Liquid Crystals
Liquid Crystals
In 1888 the Austrian botanist and chemist Friedrich Reinitzer, interested in the chemical function of cholesterol in plants, noticed that the cholesterol derivative cholesteryl benzoate had two distinct melting points. At 145.5°C (293.9°F) the solid compound melted to form a turbid fluid, and this fluid stayed turbid until 178.5°C (353.3°F), at which temperature the turbidity disappeared and the liquid became clear. On cooling the liquid, he found that this sequence was reversed. He concluded that he had discovered a new state of matter occupying a niche between the crystalline solid and liquid states: the liquid crystalline state. More than a century after Reinitzer's discovery, liquid crystals are an important class of advanced materials, being used for applications ranging from clock and calculator displays to temperature sensors.
Mesophases
In a crystalline solid, the molecules are well ordered in a crystal lattice . When a crystal is heated, the thermal motions of the molecules within the lattice become more vigorous, and eventually the vibrations become so strong that the crystal lattice breaks down and the molecules assume a disordered liquid state. The temperature at which this process occurs is the melting point . Although the transition from a fully ordered structure to a fully disordered one takes place in one step for most compounds, this
transition is not a universal behavior. For some compounds, this process of diminishing order as temperature is increased occurs via one or more intermediate steps. The intermediate phases are called mesophases (from the Greek word mesos, meaning "between"), or liquid crystalline phases. Liquid crystalline phases have properties intermediate between those of fully ordered crystalline solids and liquids. Liquid crystals are fluid and can flow like liquids, but the magnitudes of some electrical and mechanical properties of individual liquid crystals depend on the direction of the measurement (either along the main crystal axis or in another direction not along the main axis). Typical liquid crystals have rodlike or disklike shapes. Classes of liquid crystalline states or mesophases can be distinguished according to degrees of internal order.
The least ordered liquid crystalline phase for rodlike molecules is the nematic phase (N), in which the long axes of individual molecules have an approximate direction (which is called the director, n). A nematic phase material has a low viscosity and is therefore very fluid. The term "nematic" is derived from the Greek word for thread (after the threadlike microscopic textures exhibited by nematic phase substances). In the smectic phases, the molecules have more order than molecules existing in the nematic phase. Just as in the nematic phase, the molecules have their long axes more or less parallel to the director. Additionally, the molecules are more or less confined to layers. The term "smectic" is derived from the Greek word for soap (owing to the fact that smectic liquid crystals have mechanical properties similar to those of concentrated aqueous soap solutions). The smectic phases are divided into classes based on degree of molecular order; the smectic A phase (SmA) and the smectic C phase (SmC) are the most studied ones. In the SmA phase, the molecules are perpendicular to the smectic layer planes, whereas in the SmC phase they are tilted. Substances assuming these phases have some fluidity, but their viscosities are much higher than that of a nematic phase substance. In Figure 1, the arrangements of molecules in the nematic, smectic A, and smectic C phases are shown schematically. Chiral molecules (molecules lacking a center of symmetry) can assume a cholesteric phase, also called a chiral nematic phase. In this mesophase, the molecules have helical arrangements. The pitch is the distance along a longitudinal axis corresponding to a full turn of the helix .
Typical mesophases formed by disklike molecules are columnar, wherein the molecules are "stacked" into columns. The columns are in turn packed together to form two-dimensional arrays. In addition to the columnar arrangements, the molecules can become ordered in a way that is comparable to a heap of coins spread on a flat surface—the discotic nematic phase.
Displays
By far the most important application of liquid crystals is display devices. Liquid crystal displays (LCDs) are used in watches, calculators, and laptop computer screens, and for instrumentation in cars, ships, and airplanes. Several types of LCDs exist. In general their value is due to the fact that the orientation of the molecules in a nematic phase substance can be altered by the application of an external electric field, and that liquid crystals are anisotropic fluids, that is, fluids whose physical properties depend on the direction of measurement. It is not pure liquid crystalline compounds that are used in LCDs, but liquid crystal mixtures having optimized properties.
The simplest LCDs that display letters and numbers have no internal light source. They make use of surrounding light, which is selectively reflected or absorbed. An LCD is analogous to a mirror that is made nonreflective at distinct places on its surface for a certain period. The main advantage of an LCD is low energy consumption. More advanced LCDs need back light, color filters, and advanced electronics to display complex figures. The best-known LCD is the so-called twisted nematic display.
