Liquid Crystals

The focus of our research activities is the physical chemistry of the liquid-crystalline state of matter. Classical topics of physical chemistry like the relation between structure and properties, thermodynamics and kinetics of phase transitions, electrical and optical properties of matter as well as structure and dynamics of low-dimensional and biological systems can be studied by means of liquid-crystalline systems in an excellent way. The following sections should deliver introductory insight into the research field of liquid crystals and its technical application.

Liquid crystals are liquids with long-range orientational order (anisotropic fluids), which combine the fluidity of ordinary liquids with the interesting electrical and optical properties of crystalline solids. They are observed as thermodynamically stable phases between the crystalline solid and ordinary isotropic liquid states (thermotropic liquid crystals). Liquid-crystalline structures result from self-organization of strongly anisometric molecules (Figure 1): The majority of liquid crystals are formed by rod-like (calamitic) molecules with a length of approximately 20 to 40 Ångströms. However disc-like (discotic) molecules, such as Phthalocyanincomplexes, Phospholipids as well as rigid DNA-double-helices also form liquid-crystalline systems.

Figure 1: Example of the self-organization of anisometric molecules in liquid-crystalline phases. On the left: rod-like molecules form a nematic liquid, in which the longitudinal axes of the molecules are parallelly aligned to a common preferred direction ("director"). On the right: disc-like (discotic) molecules arrange to molecule-stacks (columns), in which the longitudinal axes are also aligned parallely to the director. As a result of their orientational order, liquid crystals exhibit anisotropic physical properties, just like crystals.

Figure 2: Polarizing microscope picture of the formation of a nematic liquid crystal upon cooling out of the isotropic melt. Because of its optical anisotropy (birefringence) the liquid crystal appears bright between the crossed polarizers of the microscope. In the black areas (left side) we still have an optical isotropic melt.

A fascinating and characteristic feature of liquid-crystalline systems is, that they change their molecular and supermolecular organization drastically as an effect of very small external perturbations: The molecules in liquid crystal displays for instance are reoriented by relatively weak electrical fields. If one dissolves a small amount of chiral molecules in an achiral liquid-crystalline host phase, this results in remarkable macroscopic chirality effects, ranging from helical superstructures to the appearence of ferroelectricity. For this and other reasons liquid crystals - combined with polymers and colloids - are therefore summed up under the generic term ''Soft Matter" and treated under the branch of physical chemistry of condensed matter.

Figure 3: In liquid-crystalline systems elastic deformations are already induced by relatively weak perturbations (e.g. an electric field E). The scale of length of those deformations lies within the range of optical wave lenghts.

Figure 4: Schematic classification of the branch of "liquid crystals" into the physical chemistry of condensed matter.

Nombre: Franklin J. Quintero C.
Asignatura: CRF
Dirección:http://www.ipc.unistuttgart.de/~giesselm/AG_Giesselmann/Forschung/Fluessigkristalle/Fluessigkristalle.html
Ver Blog: http://franklinqcrf.blogspot.com/