Thermotropic liquid crystal is a liquid crystal phase caused by temperature changes. From the order of molecular arrangement to distinguish the liquid crystal phase, thermotropic liquid crystal can be divided into three categories: nematic phase, cholesteric phase and smectic phase. Nematic liquid crystal means that the liquid crystal molecules remain parallel to each other along the long axis direction, but the position of the center of gravity is disordered and cannot form a layered shape. Nematic liquid crystal is a one-dimensional orderly arrangement, and the molecules can move up and down, left and right, and back and forth. Among them, the relative movement along the long axis direction will not affect the crystal phase structure, and has good fluidity. Smectic liquid crystals are the closest in structure to the solid crystal phase structure, the molecules are arranged in layers, and the long axes of the molecules in the layer are parallel to each other. However, the molecular center of gravity is disordered in the layer, and the long axis of the molecule is arranged vertically or obliquely to the layer. The molecules can interact back and forth, left and right in the layer, but cannot move between the upper and lower layers. Compared with nematic liquid crystals, smectic liquid crystals have higher viscosity, difficult to rotate molecules, and lower fluidity, so the two have their own emphasis in application.
Figure 1. Molecular arrangement of nematic liquid crystal and smectic liquid crystal
- Intelligent dimming glass: Smectic liquid crystals can be combined with ionic salts to form bistable liquid crystals. When a high-frequency voltage is applied, the liquid crystal molecules are driven to maintain the same orientation, but the ionic salt cannot be driven, and it is in a transparent state at this time. When a low-frequency voltage is applied, the ionic salt is driven, and the random movement disturbs the arrangement of the liquid crystal molecules, and it presents a foggy state at this time. Therefore, the transparency of the glass can be controlled by controlling the voltage, and it has been applied in the fields of outdoor projection, indoor partition, home decoration and so on. Intelligent dimming glass has excellent functions such as protection of personal privacy, projection, safety, environmental protection, and sound insulation.
- Display materials: The smectic liquid crystal display has a bistable display effect on the image, that is, the display is realized under the condition of applying an electric field, and the effect can be maintained for a long time after the electric field is removed. The smectic liquid crystal display is mainly a reflective display, and its reflection of light complies with Lambert's law of reflection and has a larger viewing angle. In addition, the molecular arrangement of smectic liquid crystals can also stay in various states with different light transmission conditions, so that various gray-scale displays can be realized and multi-stable characteristics can be realized. In addition to ordinary display applications, smectic liquid crystal materials can be mixed with high-scattering materials to improve contrast and be used in polarizing microscopes.
- Laser: Nematic liquid crystal tunable laser has a wide-band tuning range from near-ultraviolet to near-infrared, and has the advantages of small size, narrow line width and high optical efficiency. It has important applications in single-chip laboratories, medical diagnosis, dermatology and other fields.
- Jacques Prost, P. S. Pershan. Flexoelectricity in nematic and smectic‐A liquid crystals [J]. Journal of Applied Physics, 1976, 47, 2298.