Liquid Crystal Thermometers
The use of liquid crystals as temperature sensors is possible because of the selective reflection of light by chiral nematic (cholesteric) liquid crystals. A chiral nematic liquid crystal reflects light having a characteristic wavelength determined by its pitch and by the viewing angle (the angle between the eye of the observer and the surface of the liquid crystal). Because the pitch of a chiral nematic compound is temperature-dependent, observed color is a function of temperature. Liquid crystals can therefore serve as thermometers. By mixing chiral nematic compounds, thermometers can be customized to be effective in a desired temperature range. The color variation of some liquid crystal thermometers extends across the entire visible light spectrum within changes of a few tenths of a degree centigrade. For use in devices, microcapsules containing chiral nematic mixtures are mixed with binder materials. Liquid crystal thermometers find application in medicine (medical thermography). A liquid crystal thermometer attached to the skin can measure temperature variations of the skin. This can be useful in the detection of skin cancer, as tumors have different temperatures than surrounding tissues. In electronics, liquid crystal temperature sensors can pinpoint bad connections within a circuit board by detecting the characteristic local heating. The color changes of gadgets such as "mood rings" are a manifestation of chiral nematic mixtures.
see also Inorganic Chemistry.
Koen Binnemans
Bibliography
Collings, Peter J. (1990). Liquid Crystals: Nature's Delicate Phase of Matter. Princeton, NJ: Princeton University Press.
Collings, Peter J., and Hird, Michael (1997). Introduction to Liquid Crystals: Chemistry and Physics. London: Taylor and Francis.
Demus, Dietrich; Goodby, John; Gray, George W.; et al., eds. (1998). Handbook of Liquid Crystals, Vols. 1–3. Weinheim, Germany: Wiley-VCH.
Liquid Crystals
Liquid Crystals
Liquid crystals are pure substances in a state of matter that shows properties of both liquids and solids over a specific temperature range. At temperatures lower than this range, the liquid crystals are only like solids. They do not flow and their molecules maintain a regular arrangement. At temperatures above this range, the liquid crystals behave only like liquids. They can flow and the molecules have no special arrangement. Within the temperature range, different for every liquid crystal, liquid crystals are able to flow but they still keep their molecules in a specific arrangement.
The molecules of liquid crystals are usually much longer than they are wide. When light waves pass through these molecules, the speed of the light depends on whether it is traveling along the short direction or along the long direction. Depending on the specific liquid crystal, one direction will be faster than the other. The direction in which a light wave vibrates is called its polarization. When the light wave emerges from the liquid crystal, the direction of polarization may have been changed due to the difference in light speed along different directions. Human eyes cannot detect the direction of light polarization but a device called a polarizer can. Many of the first liquid crystals discovered were chemically made from cholesterol and showed this twisting effect. Cholesterol itself is not a liquid crystal, but any liquid crystal that shows this spiral, even if it not made from cholesterol, is still called cholesteric.
The cholesteric class of liquid crystals shows some color effects that do not require a polarizer to see. The twist of the spiral structure is very regularly spaced, almost like the steps of a spiral staircase. When white light falls on this spiral, most of it passes through. But white light is actually composed of many different colors of light waves. Light waves of different colors have different lengths. The length of a wave, called the wavelength, is measured from one point of the wave to another identical point. If the light wave is just the right length to match the regular spacing of the spiral, it will be reflected instead. So depending on the size of the helix spacing, only certain colors will be reflected. One way to control the size of the helix spacing is by choosing liquid crystals that twist a lot or a little from one layer to the next. Another way is by controlling the temperature. When a cholesteric helix is warmed, the layers twist a little more. This means that the regular spacing of the steps of the spiral is closer. The light waves that are reflected will be the short light waves that are blue in color. When the cholesteric is cooled, there is less twisting and a longer spacing, so longer light waves are
KEY TERMS
Light speed —How fast light gets from one place to another. The speed of light depends on the material through which it must travel.
Light wave —A way of picturing the energy in light as a wiggling rope.
Molecule —A combination of atoms. Molecules are the smallest units of compounds.
Polarization —The direction of vibration or “wiggle” of a light wave.
Polarizer —A device that allows only one direction of light vibration to pass through it.
State of matter —The condition of being a gas, liquid, or solid.
Wavelength —The distance between two consecutive crests or troughs in a wave.
reflected. Long light waves are red. This is the mechanism that makes liquid crystal thermometers work—you see red when the liquid crystals in the thermometer are cool, then yellow, green, and blue as they are warmed.
