What Is Liquid Crystal?

Some substances in the molten state or dissolved by the solvent, although losing the rigidity of the solid substance, have obtained the fluidity of the liquid, and retain the anisotropic orderly arrangement of some crystalline substance molecules, forming a combination of crystal And the intermediate state of some properties of the liquid, ^[1] This kind of oriented and ordered fluid existing in the process of conversion from solid to liquid is called liquid crystal. The definition is now relaxed to include substances that can be liquid crystalline phase at a certain temperature range and normal crystalline at lower temperatures. For example, a liquid crystal can flow like a liquid (fluidity), but its molecules are oriented and ordered (anisotropic) like a road. There are many different types of liquid crystal phases, which can be distinguished by their different optical properties, such as the phenomenon of birefringence. When using a polarized light source, different liquid crystal phases will appear with different textures when viewed under a microscope. Different textures in the texture contrast area correspond to different liquid crystal molecules. However, the molecules are well oriented. And liquid crystal materials may not always be in the liquid crystal phase (just as water can become ice or water vapor).

1850 Doctor of Prussia

Liquid crystal state ------ a mesomorphic state with long-range orientation order, partial position order or complete position disorder;

Mesomorphic state --- A state of matter between molecular solids with perfect three-dimensional, long-range position and orientation ordered solid crystals, and isotropic liquids, gases, and amorphous solids that lack long-range order. ;

Liquid crystal ------ A substance in liquid crystal state;

Crystal phase ------ long-term periodic position / shifted ordered phase;

Liquid phase --- a phase with no long-range period or orderly orientation;

Liquid crystal phase (intermediate phase) --- there is no long-range position order, but there is a long-range orientation order phase;

Thermally induced liquid crystal phase --- mesophase formed by heating solids and cooling isotropic liquids or by heating and cooling thermodynamically stable mesophases;

Lyotropic liquid crystal phase --- under the appropriate concentration and temperature conditions, by dissolving mesogenic compounds in a suitable solvent to form a mesophase;

Rod-shaped liquid crystal phase --- a liquid crystal phase formed by molecules or macromolecules of rod-like or slab-like molecular structure;

Columnar liquid crystal phase --- a phase formed by molecules stacked in a columnar shape;

Mesogenic compound --- a compound that can exist as a mesophase under suitable temperature, pressure and concentration conditions;

Rod-shaped liquid crystal --- a mesogenic compound composed of molecules of rod-like or slab-like molecular structure;

Discotic liquid crystal--a mesogenic compound composed of relatively flat, plate-like or lamellar molecules;

Cone-shaped or bowl-shaped liquid crystal--a mesogenic compound composed of molecules from a semi-rigid conical core;

Multi-vertical liquid crystals --- mesogenic compounds composed of molecules with an elongated rigid core and several flexible chains connected at its ends;

Dovetail liquid crystals --- mesogenic compounds composed of molecules with a slender rigid core and a flexible chain connected at one end and a branched flexible chain of the same length at the other end;

Mesogenic (liquid crystal) dimers, trimers, etc. --- mesogenic compounds composed of two, three, or more molecules connected to mesogen units, usually of the same structure

Plate-shaped liquid crystal --- a mesogenic compound composed of plate-shaped molecules;

Amphoteric liquid crystals --- compounds composed of molecules with opposite properties, namely hydrophilic and hydrophobic or lipophilic and lipophobic;

Bidirectional materials --- compounds that can exhibit thermally and lyotropic mesophases (liquid crystal phases). ^[1]

The nomenclature of LCD systems, like any other modern language, is still a very dynamic non-systematic language. Therefore, since the current naming system, many changes have been made to the terms that are currently acceptable, new notations have been introduced, and obsolete notations have been deleted. Because the naming system is in a state of constant change, all definitions and the corresponding notation can be changed. However, in some regions, the topic of naming has naturally become internationally accepted, except for notations that are not recognized by the (Academy of Sciences), while in other regions where research is still very active, changes in representations are very common . However, members of the International Liquid Crystal Association (ILCS) and the International Union of Theoretical and Applied Chemistry (IUPAC) are trying to create the first ever widely accepted naming system for liquid crystals. This description and insight is consistent with the ILCS and IUPAC proposals. ^[1]

When it is energized, it is turned on, and the arrangement becomes orderly, so that light can easily pass through; when not energized, it is arranged.

There are many types of liquid crystals, usually according to the center of the liquid crystal molecules

1. Influence of applied field on liquid crystal

Scientists and engineers are able to use liquid crystals for a variety of applications because interference from external electric fields can cause meaningful changes in the microscopic properties of liquid crystal systems. Both electric and magnetic fields can be used to induce these changes. The size of the external field is the same as the speed of its change, which is a very important characteristic in its application in industrial processing. Special surface treatments can be used in liquid crystal devices to give liquid crystals a specific orientation.

