Liquid crystal is a very familiar term to people today. Many computers, televisions, mobile phones, and large screens on the street use LCD screens to realize display functions.
At the end of the 19th century, chemists discovered that some solid organic compounds did not directly turn into liquid after heating, but fell between solid and liquid within a certain temperature range, and then turned into liquid at high temperatures. It is different from liquids that can flow at will, and from regular and orderly crystals. It has structural characteristics that are in between. Because it had both fluidity and crystal-like optical and electrical properties, it was thought to be a liquid crystal, referred to as liquid crystal for short. There is also a view that in addition to the three most common states of solid, liquid and gas, the two states of liquid crystal and plasma should also be added to the state of matter.
Now chemists have discovered that the molecules of compounds that can form liquid crystals often have specific geometric shapes. The common ones are long rod-shaped molecules with a large aspect ratio and polar groups. Within a certain temperature range, long rod-shaped molecules of liquid crystals do not flow at will like liquid molecules, but tend to align and flow parallel to each other in the same direction. The positional relationship between liquid crystal molecules is like a bunch of loose toothpicks, all nearly parallel to each other and facing in the same direction, but the head and tail are still uneven, not as strictly ordered as crystals.
The existence of polar groups in liquid crystal molecules causes an uneven distribution of charges within the molecule, causing the liquid crystal molecules to become polar. Therefore, the arrangement state of liquid crystal molecules will change due to the influence of the applied voltage, and then switch between transparent and opaque. Liquid crystal displays are made using this phenomenon. By injecting liquid crystal into the interlayer of two pieces of glass, and controlling the arrangement of liquid crystal molecules by applying an applied voltage, images of different shapes, light and dark can be output. Liquid crystal displays have the advantages of light and thin structure, low power consumption and low operating voltage, so the cumbersome cathode ray tube (CRT) displays were quickly eliminated and became one of the mainstream displays.
The orientation of liquid crystal molecules is very orderly, just like toothpicks packed in a round box. No matter how you shake, all toothpicks are oriented upward, although the position of each toothpick changes. Liquid crystals are similar to this. Although the organic molecules that make up the liquid crystal do not have a positional order, they have an orientation order.