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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.
The major technical application of liquid crystals since
the mid-1960s is their use in the nowadays widespread liquid
crystal displays (''LCDs''). With a global market volume of
about 28 billion US-$ in the year 2002, liquid crystal displays have
emerged into an established key-technology.
Figure 3: The technical application of
liquid crystals in flat panel displays for desktop- and
notebook-computers or in the displays of cellular phones has
become an indispensable part of modern information and
communication technologies.
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 4: 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 5: Schematic classification of the
branch of "liquid crystals" into the physical chemistry of
condensed matter.
Further information about liquid crystals are for instance
available at
LCI OnLine, as well as in the
Online tutorial of the Heppke-group, TU Berlin, or in the
Online tutorial "What are Liquid Crystals?" of the Chalmers
Liquid Crystal Group, Göteborg.
Information about our ongoing research focuses and projects can be
found
 here.
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