Effect of optical purity on phase sequence in antiferroelectric liquid crystals

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$_{\alpha}^{*}$ 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$_{\alpha}^{\star}$ 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$$, the flexoelectric coupling is very small. The materials which exhibit the SmC$_{\alpha}^{\star}$ in a certain range of optical purity and temperature, can be expected to have a significant flexoelectric coupling that is comparable with the piezoelectric coupling. And finally, when temperature is lowered the phase sequence SmA $\to$ SmC$$ $\to$ SmC$^{\star}$ $\to$ SmC$$ is possible only in materials in which quadrupolar coupling is very strong.Comment: 17 pages including 6 figures, submitted to PR

Structure of nanoscale-pitch helical phases: blue phase and twist-bend nematic phase resolved by resonant soft X-ray scattering

Periodic structures of phases with orientational order of molecules, but homogenous electron density distribution: a short pitch cholesteric, blue phase and twist-bend nematic phase, were probed by a resonant soft x-ray scattering (RSoXS) at the carbon K-edge. The theoretical model shows that in case of a simple heliconical nematic structure two resonant signals corresponding to the full and half pitch band should be present, while only the full pitch band is observed in experiment. This suggests that the twist-bend nematic phase has complex structure with a double-helix, built of two interlocked, shifted helices. We confirm that the helical pitch in the twist-bend nematic phase is in a 10 nm range, for both, the chiral and achiral materials. We also show that the symmetry of a blue phase can unambiguously be determined through a resonant enhancement of x-ray diffraction signals, by including polarization effects, which are found to be an important indicator in phase structure determination

Short-range smectic fluctuations and the flexoelectric model of modulated nematic liquid crystals

We show that the flexoelectric model of chiral and achiral modulated nematics predicts the compression modulus that is by orders of magnitude lower than the measured values. The discrepancy is much larger in the chiral modulated nematic phase, in which the measured value of the compression modulus is of the same order of magnitude as in achiral modulated nematics, even though the heliconical pitch is by an order of magnitude larger. The relaxation of a one-constant approximation in the biaxial elastic model used for chiral modulated nematics does not solve the problem. Therefore, we propose a structural model of the modulated nematic phase, which is consistent with the current experimental evidence and can also explain large compression modulus: the structure consists of short-range smectic clusters with a fourfold symmetry and periodicity of two molecular distances. In chiral systems, chiral interactions lead to a helicoidal structure of such clusters

Dielectric response of a ferroelectric nematic liquid crystalline phase in thin cells

We studied dielectric properties of a polar nematic phase (NF) sandwiched between two gold or ITO electrodes, serving as a cell surfaces. In bulk, NF is expected to exhibit a Goldstone mode (phason), because polarization can uniformly rotate with no energy cost. However, because the coupling between the direction of nematic director and polarization is finite, and the confinement, even in the absence of the aligning surface layer, induces some energy cost for a reorientation of polarization, the phason dielectric relaxation frequency is measured in a kHz regime. The phason mode is easily quenched by a bias electric field, which enables fluctuations in the magnitude of polarization to be followed in both, the ferronematic and nematic phases. This amplitude (soft) mode is also influenced by boundary conditions. A theory describing the phase and amplitude fluctuations in the NF phase shows that the free energy of the system and, consequently, the dielectric response are dominated by polarization-related terms with the flexoelectricity being relevant only at a very weak surface anchoring. Contributions due to the nematic elastic terms are always negligible. The model relates the observed low frequency mode to the director fluctuations weakly coupled to polarization fluctuations

Critical behavior of the optical birefringence at the nematic to twist bend nematic phase transition

This research was supported by the National Science Centre (Poland) under the grant no. 2016/22/A/ST5/00319. NV acknowledges the support of the Slovenian Research Agency (ARRS), through the research programme P1-0055.Peer reviewedPublisher PD

Multi-level chirality in liquid crystals formed by achiral molecules

M.S., D.P., and N.V. acknowledge the support of the National Science Centre (Poland) under the grant no. 2016/22/A/ST5/00319. E.G. acknowledges the funding from the Foundation for Polish Science through the Sabbatical Fellowships Program. N.V. acknowledges the support of the Slovenian Research Agency (ARRS), through the research core funding no. P1-0055. R.W. gratefully acknowledges the Carnegie Trust for the Universities of Scotland for funding the award of a PhD scholarship. The beamline 11.0.1.2 at the Advanced Light Source at the Lawrence Berkeley National Laboratory is supported by the director of the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.Peer reviewedPublisher PD

THE HISTORY OF AUTOMOBILISM

Zgodovina avtomobilizma ni le zgodovina avtomobila kakršnega poznamo danes, ampak sega mnogo dlje nazaj v čas, ko so ljudje za prenos svojih bremen uporabljali vozičke in vozove, za pogon pa veter, roke in noge. Šele kasneje so iznašli oziroma dodali motor z notranjim zgorevanjem. Prvi pravi začetki avtomobila segajo v leto 1769, ko je bilo predstavljeno prvo samohodno vozilo in je bilo namenjeno za vojaške potrebe. Že v tem obdobju in kasneje so bile prav vojaške razmere gonilna sila razvoja avtomobilizma, saj so prav te narekovale hitrejši in učinkovitejši razvoj avtomobila. Največja »revolucija« v zgodovini avtomobilizma se je zgodila proti koncu 19. stoletja, ko je Karl Benz izumil bencinski motor in ga kasneje vgradil v vozilo. Takrat avtomobili zaradi previsoke cene niso bili dostopni širši množici, vendar so bili privilegij premožnejšega sloja ljudi. Ta privilegij družbe je končal Henry Ford, ko je širši množici ponudil prvi serijski avtomobil - model T. Tako je postal glavni cilj izdelovalcev avtomobilov ustvariti »ljudsko vozilo«, ki bo dostopno širši množici ljudi.The history of automobilism is not only the history of automobile, known in modern times, but it goes back to the time when people used carriages and handcarts saddled with a burden. They used hand, wind and treadle drive. The first real origin goes back to the year 1769 when the first self-propelled vehicle was presented and used for military purposes. In this period and later on, the military conditions were the ones who dictated the development of automobilism fast and effectivly. The greatest »revolution« in the history of automobilism happened at the end of the 19th century when Karl Benz invented the petrol engine which was later built in a vehicle. At that time the cars weren\u27t accessible to the large crowd of people, however, they were the privilege of the wealthy class. Henry Ford ended this privilege when he offered the first serial car – T model. Accordingly, the main goal of car makers was to create a »people\u27s vehicle« which would be easy of access to the large crowd of people