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.