Liquid Crystals

From Chempedia

Liquid Crystals and LCDs By Ben Lodin, Shalini Latchman, Alan Lichtenheld, Meryl Larson

A liquid crystal lives up to its name by behaving as both a liquid and a solid. The molecules of liquid crystals move like a liquid, but maintain orientation as if they were solids. They are the technology behind how we view our information, our calculations, and our time. LCDs (liquid crystal displays) have unique properties, which are found in such devices as laptops, cell phones, calculators and watches, giving them qualitative advantages.

History "In 1888, while experimenting with the molecular weight of cholesterol, Austrian chemist Friedrich Reinitzer discovered that at 145.5°C the solid crystal melted into a cloudy liquid which existed until 178.5°C where the cloudiness suddenly disappeared, giving way to a clear transparent liquid"^[1]. With help from German physicist Otto Lehmann, Reinitzer realized the transparent liquid was similar to other liquids, but the cloudy liquid had characteristics unlike any of the three classic forms of a substance, which they named a "liquid crystal". RCA was the first company to use a liquid crystal display with intentions of commercial use. In 1968, they produced a digital clock as a prototype^[2]. Today, liquid crystal display technology is used for computer screens, compact disc players, thermometers, microwave ovens, mood rings, and digital clocks.

How LCDs Work Liquid crystals work extremely well in producing displays because of their highly predictable reactions with electricity and light manipulation^[3]. The displays work by layering many different materials and passing electricity through specific parts of the panel. The first layers necessary to an LCD are the glass substrates. Each layer of glass has microscopic grooves made with a transparent, conductive material on it to designate where the pixels will be. The substrates are laid out so the grooves are perpendicular to each other. Liquid crystal material is put between the two layers of substrate and then a polarizing film is placed on the outer-side of each substrate. The rows and columns on the substrates are connected to integrated circuits, which control the electrical currents that are sent through the columns and rows to activate a specific pixel. When the integrated circuit does allow an electrical current to pass through to a pixel, the voltage that the liquid crystals receive causes them to untwist. This untwisting of liquid crystals causes the light passing through the display to show at a different angle or intensity.^[4] Liquid crystals do not emit any light on their own. Some LCD's are reflective, which means they rely on other light sources to make their displays visible. An example of a reflective LCD would be a calculator, which is why using a calculator in a dimly lit room causes you to rotate your calculator to different angles to view the numbers. Other LCD's have built in fluorescent tubes either beside or behind the LCD. When there is a built in light source another layer is added to the LCD, which is called the diffusion panel. The diffusion panel is the back layer and ensures that the light is scattered evenly across the LCD. Also, temperature is a factor in the effectiveness of an LCD, because the state of liquid crystals is very indefinite. Liquid crystals are mostly considered to be liquid with a structured molecular system. However, when you change the temperature, it changes the state of the liquid crystals. If the temperature gets too hot the liquid crystals will be considered more liquid than solid, and if the temperature gets too cold the liquid crystals will be more solid than liquid. When the delicate balance of the liquid crystal's state is changed it causes the LCD to malfunction. This explains why cell phones or computers do not work well if they have been left in a car in the winter or during a hot summer day.

Phases of Liquid Crystal Liquid crystals can be divided into two main phases, the nematic phase and the smectic phase. The nematic phase resembles more of the liquid phase and is simpler. The liquid crystal molecules float as though they are liquid, but are uniform in direction. The smectic phase resembles the solid phase more than the liquid phase because the liquid crystals are layered. Within the layers they are free to move around, but are confined to that specific layer. The molecules tend to align together regardless of the confinement between layers. There are many variations of smectic phases; in fact there are already twelve that have been identified. Figure 2 Smectic Phase: Mesophases A (left) and C (right) Two very common examples are shown in Figure 2, the left picture of smectic A mesophase, and the right of smectic C mesophase[5]. As shown, in the smectic A mesophase, the liquid crystals orientate themselves perpendicular to the layers. While in the smectic C mesophase the crystals arrange themselves at an angle to the layers. The nematic phase is what makes LCD's possible. Figure 3 shows that in the nematic phase the liquid crystals twist in infinitesimally thin layers. While each layer's direction differs from the last, in each layer all liquid crystal molecules are aligned in the same way. Figure 3 Nematic Phase Future of Liquid Crystals and LCDS As discussed, liquid crystals are a combination of properties from both liquids and solids which allow it to control light passage through the orientation of its molecules. Using these special properties, LCDs have revolutionized the way we interact with technology. In addition, liquid crystal technology is less than 20 years old and it has a long life ahead. Liquid crystal displays are making forward strides in the quality and portability of information display. Furthermore, polymer liquid crystals (PLC) are being used to make high strength fibers like Kevlar. In the future, new materials will be created using PLCs' light-weight and strong properties[6]. Fiber optic media transfer will also benefit from switching from lithium niobate to tiny, low cost liquid crystal devices at the cable's tip[7]. With so many applications and such great potential, it seems the uses for liquid crystals are never-ending. As the amount of research and time spent working with liquid crystals increases, the technology will only develop and become more cost efficient. [edit] Footnotes ^ History and Properties of Liquid Crystals, http://nobelprize.org/physics/educational/liquid_crystals/history/ (accessed 9/26/2005) ^ Corning Display Technologies, http://www.corning.com/displaytechnologies/ww/en/discovery_center/advances_breakthroughs/index.aspx (accessed 9/36/2005) ^ How Does It Work Inc., How Do LCD's Work?, http://electronics.howstuffworks.com/lcd.htm (accessed 9/27/2005) ^  Encarta Encyclopedia 2004, Liquid Crystal ^ Phases of Liquid Crystals, http://plc.cwru.edu/tutorial/enhanced/files/lc/phase/phase.htm (accessed 9/26/2005) ^ Phases of Polymer Liquid Crystals, http://plc.cwru.edu/tutorial/enhanced/files/plc/plc_faze/plc_faze.htm (accessed 9/27/2005) ^ Scientists devise tiny liquid crystal devices for telecommunications, http://www.eurekalert.org/features/doe/2003-01/dnl-sdt012303.php (accessed 9/27/2005) Figure 1: Samsung LCD monitor, http://www.pcmag.com/category2/0,1874,1409036,00.asp?rsDis=Liquid_Crystal_Displays-Page001-26265 (accessed 9/27/2005) Figure 2 & Figure 3, http://www.mc2.chalmers.se/pl/lc/engelska/tutorial/lctypes.html (accessed 9/26/2005)