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Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal. For instance, a liquid crystal (LC) may flow like a liquid, but have the molecules in the liquid arranged and oriented in a crystal-like way. There are many different types of LC phases, which can be distinguished based on their different optical properties (such as birefringence). Viewed in a microscope under polarized light illumination, a liquid crystal material will appear to have a distinct texture. Each 'patch' in the texture corresponds to a domain where the LC molecules are oriented in a different direction. Within a domain, however, the molecules are well ordered. Liquid crystal materials may not always be in an LC phase (just as water is not always in the liquid phase: it may also be found in the solid or gas phase). Liquid crystals can be divided into thermotropic and lyotrophic LCs. Thermotropic LCs exhibit a phase transition into the LC phase as temperature is changed, whereas lyotropic LCs exhibit phase transitions as a function of concentration.
Molecules that exhibit liquid crystal phases are called 'mesogens.' For a molecule to display an LC phase, it must generally be rigid and anisotropic (i.e. longer in one direction than another). Most mesogens fall into the 'rigid-rod' class, although disk-like mesogens are also known.
Liquid crystal phases
The various LC phases can be characterized by the type of ordering that is present. One can distinguish positional order (whether or not molecules are arranged in any sort of ordered lattice) and orientational order (whether or not molecules are pointing in the same dierction), and moreover order can be either short-range (only between molecules close to each other) or long-range (extending to larger, sometimes macroscopic, dimensions). Most thermotropic LCs will have an isotropic phase at high temperature. That is, heating will eventually drive them into a conventional liquid phase characterized by random and isotropic molecular ordering (little to no long-range order), and fluid-like flow behavior. Under other conditions (for instance, lower temperature), an LC might inhabit one or more phases with significant anisotropic orientational structure and long-range orientational order while still having an ability to flow. The orientational order may be quasicrystalline. One of the most common LC phases is the nematic, where the molecules have no positional order, but they do have long-range orientational order. Thus, the molecules flow and are randomly distributed as in a liquid, but they all point in the same direction (within each domain). The smectic phase is one where in addition to orientation order, the mesogens are grouped into layers, enforcing long-range positional order in one direction. In the smetic A phase, the molecules point perpendicular to the layer planes, whereas in the smectic C phase, the molecules are tilted with respect to the layer planes.
The ordering of liquid crystalline phases is extensive on the molecular scale. This order extends up to the entire domain size, which may be on the order or microns, but usually does not extend to the macroscopic scale as often occurs in classical crystalline solids. However, some techniques (such as the use of boundaries or an applied electric field) can be used to enforce a single ordered domain in a macroscopic liquid crystal sample. The ordering in a liquid crystal might extend along only one dimension, with the material being essentially disordered in the other two directions.
Liquid crystal mesogens are divided into two groups depending on the shape of the molecules. Calamitic liquid crystals consists of rod-like molecules and have order in the direction of the longer axes of the molecules. In contrast, discotic liquid crystals are composed of flat-shaped molecules which align in the direction of the shorter axes of the molecules.
Important types of calamitic liquid crystals include
- nematics (most nematics are uniaxial but biaxial nematics are also known)
- smectics (smectic A , smectic C , and hexatic )
- cholesterics (which can have spiral and quasicrystalline orientational order)
Important types of discotic liquid crystals include
Biological membranes are a form of liquid crystal. Their rod-like molecules (e.g., phospholipids) are organized perpendicularly to the membrane surface, yet the membrane is fluid and elastic. It can also host important proteins such as receptors freely "floating" inside, or partly outside, the membrane.
Applications of liquid crystals
Liquid crystals find wide use in liquid crystal displays, which rely on the optical properties of certain liquid crystalline molecules in the presence or absence of an electric field. In the presence of electric field, these molecules align with the electric field, altering polarization of the light in a certain way.
Lyotropic liquid crystals
A lyotropic liquid crystal is a group of liquid-crystalline assemblies that consists of two or more components and exhibits liquid-crystalline properties in certain concentration ranges. In the lyotropic phases, solvent molecules fill the space around the compounds to provide fluidity to the system. In contrast to thermotropic liquid crystals, these lyotropics have another degree of freedom of concentration that enables them to induce a variety of different phases.
Even within the same phases, their self-assembled structures are tunable by the concentration: for example, in lamellar phases, the layer distances increase with the solvent volume. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is more difficult to analyze their structures and properties than those of thermotropic liquid crystals.
A compound which has two immiscible hydrophilic and hydrophobic parts within the same molecule is called an amphiphilic molecule. Many amphiphilic molecules show lyotropic liquid-crystalline phase sequences depending on the volume balances between the hydrophilic part and hydrophobic part. These structures are formed through the micro-phase segregation of two incompatible components on a nanometer scale.
The content of water or other solvent molecules changes the self-assembled structures as follows:
- Discontinuous cubic phase (micellar phase)
- Hexagonal columnar phase (middle phase)
- Bicontinuous cubic phase
- Lamellar phase
- Bicontinuous cubic phase
- Reverse hexagonal columnar phase
- Inverse cubic phase (Inverse micellar phase)
The same characteristics can be observed in immiscible diblock copolymers.
These lyotropic liquid-crystalline nanostructures are abundant in living systems such as DNA, polypeptides, and cell membranes. Accordingly, lyotropic liquid crystals attract particular attention in the field of biomimetic chemistry.
Effect of chirality
When the molecules that form liquid crystals have asymmetric carbon atoms and when the system has not chirality but racemic modification, the orientation vector of the molecular axis of the liquid crystals changes continuously and a macroscopic spiral structure appears in the system as a result. The cycle of the spiral structure is different for each molecule, but each molecule has the property that it reflects the light corresponding to its cycle. From this property, the liquid crystals change color when the cycle of the spiral structure agrees with the visible rays of light. Some kinds of liquid crystals change the cycle of their spiral structure when the temperature changes. This principle is applied in liquid crystal thermometers.
Nematic liquid crystals, which have spiral structures, are called cholesteric liquid crystals. Cholesteric liquid crystals are not distinguished from nematic liquid crystals thermodynamically; hence cholesteric liquid crystals are sometimes called chiral nematic liquid crystals.
Although almost all chiral liquid crystals include asymmetric carbon atoms in their molecules, it has recently been discovered that macroscopic chirality appears in liquid crystals that consist of bent-core molecules which do not have asymmetric carbon atoms. However, the appearance mechanism of this macroscopic chirality is not yet clear.
- de Gennes, P.G. and Prost, J. The Physics of Liquid Crystals, Claredon Press (1993).
- Chandrasekhar, S. Liquid Crystals 2ND edition, Cambridge Univ Pr Published (1993).
- Kleinert, H. and Maki, K., Lattice Textures in Cholesteric Liquid Crystals, Fortschritte Physik 29, 1 (1981).
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