The electronic properties of the molecules cause the liquid crystal to have the ability to be oriented along an applied field. A permanent electric dipole causes a net negative charge to appear on the other end of a molecule when it has a net positive charge. When an external electric field is applied to the liquid crystal, the dipole molecules tend to be aligned in the direction of the electric field. Even if a molecule does not form a permanent electric dipole, it will still be affected by the electric field. In some cases, the applied field causes a slight rearrangement of the electrons and protons in the molecule. This is the result of the excited protons. Although not as strong as a permanent dipole, the orientation of the molecule along the applied field Will still happen.

The effect of a magnetic field on liquid crystal molecules is similar to that of an electric field, because a magnetic field is generated by a moving charge, while a permanent magnetic dipole is generated by an electron that moves around an atom. When a magnetic field is applied to the liquid crystal, the molecules tend to line up in the direction of the field or in the opposite direction.

2. The effect of surface treatment on liquid crystal

Without the effect of an external field, the liquid crystal molecules will be aligned in any direction. In any case, it is possible to introduce a specific orientation to the molecule by introducing an external effect on the system. For example, when a thin polymer coating (usually polyimide) is spread on a glass substrate and rubbed with a cloth in one direction, the liquid crystal molecules line up in the rubbing direction. The acceptable mechanism for this phenomenon is that people believe that the liquid crystal layer will be epitaxially grown near the surface of the polyimide layer on the partially aligned polymer chains.

3. The effect of chirality on liquid crystal

Chiral liquid crystal molecules usually produce a chiral liquid crystal phase. This means that the liquid crystal molecules have some asymmetry, such as creating a stereocenter. An additional condition for this property is that the system cannot be racemic (a mixture of left and right chiral molecules will offset the effect of chirality). However, due to the synergy of liquid crystal orientation, adding a small amount of chiral dopants to the achiral mesophase will cause the liquid crystal molecules to exhibit chirality.

Chiral phase molecules usually rotate helically. If the pitch of the rotation is similar to the wavelength of visible light, we will observe the interference effect of light waves. The chiral rotation of the chiral phase of the liquid crystal causes the system to emit different circularly polarized light to the left or right. This material can be used to make polarizing filters. ^[3]

The working principle of the blue phase liquid crystal is based on the Kerr effect. A blue phase liquid crystal is placed between two parallel electrode plates to form a Kerr cell. An external electric field acts on the blue phase liquid crystal through the parallel electrode plate. Under the action of an external electric field, the blue phase liquid crystal becomes an optical uniaxial crystal. The direction of the optical axis is parallel to the direction of the electric field. When linearly polarized light passes through the blue-phase liquid crystal in a direction perpendicular to the electric field, it is decomposed into two linearly polarized light, one light vector follows the direction of the electric field, and the other light vector is perpendicular to the electric field.

Their refractive indices are called normal refractive index n0 and abnormal refractive index ne, respectively. Blue phase liquid crystals are positive or negative birefringent materials, depending on whether the value of n-n0 is positive or negative.

· However, the structure of the Kerr box is not suitable for displays, because according to the standard Kerr box structure, the voltage is applied between two parallel electrode plates, that is, the electric field is perpendicular to the electrode plate, and the incident light must be parallel to the electric field. Incidence between electrode plates. As a display, incident light is incident perpendicular to two parallel transparent electrode plates. To generate an electric field perpendicular to the incident light, parallel electrodes can only be made on the lower transparent electrode plate. In order to enhance the electric field, the two parallel electrodes of each group must be close to each other, that is, made as coplanar

The liquid crystal must be fully stirred before use before being poured. The liquid crystal with solid chiral agent added must be heated to 60 degrees Celsius, then quickly cooled to room temperature and fully stirred. And it should not be left for too long during use. Especially for low threshold voltage liquid crystals, because of the different characteristics of low threshold voltage liquid crystals, the following aspects should be paid attention to when using these liquid crystals:

1. The liquid crystal should be fully stirred before use, and the prepared liquid crystal should be put into production immediately, to minimize the storage time to avoid the occurrence of chromatography.

Liquid crystal is a very novel intermediate state appearing in nature, which has triggered a whole new research field. Nature is made up of a variety of different substances. Previously, it was well known that matter exists in three states: solid, liquid, and gaseous. The solid state can be divided into crystalline and amorphous. In crystalline solids, molecules have orientation order and position order, so-called long-range order. Of course, these molecules vibrate a little at the equilibrium position, but on average, they have maintained this highly ordered arrangement. In this way, the forces between individual molecules are superimposed, and a large external force is needed to destroy this ordered structure of the solid, so the solid is hard and has a certain shape. It is difficult to deform. When a solid solid is heated, in general it will transform into an isotropic liquid at the melting point. This isotropic liquid does not have long-range ordering of molecular arrangements. That is, the molecules do not occupy a defined position and are not oriented in a particular way. The liquid does not have a fixed shape, but usually takes the shape of a container and has fluidity. However, the interaction force between molecules is still quite strong. The molecules maintain a specific distance from each other, so the liquid has a constant density and is difficult to compress. At higher temperatures, substances usually appear gaseous. At this time, the order of the molecular arrangement is smaller than that of the liquid. The intermolecular interaction is even smaller, and the molecules take chaotic movements, causing them to eventually spread throughout the container. Therefore, the gas does not have a certain shape, has no constant density, and is easy to compress. ^[4]

1972 Gruen Teletime, the first watch to use an LCD monitor.

1973 Sharp EL-805, the first calculator to use an LCD monitor. In 1973, Japan's Sonic Corporation applied liquid crystals for the first time to make digital displays for electronic calculators. Liquid crystal is the main display device for laptops and palmtop computers, and it also plays a very important role in projectors.

