arXiv subjects
Publications and source records attributed to Mojca Cepic.
The article presents a straightforward experiment that directly and illustratively demonstrates double refraction. For this purpose, two liquid crystalline cells were designed, which enable qualitative and quantitative measurements of the extraordinary refractive index direction dependency in a uniaxial nematic liquid crystal.
The paper presents a teaching module about liquid crystals. Since liquid crystals are linked to everyday student experiences and are also a topic of a current scientific research, they are an excellent candidate of a modern topic to be introduced into education. We show that liquid crystals can provide a file rouge through several fields of physics such as thermodynamics, optics and electromagnetism. We discuss what students should learn about liquid crystals and what physical concepts they should know before considering them. In the presentation of the teaching module that consists of a lecture and experimental work in a chemistry and physics lab, we focus on experiments on phase transitions, polarization of light, double refraction and colours. A pilot evaluation of the module was performed among pre-service primary school teachers who have no special preference for natural sciences. The evaluation shows that the module is very efficient in transferring knowledge. A prior study showed that the informally obtained pre-knowledge on liquid crystals of the first year students on several different study fields is negligible. Since the social science students are the ones that are the least interested in natural sciences it can be expected that students in any study programme will on average achieve at least as good conceptual understanding of phenomena related to liquid crystals as the group involved in the pilot study.
We use the discrete phenomenological model to study theoretically the phase diagrams in antiferroelectric liquid crystals (AFLCs) as a function of optical purity and temperature. Recent experiments have shown that in some systems the number of phases is reduced if the optical purity is extremely high. In some materials the SmC$_{A}^{\star}$ phase is the only stable tilted smectic phase in the pure sample. In the scope of the presented model this high sensitivity of the phase sequence in the AFLCs to optical purity is attributed to the piezoelectric coupling which is reduced if optical purity is reduced. We limit our study to three topologically equal phases - SmC$^{*}$, SmC$_α^{*}$ and SmC$_{A}^{*}$ and show that the reduction of optical purity forces the system from the antiferroelectric to the ferroelectric phase with a possible SmC$_α^{\star}$ between them. The effect of the flexoelectric and quadrupolar coupling is considered as well. If the phase diagram includes only two phases, SmC$^{\star}$ and SmC$%_{A}^{\star}$, the flexoelectric coupling is very small. The materials which exhibit the SmC$_α^{\star}$ in a certain range of optical purity and temperature, can be expected to have a significant flexoelectric coupling that is comparable with the piezoelectric coupling. And finally, when temperature is lowered the phase sequence SmA $\to $ SmC$%_α^{\star}$ $\to $ SmC$^{\star}$ $\to $ SmC$%_{A}^{\star}$ is possible only in materials in which quadrupolar coupling is very strong.
The behavior of polar columnar phases made of bent shaped molecules (B_1Rev and B_1RevTilted) was studied under applied electric field. There are two competing mechanisms of switching in polar B_1RevTilted columnar phase: the collective rotation around the long molecular axis and the collective rotation around the tilt cone. Respectively monostable and bistable optical switching is observed for antiferroelectric B_1RevTilted phase. The main factor discriminating the type of the switching is a width of the column cross-section.
A simple experiment is presented that enables qualitative and quantitative measurement of the extraordinary refractive index direction dependency in an uniaxial nematic liquid crystal. Three liquid crystaline cells were designed in which elongated molecules of nematic liquid crystal align in directions which enable to observe the variation of extraordinary refractive index as a function of the direction of light.
The free energy of antiferroelectric liquid crystal which takes into account polar order explicitly is presented. Steric, van der Waals, piezoelectric and flexoelectric interactions to the nearest layers and dipolar electrostatic interactions to the nearest and to the next nearest layers induce indirect tilt interactions with chiral and achiral properties, which extend to the third and to the fourth nearest layers. Chiral indirect interactions between tilts can be large and induce helicoidal modulations even in systems with negligible chiral van der Waals interactions. If indirect chiral interactions compete with chiral van der Waals interactions, the helix unwinding is possible. Although strength of microscopic interactions change monotonically with decreasing temperature, effective interlayer interactions change nonmonotonically and give rise to nonmonotouous change of modulation period through various phases. Increased enatiomeric excess i.e. increased chirality changes the phase sequence.
Phenomenological model of chiral polar smectics is introduced with interactions up to the fourth neighboring layers. The minimization of the free energy gives three stable structures: the ferroelectric Sm C$^*$ phase, the antiferroelectric Sm C$^*_A$ phase and the hellicoidally modulated structure of the Sm C$^*_H$ phase. The phase Sm C$^*_α$ can be recognized as the Sm C$^*_H$ phase below Sm A phase and as the Sm C$^*_{FI2}$ phase or as the Sm C$^*_{FI1}$ phase when appearing between the Sm C$^*$ phase and the Sm C$^*_A$ phase. Stability of these phases is analyzed and the phase diagram in the space of model parameters is presented.