a crystalline solid to a material in a turbid, cloudy state when it was heated. This form of the chemical, which is now called a cholesteric liquid crystal, changed to a clear liquid on further heating. When cooled, the liquid changed back to the liquid crystal state and then to a crystalline solid at appropriate temperatures. Reinitzer also noted that the chemical changed color from red to blue when heated and that the color change was reversed upon cooling.

About half of all known organic chemicals become liquid crystals when heated, with consistencies ranging from free-flowing liquids to semisolids. At first, liquid crystals were curiosities that blended the mechanical properties of liquids with the optical properties of crystalline solids. Then, near the middle of the twentieth century, it became clear that they had many properties of use to industry and to medicine. One of the best known of these properties, their utility in digital displays, is discussed at length above.

A second useful property of liquid crystals is the ability of cholesteric nematic liquid crystals to change color when subjected to changed temperature, pressure, electrical field strength, or magnetic field strength. This type of liquid crystal has numerous uses, including medical diagnosis. Paintlike solutions that contain appropriate liquid crystals are applied to people's bodies for use in identifying abnormal body temperatures, which can aid in diagnosis. One example of such a diagnosis is identification of blocked surface blood vessels. This type of methodology very often yields more sensitive, detailed, and easily interpreted data than more conventional types of thermometers. In industry, such temperature-induced color changes have also been used to identify surface defects in metal parts and castings. Here, the object to be tested is again painted with an appropriate cholesteric liquid crystal. The differences in the surface distribution of temperature allow the identification of small imperfections in part or casting preparation via color differences. This testing is particularly useful because it is nondestructive.

In 2011, the Crystal Diagnostics company introduced a liquid crystal biosensor for use in detecting pathogens in food. The device works by combining a liquid crystal matrix with two polarized filters. First, the sample to be tested is mixed with various agents that bind to harmful microbes, and this mixture is introduced to the liquid crystal matrix. The matrix is placed between the filters, which are oriented with their polarization perpendicular to one another, and a light source is introduced. If no pathogens are present, light waves oriented in the direction of the first filter pass through the liquid crystal uninterrupted but cannot pass through the second, perpendicular filter. However, if pathogens are present, they bind with the appropriate antibodies and form aggregates, which distort the liquid crystal matrix and thus cause the polarized light to bend. Some of these light waves are realigned with the second polarizer and pass through unimpeded, creating a spot of light that indicates the presence of the pathogens.

Liquid crystals, once a curiosity, have become important in many aspects of science, medicine, and industry as well as people's everyday lives. It is expected that they will become even more useful in the future and that many new uses for these fascinating chemicals will become evident.

Principal terms

CHOLESTERIC NEMATIC: a type of liquid crystal whose component particles are arranged in layers with their long axes parallel to one another and to the plane of the layer

DIFFRACTION GRATING: a polished glass or metal surface with many fine, parallel grooves cut into its surface that produces optical spectra by the diffraction of transmitted or reflected light

LIQUID CRYSTAL: a state form of a substance that possesses the properties of both a liquid and a solid crystal

LYOTROPIC: a type of liquid crystal produced by mixing several components, usually organic chemicals (such as fatlike molecules) and a polar solvent such as water

ORDINARY NEMATIC: a type of liquid crystal that is arranged so that the long axes of all component particles are parallel to one another but are free to slide or roll around

ORGANIC CHEMICAL: a chemical produced by the chemical reaction between carbon and another element

PARACRYSTALLINE: describes material whose structure is similar to that of a crystal but only features long-range crystalline ordering in one or two dimensions

POLAROID PLATES: plates made of transparent materials that only allow the passage of light waves vibrating in one direction; two polaroid plates placed at right angles to each other (crossed) do not allow any light to pass through them

SMECTIC: a type of liquid crystal that is arranged so that its particles form many individual layers, with the long axis of each particle parallel to all others in the layer and perpendicular or otherwise inclined to the plane of the layer

STATE FORM: the physical form in which a chemical exists under a given set of conditions, usually solid, liquid, gas, or plasma; some scientists suggest liquid crystals as another state form of matter

THERMOTROPIC: a type of liquid crystal that is obtained by heating a solid capable of attaining the paracrystalline state

Bibliography

Brady, James E., and John R. Holum. Fundamentals of Chemistry. 3rd ed. New York: Wiley, 1988. Print. This clear chemistry text, aimed at college science majors, does a good job of explaining the state forms of matter and presents some useful basic information on liquid crystals. Many fundamental concepts germane to this article are developed throughout the book, and it is a good source for readers wanting some technical information.

Brown, Glenn H., ed. Liquid Crystals. No. 2. 2 pts. London: Gordon, 1969. Print. This edited work contains the papers presented at the Second International Conference on Liquid Crystals in 1968. Topics include properties of liquid crystals, liquid crystals in biology, synthetic work in the field, and the use of liquid crystals in magnetic resonance. A very extensive list of references is included throughout this valuable reference work.

Brown, Glenn H., J. W. Doane, and Vernon D. Neff. A Review of the Structure and Physical Properties of Liquid Crystals. Cleveland: CRC, 1971. Print. This technical book reviews the structure and physical properties of thermotropic and lyotropic liquid crystals, covering topics such as molecular geometry, structural theory, thermodynamics, methods of liquid crystal examination, and special physical properties such as viscosity and birefringence. Best suited to readers who wish in-depth coverage. Almost three hundred references are included.

Choudhury, Pankaj Kr., ed. New Developments in Liquid Crystals and Applications. Hauppauge: Nova Sci., 2013. Print.

Hill, John W., Terry W. McCreary, and Doris K. Kolb. Chemistry for Changing Times. 13th ed. Boston: Pearson, 2013. Print. This chatty liberal-arts chemistry textbook is particularly useful for basic concepts related to the three states of ordinary matter and their interconversions. These concepts are dealt with in a pleasant, disarming fashion that will educate the beginning reader in the basic concepts covered. A good beginning for the development of a factual base.

Kwok, Hoi-Sing, Shohei Naemura, and Hiap Liew Ong, eds. Progress in Liquid Crystal Science and Technology: In Honor of Shunsuke Kobayashi's 80th Birthday. Singapore: World Scientific, 2013. Print.

Li, Quan, ed. Liquid Crystals beyond Displays: Chemistry, Physics, and Applications. Hoboken: Wiley, 2012. Print.

Moore, Walter J. Physical Chemistry. 5th ed. London: Longman, 1972. Print. This well-crafted physical chemistry textbook covers solids, liquids, gases, crystals, and liquid crystals at the level of the advanced chemistry student. It is most useful to those wishing a detailed explanation of the state forms of matter and their interconversions and a description of the properties of liquid crystals from the viewpoint of the physical chemist.

"New Liquid Crystal–Based Technology Rapidly Detects Multiple Pathogens in Food." Kent State University. Kent State U, 26 Oct. 2011. Web. 18 Dec. 2013.

Ostdiek, Vern J., and Donald J. Bord. Inquiry into Physics. 7th ed. Boston: Brooks, 2013. Print. This elementary physics text covers many aspects of the state forms of matter, electricity, and optics related to the applications of liquid crystals. The book is clear, simple, and very readable. Particularly useful is the explanation of polarization and its use in liquid crystal displays.