What are electrodes usually made of?

Liquid crystals

You have probably heard of so-called LCDs - these screens are used today in most cell phones, digital cameras, televisions, calculators and watches. But do you also know what they are made of and how they work?

LCD stands for "liquid crystal display", ie a screen that makes use of the properties of liquid crystal materials. The liquid crystalline state, as the name suggests, combines properties of liquids and crystals and was discovered in a cholesterol compound in 1888.

We now know more than 50,000 chemical compounds that do not change directly into a liquid state when they melt, but rather go through one or more liquid-crystalline phases. What all these molecules have in common is a relatively rigid, often rod-shaped structure, such as that found in pentylcyanobiphenyl (5CB), for example. In the solid state, these molecules are in a crystal lattice; in the liquid state, the rods are randomly aligned. In the intermediate liquid-crystalline phase, these rods are preferably arranged parallel to one another, which means that the properties of a liquid crystal differ from one another in the direction parallel to the rods and perpendicular to them.

Screens make use of the fact that, due to their chemical structure, the rods can also be aligned by applying an electric field. An LCD usually consists of two transparent electrodes made of indium tin oxide and an approximately 10 μm thin liquid crystal layer in between. The surfaces of the electrodes are textured, i.e. pretreated so that the rods align in one direction. By applying the two textured electrodes at a 90 ° angle to each other, the molecules are made to arrange themselves helically in the layer. In addition, two polarization filters are applied, the polarization of which is aligned in the same way as the rods. In this state, light can pass through the layer, since the plane of polarization of the light follows the screw structure. If you apply an electric field, the liquid crystal molecules align themselves parallel to the field and the polarized light can no longer pass through the layer. An intermediate layer with color filters and the division of the screen into pixels enables multicolored imaging.

Today, many other areas of application for liquid crystals are also being researched. In the Polymer Physics Group At the Department of Materials Science, for example, liquid crystals are simulated on the computer. These simulations help to understand the direction-dependent properties of liquids not only under the influence of an electromagnetic field, but also under the influence of currents and as a component of composite materials, and are generally used to test theoretical concepts for the description of anisotropic liquids. The Research group for soft materials couples liquid crystals with magnetic nanoparticles with the aim of developing artificial muscles or new concepts for data storage.

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