Sexy liquid crystal

Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystall. 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 optical properties. When viewed under a microscope using a polarized light source, different liquid crystal phase will appear to have a distinct texture. The contrasting ares in the texture each correspond to a domain where the LC molecules are oriented in a different direction. Within a domain, however, the molecules are well ordered. LC 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 solid and gaseous phase)

Liquid crystal phases

a) Thermotropic liquid crystals

Thermotrophic phases are those that occur in a certain temperatur range. If the temperature is raised too high, termal motion will destroy the delicate cooperative ordering of the LC phase, pushing the material into a conventional isotropic liquid phase. At too low temperature, most LC materials will form a conventional (though anisotropic) crystall. Many thermotropic LCs exhibit a variety of phases as temperature is changed. For instance, a particular mesogen may exhibit various smectic and nematic (and finally anisotropic) phases as temperature is increased. An example of a compound displaying thermotropic LC behaviour is para-azoxyanisole.

- Nematic phase

One of the most common LC phases is the nematic, where the molecules have no positional order, but they have long-range orientational order. Thus the molecules flow and their center of mass positions are randomly distributed as in a liquid, but they all point in the same direction (within each domain). Most nematics are unaxial; they have one axis that is longer and preferred, with the the other two being equivalent (can be approximated as cylinders). The word nematic comes from the Greek, which mean 'thread'. Nematics have fluidity similar to that of ordinary (isotropic) liquids but they can easily aligned by an external magnetic or electric field. An aligned nematic has the optical properties of a uniaxial crystal and this makes them extremly useful in liquid crystal displays (LCD).

- Smectic phase

The smectic phases, which are found at lower temperatures than the nematic, form well-defined layers that can slide over one another like soap. The smectics are thus positionally ordered along one direction.

- Chiral phases

The chiral nematic phase exhibits chirality (handedness). This phase is often called the cholesteric phase because it was first observed for cholesterol derivatives. Only chiral molecules can give rise to such a phase. This phase exhibits a twisting of the molecules perpendicular to the director, with the molecular axis parallel to the director. The finite twist angle between adjacent molecules is due to their asymmetric packing, which results in longer-range chiral order.

- Blue phases

Blue phases are special types of liquid crystal phases that appear in the temperature range between a chiral nematic phase and an isotropic liquid phase. Blue phases have a regular three dimensional cubic structure of defects with lattice periods of several hundred nanometers, and thus they exhibit selective Bragg reflections in the wavelength range of light corresponding to the cubic lattice.

- Discotic phases

Disk shaped mesogens can orient themselves in a layer-like fashion known as the discotic nematic phase. If the disk pack into stacks, the phase is called a discotic columnar. The columns themselves may be organized into rectangular or hexagonal arrays. Chiral discotic phases, similar to the chiral nematic phase, are also known.

b) Lyotropic liquid crystals

A lyotropic liquid crystal consists of two or more components that exhibit 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 lyotropic have another degree of freedom of concentration that enables them to induce a variety of different phases.

c) Metallotropic liquid crystals

Liquid crystal phases can also be based on low-melting inorganic phases like Zink chloride that have a structure formed of linked tetrahedra an easily form glasses. The addition of long chain soaplike molecules lead to a series of new phases that show a variety of liquid crystalline behaviour both as a function of the inorganic-organic composition ratio and of temperature. This class of materials has been named metallotropic.