The most important use of liquid crystals is in displays because the molecules of a liquid crystal can control the amount, color, and direction of vibration of the light that passes through them. This means that by controlling the arrangement of the molecules, an image in light can be produced and manipulated. Liquid crystal displays, or LCDs, are used in watch faces, laptop computer screens, camcorder viewers, virtual reality helmet displays, and even television screens.
Current research in liquid crystals is focused on mixing liquid crystals with other materials like polymers. Scientists hope to make mixtures for liquid crystal displays so that these displays can show more detail, more color, and change image faster, but use less energy.
Resources
BOOKS
Oswald P. and P. Pieranski. Nematic and Cholesteric Liquid Crystals. Boca Raton, FL: CRC Press, 2005.
Wu, S.-T. and D.-K. Yang. Fundamentals of Liquid Crystal Devices. Chichester, United Kingdon: Wiley, 2006.
Eileen M. Korenic
Liquid Crystals
Liquid crystals
Liquid crystals are pure substances in a state of matter that shows properties of both liquids and solids over a specific temperature range. At temperatures lower than this range, the liquid crystals are only like solids. They do not flow and their molecules maintain a regular arrangement. At temperatures above this range, the liquid crystals behave only like liquids. They can flow and the molecules have no special arrangement. Within the temperature range, different for every liquid crystal , liquid crystals are able to flow but they still keep their molecules in a specific arrangement.
The molecules of liquid crystals are usually much longer than they are wide. You can think of them like pencils. When light waves pass through these molecules, the speed of the light depends on whether it is traveling along the short direction or along the long direction. Depending on the specific liquid crystal, one direction will be faster than the other. Imagine the light wave as a wiggling rope. The direction of wiggle or vibration is called the polarization of the light wave. When the light wave emerges from the liquid crystal, the direction of polarization may have been changed due to the difference in light speed along different directions. Our eyes can not detect the direction of light polarization but a device called a polarizer can. Many of the first liquid crystals discovered were chemically made from cholesterol and showed this twisting effect. Cholesterol itself is not a liquid crystal, but any liquid crystal that shows this spiral, even if it not made from cholesterol, is still called cholesteric.
The cholesteric class of liquid crystals shows some color effects that do not require a polarizer to see. The twist of the spiral structure is very regularly spaced, almost like the steps of a spiral staircase. When white light falls on this spiral, most of it passes through. But white light is actually composed of many different colors of light waves. Light waves of different colors have different lengths. The length of a wave, called the wavelength, is measured from one point of the wave to another identical point. If the light wave is just the right length to match the regular spacing of the spiral, it will be reflected instead. So depending on the size of the helix spacing, only certain colors will be reflected. One way to control the size of the helix spacing is by choosing liquid crystals that twist a lot or a little from one layer to the next. Another way is by controlling the temperature. When a cholesteric helix is warmed, the layers twist a little more. This means that the regular spacing of the "stairs" of the spiral is closer. The light waves that are reflected will be the short light waves which are blue in color. When the cholesteric is cooled, there is less twisting and a longer spacing, so longer light waves are reflected. Long light waves are red. This is the mechanism that makes liquid crystal thermometers work—you see red when the liquid crystals in the thermometer are cool, then yellow, green, and blue as they are warmed.
The most important use of liquid crystals is in displays because the molecules of a liquid crystal can control the amount, color, and direction of vibration of the light that passes through them. This means that by controlling the arrangement of the molecules, an image in light can be produced and manipulated. Liquid crystal displays, or LCDs, are used in watch faces, laptop computer screens, camcorder viewers, virtual reality helmet displays, and even television screens.
Current research in liquid crystals is focused on mixing liquid crystals with other materials like polymers. Scientists hope to make mixtures for liquid crystal displays so that these displays can show more detail, more color, and change image faster, but use less energy .
Resources
books
Chandrasekhar, S. Liquid Crystals. 2nd ed. Cambridge University Press, 1992.
Collings, Peter J. Liquid Crystals: Nature's Delicate Phase of Matter. Princeton University Press, 1990.
De Gennes, P.G., and J. Prost. The Physics of Liquid Crystals. 2nd ed. Oxford Science Publications, 1993.
Eileen M. Korenic
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Light speed
—How fast light gets from one place to another. The speed of light depends on the material through which it must travel.
- Light wave
—A way of picturing the energy in light as a wiggling rope.
- Molecule
—A combination of atoms. Molecules are the smallest units of compounds.
- Polarization
—The direction of vibration or "wiggle" of a light wave.
- Polarizer
—A device that allows only one direction of light vibration to pass through it.
- State of matter
—The condition of being a gas, liquid, or solid.
- Wavelength
—The distance between two consecutive crests or troughs in a wave.