Behavior of Inclusions with Different Value and Orientation of Topological Dipoles in Ferroelectric Smectic Films

Abstract—Cholesteric droplets in ferroelectric free standing films with tunable anchoring on the droplet boundary are investigated. A droplet and satellite topological defect(s) form a topological dipole. We obtained droplets with different angles α between two radial lines from the droplet center to –1/2 topological defects. Droplets with parallel dipoles form linear chains in which the interparticle distances decrease with increasing the defect angle α. For the first time, the dependence of the interparticle distance on the angle between topological defects was measured. We can adjust the magnitude and orientation of topological dipoles formed by the droplets. For the first time, the droplets with antiparallel topological dipoles were prepared in a smectic film. Interaction of the droplets with parallel and antiparallel dipoles differs drastically. Formation of antiparallel dipoles leads to a decomposition of the droplet pairs and chains of droplets. Our observations may be used to change the magnitude, anisotropy of the interparticle interaction, and structures of inclusions in liquid crystal media

The Effect of Spontaneous Polarization on Two Dimensional Elasticity of Smectic Liquid Crystals

The influence of polarity on orientational elasticity and on structures formed in the director field is studied in free standing smectic films. Periodic stripe patterns and 2π walls in a magnetic field are investi gated. Measurements are performed on a nonpolar racemic mixture, on an optically pure ferroelectric com pound, and in mixtures with different concentrations of the chiral isomers of opposite signs. The structure of periodic stripes changes drastically with the polarity of the film. The ratio of the bend KB and splay KS elastic constants for smectic films is determined as a function of polarization from the structure of periodic stripe patterns and 2π walls. We find that the elastic anisotropy KB/KS increases essentially with increasing the polarity of the film. Changes of the elasticity and the structure of periodic stripes are explained by polarization charge effects

Rearrangement of topological defects and anchoring on the inclusion boundary in ferroelectric smectic membranes

We report experiments on a ferroelectric membrane and droplets with tunable surface properties. In smectic membranes the configuration of the c-director field near inclusions may be rearranged drastically with temperature. The transformation of the c-director field results from the competition between the elastic and polar properties of the membranes.We demonstrate that anchoring conditions on the inclusion boundary are not fixed but depend on the temperature. A dipolar c-director configuration near droplets can evolve to a mixed configuration and to a quadrupolar one. These modifications of the c-director field near the inclusions lead to a change of the interaction between the inclusions, their self-organization, and even to the destruction of structures already formed by the inclusions. Our observations open new possibilities for manipulating inclusions and controlling their self-organization

Shape of nematic droplets in smectic membranes

We found that the nematic droplets in smectic membranes and the configuration of the c-director field near droplets may rearrange drastically with droplet size and temperature. Investigations were made in the membranes with quadrupolar droplets. Large droplets are nearly circular, whereas small droplets become elongated. The elongation of the droplets increases with decreasing droplet size. At high temperature the droplets are found to be spindlelike with two cusps on their boundary. The observed transformations are explained by the competition between the elastic energy of the director field and the surface tension of the interface between the droplets and the smectic membrane

Optical reflectivity study of synclinic and anticlinic structures in thin freely suspended smectic films

Optical reflectivity studies have been conducted on freely suspended films with synclinic and anticlinic structures. For the first time quantitative data were obtained on orientational ordering in films with anticlinic structure. In the same wide temperature range of more than 30 K we observed both transverse and longitudinal ferroelectricity in the films of the same thickness. Equilibrium transition temperatures between structures with transverse and longitudinal polarization were above the bulk transition temperature to the SmC$_{\rm A}^*$ phase. The molecule tilt was determined in superthin films with synclinic and anticlinic ordering

Stepwise transition of a topological defect from the smectic film to the boundary of a dipolar inclusion

Cholesteric droplets accompanied by a topological defect are studied in free standing smectic C* films. We observed a transition between two droplet-defect configurations with the defect in the film and on the droplet boundary. We found that the distance between the droplet surface and the topological defect decreases continuously with increasing temperature and above a certain critical temperature the defect jumps to the droplet boundary. We relate this stepwise change in the defect position to the change in the anchoring on the droplet boundary. This transformation leads to a decrease in the interparticle distances in self-organized chains from droplets. Our simple theory allows us to estimate the value of the anchoring energy

Application of X-ray resonant diffraction to structural studies of liquid crystals

Liquid crystals are soft materials that combine the fluidity of disordered liquids and the long range orientational or positional order of crystalline solids along one or two directions of space. X-ray scattering is widely and generally successfully used to investigate and characterize the microscopic structure of most liquid crystals. In many cases however, the Bragg reflections are forbidden by special symmetries of the unit cell and the low dimensional structure of the liquid crystalline phases are out of reach of conventional X-ray experiments. We show in this paper that this problem can be overcome by resonant scattering of X-rays as it reveals the anisotropy of the tensor structure factor. We review various examples in which the restored forbidden reflections reveal unambiguously the hidden structure of liquid crystalline phases. Moreover, we show that in some cases, a fine analysis of the polarization of the Bragg reflections enables one to discriminate between different structural models. These studies solved long standing questions about biaxial liquid crystal structures and provided new insights into physical phenomena such as supercritical behaviour or commensurate-incommensurate transitions