Cholesteric liquid crystal is a special form of nematic liquid crystal. Such liquid crystal molecules are flat and arranged in parallel to each other into a layered structure by virtue of the interaction of the end groups. In each plane layer, the long axis of the molecule is parallel to the plane of the layer, and the long axis of the molecule between the layers is gradually deflected and arranged in a spiral structure along the normal direction of the layer. In cholesteric liquid crystals, the orientation of the long axis of the molecule is restored after being rotated 360°, and the distance between the two nearest layers with the same orientation is called the pitch. The thread pitch of cholesteric liquid crystals is about 300nm, which is the same order of magnitude as the wavelength of visible light. This thread pitch will vary with the external temperature and electric field conditions. Therefore, the external light can be modulated by adjusting the thread pitch. In addition, most of the cholesteric liquid crystals are derivatives of cholesterol, which are usually chiral, and therefore have extremely high optical rotation. The spiral plane selectively reflects light and can scatter white into brilliant colors. According to the properties of cholesteric liquid crystals, it can be used in detection, display and other fields.
Figure 1. Internal arrangement of cholesteric liquid crystal molecules
- Metal detector: The pitch of cholesteric liquid crystals is very sensitive to temperature. When the pitch is consistent with the wavelength of light, strong and selective reflection occurs. Under sunlight, as the temperature increases, the color changes in the order of red, orange, yellow, green, blue, and purple, and when the temperature decreases, the color changes in the reverse order. The entire chromatogram can be displayed within a temperature difference of less than one degree, with high sensitivity. Using this temperature effect, it can be used as a metal detector to detect the internal structure quality of metal materials and parts, detect short circuits in microelectronics, check the heat leakage of refrigerators, and diagnose diseases.
- Display technology: Cholesteric liquid crystals have bistable characteristics, which are the basis for display technology. In the case of applying external electric fields of different strengths, the helical structure of the cholesteric molecule changes and presents a bright and dark state respectively. In the bright state, the liquid crystal molecules are in a planar state, and the spiral axis of the liquid crystal molecules is perpendicular to the surface of the display substrate. At this time, if the spiral pitch is close to or equal to the wavelength of the incident light, and the Bragg reflection condition is satisfied, the incident light of this wavelength will be reflected. On the contrary, if the spiral axis is substantially parallel to the surface of the display substrate in the dark state, when the liquid crystal molecules are arranged irregularly, the incident light is scattered or absorbed by the absorption layer on the surface of the lower substrate, and no light is emitted. In this way, the reflective display technology is realized without a backlight and a polarizer. If it is necessary to obtain a color display, the reflection of light of different wavelengths can also be obtained by adding optical rotators with different pitches to achieve color display. Such low-power color display technology can be widely used in flexible display fields such as flash drives, e-book readers, packaging and labels.
- Michel Mitov. Cholesteric liquid crystals in living matter [J]. Soft Matter, 2017,13, 4176-4209.