Optical-Thermal Phenomenon and Structural Study of Liquid Crystal Thin Films

Defect-mediated phase transition has been generally considered as one of the most exciting topics in condensed matter physics in recent years. The phase transitions in reduced-dimensional system have attracted great attention because it involves the research of fundamental physics and technical applications. Free-standing liquid crystal thin films provide an ideal experimental system for the research of reduced-dimension.We have performed AC calorimetry and optical reflectivity studies on various liquid- crystal thin films and reported the following discoveries：
(1) I have investigated the Sm-A- Cry-B Phase transition of 14S5 in a liquid-crystal material, using electron diffraction and optical reflectivity have provided the direct confirmation of the existence of layer-by-layer surface transitions, without going through an intermediate hexatic phase. The molecular interactions are found to be through retarded van der waals forces.
(2) We have also studied heat-capacity measurements near the smectic-A- smectic-C* phase transition in free standing films of one chiral liquid-crystal compound-C7. The heat-capacity behavior confirms the evolution of the transition in the interior layers from first to second order as function of film thickness suggested earlier in tilt angle measurements. The asymmetry in the heat-capacity anomaly exhibits an interesting crossover in thinner films. The surface ordering strength in these films is found to be much larger than that in other orthogonal-smectic films, which we speculate is due to the molecular tilt.
In this thesis, by using our state-of-the-art ac calorimeter and high-resolution optical reflectivity setup, I have systematically studied several phase transitions in 14S5 and C7 liquid-crystal materials.
It can’t be denied that the ac calorimetric and optical-reflectivity studies provide a new and important field for the study of the free-standing liquid-crystal films. Many interesting compounds including their phase diagrams remain to be investigated. The substrate-free property also with the quantized layer number makes free-standing liquid-crystal films unique and suitable for the study of the reduced-dimensional system. Chiral LC compounds have many new phase and some interesting physical phenomena, and of course, need further studies.