1981 EPSON HX-20, the first portable computer using an LCD monitor.

NEC UltraLite in 1989, the first notebook computer. ^[4]

Liquid crystal display materials have obvious advantages: low driving voltage, small power consumption, high reliability, large amount of display information, color display, no flicker, no harm to the human body,

The electro-optical effect of liquid crystal refers to its interference, scattering, diffraction, optical rotation, absorption and other optical phenomena modulated by electric fields.

According to the characteristics of the liquid crystal's discoloration, people use it to indicate temperature, alarm gas and so on. For example, liquid crystals can change color from red to green and blue with changes in temperature. This will indicate the temperature in an experiment. Liquid crystals also change color when they encounter toxic gases such as hydrogen chloride and hydrocyanic acid.

The widespread use of liquid crystals in liquid crystal displays depends on the presence or absence of an electric field in the optical properties of certain liquid crystal substances. In a typical device, a liquid crystal layer (usually 10 m thick) sits on two polarizers, passing through (facing the other at 90 °). The choice of liquid crystal orientation is such that the relaxation phase is a twisted person (see Twisted Nematic Field Effect). This twisted phase-adjusted light passes through the first polarizer, allowing it to pass through the second polarizer (and reflect back to the viewer if a mirror is provided). The transparency of the device thus emerges. When an electric field is applied to the liquid crystal layer, the long molecular axes are often aligned parallel to the electric field to gradually unravel the center of the liquid crystal layer. In this state, the liquid crystal molecules do not adjust the light, so that the polarization of the light is absorbed by the first polarizer and the second polarizer, and the device loses transparency with voltage. In this way, the electric field can be used to direct the pixel switch between transparent or opaque. Color LCD systems use the same technology to generate color filters for red, green, and blue pixels. Similar principles can be used to make other liquid crystal optics.

Liquid crystal tunable filters are used as electro-optic devices, for example, in hyperspectral imaging.

The pitch and thermal temperature of chiral liquid crystals can be used as coarse liquid crystal thermometers because the color of the material changes with the pitch. LCD color transitions are thermometers used in many aquariums and swimming pools as well as baby or bath thermometers. Other liquid crystal materials change color when stretched or emphasized. Therefore, LCD panels are commonly used in industry to find hot spots, map heat flow, measure stress distribution patterns, and so on. The formation of liquid crystals in a fluid is used to detect failures caused by electrical hot spots in the semiconductor industry.

Liquid crystal lasers use one of the liquid crystals in the laser medium instead of an external mirror distributed feedback mechanism. The photonic band gap created by the liquid crystal periodic dielectric structure has a low threshold for high-output devices that provide stable monochromatic emission.

Polymer-dispersed liquid crystal (PDLC) watches and rolls can be used as adhesives for smart films that are electrically transparent and provide privacy between opaque.

Many common liquids, such as soapy water, actually have many liquid crystal phases depending on their concentration in water.

The production of liquid crystal displays (LCDs) is based on twisted nematic liquid crystal displays. Nematic liquid crystals are designed to have two exactly opposite components at the ends of the molecular structure to produce a strong positive dielectric anisotropy, and the structure is designed to be a linear body. Similarly, LCD TVs use a coplanar conversion mode and a wide viewing angle, while using a linear liquid crystal structure with positive dielectric anisotropy. In contrast, competing LCD TV technologies give nematic liquid crystals with vertical alignment and have negative dielectric anisotropy.

Liquid crystal displays (LCDs) have undergone a series of innovations in recent years. For example, light emitting diodes (LEDs) are increasingly used as background light sources, because LEDs have improved performance, lower cost, and longer life than ordinary fluorescent lamps, and the most important thing is that LEDs consume less energy than fluorescent lamps. The color filter of the traditional liquid crystal display (LCD) will waste more than half of the light energy, and the LED reduces the energy loss by generating the color frame (FSC) sequence.

The benefits brought by FSC will be huge. The level of energy loss caused by this technology is lower than any other display; simple, environmentally friendly, and the cost is cheaper due to the elimination of color filters; the equipment can be used at lower temperatures, Eliminates the possibility of motion blur, highlighting, realistic 3D display, and achievements in high-resolution multi-screen reflection. ^[4]