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Liquid crystal (LC) materials continue to attract significant attention both from the fundamental and from the applications point of view because of richness of their phase diagrams, different types of orientational and translational order and a unique possibility to control the phase behavior and their properties using external fields, temperature, concentration in mixtures, special dopants and sample geometry. In contrast to thermotropic LCs, which are famous for their large scale applications in LC display devices, lyotropic LCs are very promising materials for applications in biotechnology, medicine, and food industry and they are also extremely important because of their role in biological membranes.

Depending on the shape of the aggregates, composed of amphiphilic molecules, lyotropic LCs exhibit different anisotropic phases. The most common lyotropic LC mesophases are lamellar, hexagonal, nematic and cholesteric ones. Lamellar phases are composed of bilayers which are separated by water layers, while hexagonal phases are composed of infinitely long cylinders. In contrast, the building blocks of nematic and cholesteric lyotropic phases are finite size micelles. Thus, finding novel mixtures exhibiting different lyotropic phases and studies of their properties and phase transitions between them are very important for the development of new potential applications in biotechnology including drug delivery systems. Recently a number of mixtures which exhibit biaxial lyotropic nematic phase have been discovered. Such biaxial materials have different properties along all three different directions which may be important for applications.

In our research group, we mainly concentrate on finding novel lyotropic mixtures presenting uniaxial and biaxial nematic and cholesteric phases. The main experimental techniques are

• Polarising microscopy to characterize the textures of lyotropic structures and to determine the uniaxial-to-biaxial nematic and cholesteric phase transitions

Textures of lyotropic (from left to right) discotic nematic, biaxial nematic, calamitic nematic, and discotic cholesteric phases.

• Laser conoscopy to determine the birefringences of nematic phases as a function of temperature and also nematic-nematic phase transitions

The interference pattern of lyotropic (from left to right) discotic nematic, biaxial nematic (near to its phase transition to discotic nematic phase), biaxial nematic (near to its phase transition to calamitic nematic phase) and a calamitic nematic phase.

• Small-angle x-ray scattering (in Sao Paulo University under collaboration) to determine the micelle sizes in lyotropic nematic and cholesteric phase

Saxs pattern of lyotropic (from left to right) discotic nematic, biaxial nematic (near to its phase transition to discotic nematic phase), biaxial nematic (near to its phase transition to calamitic nematic phase) and a calamitic nematic phase.

In addition, the micellization thermodynamics of surfactants are also studied in the research laboratories. For this purpose we have some instruments:

• Surface tensiometer
• Electrical conductivity meter
• Density meter